diff --git a/Jakefile.js b/Jakefile.js
index 0da8e524686..ebea3375994 100644
--- a/Jakefile.js
+++ b/Jakefile.js
@@ -962,6 +962,7 @@ function lintFileAsync(options, path, cb) {
 
 var servicesLintTargets = [
     "navigateTo.ts",
+    "navigationBar.ts",
     "outliningElementsCollector.ts",
     "patternMatcher.ts",
     "services.ts",
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 6a720d947df..83dfdab423a 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -79,6 +79,7 @@ namespace ts {
             getIndexTypeOfType,
             getBaseTypes,
             getReturnTypeOfSignature,
+            getNonNullableType,
             getSymbolsInScope,
             getSymbolAtLocation,
             getShorthandAssignmentValueSymbol,
@@ -120,9 +121,9 @@ namespace ts {
         const nullType = createIntrinsicType(TypeFlags.Null | nullableWideningFlags, "null");
         const emptyArrayElementType = createIntrinsicType(TypeFlags.Undefined | TypeFlags.ContainsUndefinedOrNull, "undefined");
         const unknownType = createIntrinsicType(TypeFlags.Any, "unknown");
+        const neverType = createIntrinsicType(TypeFlags.Never, "never");
 
         const emptyObjectType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
-        const nothingType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
         const emptyGenericType = <GenericType><ObjectType>createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
         emptyGenericType.instantiations = {};
 
@@ -2029,12 +2030,7 @@ namespace ts {
                         writeUnionOrIntersectionType(<UnionOrIntersectionType>type, flags);
                     }
                     else if (type.flags & TypeFlags.Anonymous) {
-                        if (type === nothingType) {
-                            writer.writeKeyword("nothing");
-                        }
-                        else {
-                            writeAnonymousType(<ObjectType>type, flags);
-                        }
+                        writeAnonymousType(<ObjectType>type, flags);
                     }
                     else if (type.flags & TypeFlags.StringLiteral) {
                         writer.writeStringLiteral(`"${escapeString((<StringLiteralType>type).text)}"`);
@@ -2884,6 +2880,10 @@ namespace ts {
             return undefined;
         }
 
+        function addOptionality(type: Type, optional: boolean): Type {
+            return strictNullChecks && optional ? addNullableKind(type, TypeFlags.Undefined) : type;
+        }
+
         // Return the inferred type for a variable, parameter, or property declaration
         function getTypeForVariableLikeDeclaration(declaration: VariableLikeDeclaration): Type {
             if (declaration.flags & NodeFlags.JavaScriptFile) {
@@ -2915,8 +2915,7 @@ namespace ts {
 
             // Use type from type annotation if one is present
             if (declaration.type) {
-                const type = getTypeFromTypeNode(declaration.type);
-                return strictNullChecks && declaration.questionToken ? addNullableKind(type, TypeFlags.Undefined) : type;
+                return addOptionality(getTypeFromTypeNode(declaration.type), /*optional*/ !!declaration.questionToken);
             }
 
             if (declaration.kind === SyntaxKind.Parameter) {
@@ -2938,13 +2937,13 @@ namespace ts {
                     ? getContextuallyTypedThisType(func)
                     : getContextuallyTypedParameterType(<ParameterDeclaration>declaration);
                 if (type) {
-                    return strictNullChecks && declaration.questionToken ? addNullableKind(type, TypeFlags.Undefined) : type;
+                    return addOptionality(type, /*optional*/ !!declaration.questionToken);
                 }
             }
 
             // Use the type of the initializer expression if one is present
             if (declaration.initializer) {
-                return checkExpressionCached(declaration.initializer);
+                return addOptionality(checkExpressionCached(declaration.initializer), /*optional*/ !!declaration.questionToken);
             }
 
             // If it is a short-hand property assignment, use the type of the identifier
@@ -3215,7 +3214,9 @@ namespace ts {
         function getTypeOfFuncClassEnumModule(symbol: Symbol): Type {
             const links = getSymbolLinks(symbol);
             if (!links.type) {
-                links.type = createObjectType(TypeFlags.Anonymous, symbol);
+                const type = createObjectType(TypeFlags.Anonymous, symbol);
+                links.type = strictNullChecks && symbol.flags & SymbolFlags.Optional ?
+                    addNullableKind(type, TypeFlags.Undefined) : type;
             }
             return links.type;
         }
@@ -3670,6 +3671,7 @@ namespace ts {
                 case SyntaxKind.VoidKeyword:
                 case SyntaxKind.UndefinedKeyword:
                 case SyntaxKind.NullKeyword:
+                case SyntaxKind.NeverKeyword:
                 case SyntaxKind.StringLiteralType:
                     return true;
                 case SyntaxKind.ArrayType:
@@ -5005,7 +5007,7 @@ namespace ts {
                 if (type.flags & TypeFlags.Undefined) typeSet.containsUndefined = true;
                 if (type.flags & TypeFlags.Null) typeSet.containsNull = true;
             }
-            else if (type !== nothingType && !contains(typeSet, type)) {
+            else if (type !== neverType && !contains(typeSet, type)) {
                 typeSet.push(type);
             }
         }
@@ -5046,7 +5048,7 @@ namespace ts {
         // a named type that circularly references itself.
         function getUnionType(types: Type[], noSubtypeReduction?: boolean): Type {
             if (types.length === 0) {
-                return nothingType;
+                return neverType;
             }
             if (types.length === 1) {
                 return types[0];
@@ -5066,7 +5068,7 @@ namespace ts {
             if (typeSet.length === 0) {
                 return typeSet.containsNull ? nullType :
                     typeSet.containsUndefined ? undefinedType :
-                    nothingType;
+                    neverType;
             }
             else if (typeSet.length === 1) {
                 return typeSet[0];
@@ -5150,7 +5152,7 @@ namespace ts {
         function getTypeFromStringLiteralTypeNode(node: StringLiteralTypeNode): Type {
             const links = getNodeLinks(node);
             if (!links.resolvedType) {
-                links.resolvedType = getStringLiteralTypeForText(node.text);
+                links.resolvedType = getStringLiteralTypeForText(unescapeIdentifier(node.text));
             }
             return links.resolvedType;
         }
@@ -5214,6 +5216,8 @@ namespace ts {
                     return undefinedType;
                 case SyntaxKind.NullKeyword:
                     return nullType;
+                case SyntaxKind.NeverKeyword:
+                    return neverType;
                 case SyntaxKind.ThisType:
                 case SyntaxKind.ThisKeyword:
                     return getTypeFromThisTypeNode(node);
@@ -5859,28 +5863,28 @@ namespace ts {
                     return isIdenticalTo(source, target);
                 }
 
-                if (target.flags & TypeFlags.Any) return Ternary.True;
-                if (source.flags & TypeFlags.Undefined) {
-                    if (!strictNullChecks || target.flags & (TypeFlags.Undefined | TypeFlags.Void) || source === emptyArrayElementType) return Ternary.True;
-                }
-                if (source.flags & TypeFlags.Null) {
-                    if (!strictNullChecks || target.flags & TypeFlags.Null) return Ternary.True;
-                }
-                if (source.flags & TypeFlags.Enum && target === numberType) return Ternary.True;
-                if (source.flags & TypeFlags.Enum && target.flags & TypeFlags.Enum) {
-                    if (result = enumRelatedTo(source, target, reportErrors)) {
-                        return result;
+                if (!(target.flags & TypeFlags.Never)) {
+                    if (target.flags & TypeFlags.Any) return Ternary.True;
+                    if (source.flags & TypeFlags.Undefined) {
+                        if (!strictNullChecks || target.flags & (TypeFlags.Undefined | TypeFlags.Void) || source === emptyArrayElementType) return Ternary.True;
+                    }
+                    if (source.flags & TypeFlags.Null) {
+                        if (!strictNullChecks || target.flags & TypeFlags.Null) return Ternary.True;
+                    }
+                    if (source.flags & TypeFlags.Enum && target === numberType) return Ternary.True;
+                    if (source.flags & TypeFlags.Enum && target.flags & TypeFlags.Enum) {
+                        if (result = enumRelatedTo(source, target, reportErrors)) {
+                            return result;
+                        }
+                    }
+                    if (source.flags & TypeFlags.StringLiteral && target === stringType) return Ternary.True;
+                    if (relation === assignableRelation || relation === comparableRelation) {
+                        if (source.flags & (TypeFlags.Any | TypeFlags.Never)) return Ternary.True;
+                        if (source === numberType && target.flags & TypeFlags.Enum) return Ternary.True;
+                    }
+                    if (source.flags & TypeFlags.Boolean && target.flags & TypeFlags.Boolean) {
+                        return Ternary.True;
                     }
-                }
-                if (source.flags & TypeFlags.StringLiteral && target === stringType) return Ternary.True;
-
-                if (relation === assignableRelation || relation === comparableRelation) {
-                    if (isTypeAny(source)) return Ternary.True;
-                    if (source === numberType && target.flags & TypeFlags.Enum) return Ternary.True;
-                }
-
-                if (source.flags & TypeFlags.Boolean && target.flags & TypeFlags.Boolean) {
-                    return Ternary.True;
                 }
 
                 if (source.flags & TypeFlags.FreshObjectLiteral) {
@@ -7485,7 +7489,7 @@ namespace ts {
 
         function getTypeWithFacts(type: Type, include: TypeFacts) {
             if (!(type.flags & TypeFlags.Union)) {
-                return getTypeFacts(type) & include ? type : nothingType;
+                return getTypeFacts(type) & include ? type : neverType;
             }
             let firstType: Type;
             let types: Type[];
@@ -7502,7 +7506,7 @@ namespace ts {
                     }
                 }
             }
-            return firstType ? types ? getUnionType(types, /*noSubtypeReduction*/ true) : firstType : nothingType;
+            return firstType ? types ? getUnionType(types, /*noSubtypeReduction*/ true) : firstType : neverType;
         }
 
         function getTypeWithDefault(type: Type, defaultExpression: Expression) {
@@ -7614,15 +7618,16 @@ namespace ts {
                 getInitialTypeOfBindingElement(<BindingElement>node);
         }
 
-        function getFlowTypeOfReference(reference: Node, declaredType: Type, initialType: Type) {
+        function getFlowTypeOfReference(reference: Node, declaredType: Type, assumeInitialized: boolean) {
             let key: string;
-            if (!reference.flowNode || declaredType === initialType && !(declaredType.flags & TypeFlags.Narrowable)) {
+            if (!reference.flowNode || assumeInitialized && !(declaredType.flags & TypeFlags.Narrowable)) {
                 return declaredType;
             }
+            const initialType = assumeInitialized ? declaredType : addNullableKind(declaredType, TypeFlags.Undefined);
             const visitedFlowStart = visitedFlowCount;
             const result = getTypeAtFlowNode(reference.flowNode);
             visitedFlowCount = visitedFlowStart;
-            if (reference.parent.kind === SyntaxKind.NonNullExpression && getTypeWithFacts(result, TypeFacts.NEUndefinedOrNull) === nothingType) {
+            if (reference.parent.kind === SyntaxKind.NonNullExpression && getTypeWithFacts(result, TypeFacts.NEUndefinedOrNull) === neverType) {
                 return declaredType;
             }
             return result;
@@ -7710,7 +7715,7 @@ namespace ts {
 
             function getTypeAtFlowCondition(flow: FlowCondition) {
                 let type = getTypeAtFlowNode(flow.antecedent);
-                if (type !== nothingType) {
+                if (type !== neverType) {
                     // If we have an antecedent type (meaning we're reachable in some way), we first
                     // attempt to narrow the antecedent type. If that produces the nothing type, then
                     // we take the type guard as an indication that control could reach here in a
@@ -7720,7 +7725,7 @@ namespace ts {
                     // narrow that.
                     const assumeTrue = (flow.flags & FlowFlags.TrueCondition) !== 0;
                     type = narrowType(type, flow.expression, assumeTrue);
-                    if (type === nothingType) {
+                    if (type === neverType) {
                         type = narrowType(declaredType, flow.expression, assumeTrue);
                     }
                 }
@@ -7942,7 +7947,7 @@ namespace ts {
                 const targetType = type.flags & TypeFlags.TypeParameter ? getApparentType(type) : type;
                 return isTypeAssignableTo(candidate, targetType) ? candidate :
                     isTypeAssignableTo(type, candidate) ? type :
-                    nothingType;
+                    neverType;
             }
 
             function narrowTypeByTypePredicate(type: Type, callExpression: CallExpression, assumeTrue: boolean): Type {
@@ -8092,11 +8097,11 @@ namespace ts {
                 return type;
             }
             const declaration = localOrExportSymbol.valueDeclaration;
-            const defaultsToDeclaredType = !strictNullChecks || type.flags & TypeFlags.Any || !declaration ||
+            const assumeInitialized = !strictNullChecks || (type.flags & TypeFlags.Any) !== 0 || !declaration ||
                 getRootDeclaration(declaration).kind === SyntaxKind.Parameter || isInAmbientContext(declaration) ||
                 getContainingFunctionOrModule(declaration) !== getContainingFunctionOrModule(node);
-            const flowType = getFlowTypeOfReference(node, type, defaultsToDeclaredType ? type : addNullableKind(type, TypeFlags.Undefined));
-            if (strictNullChecks && !(type.flags & TypeFlags.Any) && !(getNullableKind(type) & TypeFlags.Undefined) && getNullableKind(flowType) & TypeFlags.Undefined) {
+            const flowType = getFlowTypeOfReference(node, type, assumeInitialized);
+            if (!assumeInitialized && !(getNullableKind(type) & TypeFlags.Undefined) && getNullableKind(flowType) & TypeFlags.Undefined) {
                 error(node, Diagnostics.Variable_0_is_used_before_being_assigned, symbolToString(symbol));
                 // Return the declared type to reduce follow-on errors
                 return type;
@@ -8344,7 +8349,7 @@ namespace ts {
             if (isClassLike(container.parent)) {
                 const symbol = getSymbolOfNode(container.parent);
                 const type = container.flags & NodeFlags.Static ? getTypeOfSymbol(symbol) : (<InterfaceType>getDeclaredTypeOfSymbol(symbol)).thisType;
-                return getFlowTypeOfReference(node, type, type);
+                return getFlowTypeOfReference(node, type, /*assumeInitialized*/ true);
             }
 
             if (isInJavaScriptFile(node)) {
@@ -9919,7 +9924,8 @@ namespace ts {
             }
 
             const propType = getTypeOfSymbol(prop);
-            if (node.kind !== SyntaxKind.PropertyAccessExpression || !(prop.flags & (SymbolFlags.Variable | SymbolFlags.Property | SymbolFlags.Accessor)) || isAssignmentTarget(node)) {
+            if (node.kind !== SyntaxKind.PropertyAccessExpression || isAssignmentTarget(node) ||
+                !(propType.flags & TypeFlags.Union) && !(prop.flags & (SymbolFlags.Variable | SymbolFlags.Property | SymbolFlags.Accessor))) {
                 return propType;
             }
             const leftmostNode = getLeftmostIdentifierOrThis(node);
@@ -9936,7 +9942,7 @@ namespace ts {
                     return propType;
                 }
             }
-            return getFlowTypeOfReference(node, propType, propType);
+            return getFlowTypeOfReference(node, propType, /*assumeInitialized*/ true);
         }
 
         function isValidPropertyAccess(node: PropertyAccessExpression | QualifiedName, propertyName: string): boolean {
@@ -11455,6 +11461,12 @@ namespace ts {
             const links = getSymbolLinks(parameter);
             if (!links.type) {
                 links.type = instantiateType(contextualType, mapper);
+                // if inference didn't come up with anything but {}, fall back to the binding pattern if present.
+                if (links.type === emptyObjectType &&
+                    (parameter.valueDeclaration.name.kind === SyntaxKind.ObjectBindingPattern ||
+                     parameter.valueDeclaration.name.kind === SyntaxKind.ArrayBindingPattern)) {
+                    links.type = getTypeFromBindingPattern(<BindingPattern>parameter.valueDeclaration.name);
+                }
                 assignBindingElementTypes(<ParameterDeclaration>parameter.valueDeclaration);
             }
             else if (isInferentialContext(mapper)) {
@@ -11547,6 +11559,9 @@ namespace ts {
                 else {
                     const hasImplicitReturn = !!(func.flags & NodeFlags.HasImplicitReturn);
                     types = checkAndAggregateReturnExpressionTypes(<Block>func.body, contextualMapper, isAsync, hasImplicitReturn);
+                    if (!types) {
+                        return neverType;
+                    }
                     if (types.length === 0) {
                         if (isAsync) {
                             // For an async function, the return type will not be void, but rather a Promise for void.
@@ -11555,12 +11570,9 @@ namespace ts {
                                 error(func, Diagnostics.An_async_function_or_method_must_have_a_valid_awaitable_return_type);
                                 return unknownType;
                             }
-
                             return promiseType;
                         }
-                        else {
-                            return voidType;
-                        }
+                        return voidType;
                     }
                 }
                 // When yield/return statements are contextually typed we allow the return type to be a union type.
@@ -11640,7 +11652,7 @@ namespace ts {
                         // the native Promise<T> type by the caller.
                         type = checkAwaitedType(type, body.parent, Diagnostics.Return_expression_in_async_function_does_not_have_a_valid_callable_then_member);
                     }
-                    if (!contains(aggregatedTypes, type)) {
+                    if (type !== neverType && !contains(aggregatedTypes, type)) {
                         aggregatedTypes.push(type);
                     }
                 }
@@ -11648,6 +11660,9 @@ namespace ts {
                     hasOmittedExpressions = true;
                 }
             });
+            if (aggregatedTypes.length === 0 && !hasOmittedExpressions && !hasImplicitReturn) {
+                return undefined;
+            }
             if (strictNullChecks && aggregatedTypes.length && (hasOmittedExpressions || hasImplicitReturn)) {
                 if (!contains(aggregatedTypes, undefinedType)) {
                     aggregatedTypes.push(undefinedType);
@@ -11683,7 +11698,10 @@ namespace ts {
 
             const hasExplicitReturn = func.flags & NodeFlags.HasExplicitReturn;
 
-            if (returnType && !hasExplicitReturn) {
+            if (returnType === neverType) {
+                error(func.type, Diagnostics.A_function_returning_never_cannot_have_a_reachable_end_point);
+            }
+            else if (returnType && !hasExplicitReturn) {
                 // minimal check: function has syntactic return type annotation and no explicit return statements in the body
                 // this function does not conform to the specification.
                 // NOTE: having returnType !== undefined is a precondition for entering this branch so func.type will always be present
@@ -13394,7 +13412,7 @@ namespace ts {
 
             // Abstract methods can't have an implementation -- in particular, they don't need one.
             if (!isExportSymbolInsideModule && lastSeenNonAmbientDeclaration && !lastSeenNonAmbientDeclaration.body &&
-                !(lastSeenNonAmbientDeclaration.flags & NodeFlags.Abstract)) {
+                !(lastSeenNonAmbientDeclaration.flags & NodeFlags.Abstract) && !lastSeenNonAmbientDeclaration.questionToken) {
                 reportImplementationExpectedError(lastSeenNonAmbientDeclaration);
             }
 
@@ -14747,7 +14765,7 @@ namespace ts {
                 arrayType = getUnionType(filter((arrayOrStringType as UnionType).types, t => !(t.flags & TypeFlags.StringLike)));
             }
             else if (arrayOrStringType.flags & TypeFlags.StringLike) {
-                arrayType = nothingType;
+                arrayType = neverType;
             }
             const hasStringConstituent = arrayOrStringType !== arrayType;
             let reportedError = false;
@@ -14759,7 +14777,7 @@ namespace ts {
 
                 // Now that we've removed all the StringLike types, if no constituents remain, then the entire
                 // arrayOrStringType was a string.
-                if (arrayType === nothingType) {
+                if (arrayType === neverType) {
                     return stringType;
                 }
             }
@@ -14820,7 +14838,7 @@ namespace ts {
             if (func) {
                 const signature = getSignatureFromDeclaration(func);
                 const returnType = getReturnTypeOfSignature(signature);
-                if (strictNullChecks || node.expression) {
+                if (strictNullChecks || node.expression || returnType === neverType) {
                     const exprType = node.expression ? checkExpressionCached(node.expression) : undefinedType;
 
                     if (func.asteriskToken) {
@@ -18284,7 +18302,7 @@ namespace ts {
             }
 
             if (node.parent.kind === SyntaxKind.ObjectLiteralExpression) {
-                if (checkGrammarForInvalidQuestionMark(node, node.questionToken, Diagnostics.A_class_member_cannot_be_declared_optional)) {
+                if (checkGrammarForInvalidQuestionMark(node, node.questionToken, Diagnostics.An_object_member_cannot_be_declared_optional)) {
                     return true;
                 }
                 else if (node.body === undefined) {
@@ -18293,9 +18311,6 @@ namespace ts {
             }
 
             if (isClassLike(node.parent)) {
-                if (checkGrammarForInvalidQuestionMark(node, node.questionToken, Diagnostics.A_class_member_cannot_be_declared_optional)) {
-                    return true;
-                }
                 // Technically, computed properties in ambient contexts is disallowed
                 // for property declarations and accessors too, not just methods.
                 // However, property declarations disallow computed names in general,
@@ -18517,8 +18532,7 @@ namespace ts {
 
         function checkGrammarProperty(node: PropertyDeclaration) {
             if (isClassLike(node.parent)) {
-                if (checkGrammarForInvalidQuestionMark(node, node.questionToken, Diagnostics.A_class_member_cannot_be_declared_optional) ||
-                    checkGrammarForNonSymbolComputedProperty(node.name, Diagnostics.A_computed_property_name_in_a_class_property_declaration_must_directly_refer_to_a_built_in_symbol)) {
+                if (checkGrammarForNonSymbolComputedProperty(node.name, Diagnostics.A_computed_property_name_in_a_class_property_declaration_must_directly_refer_to_a_built_in_symbol)) {
                     return true;
                 }
             }
diff --git a/src/compiler/declarationEmitter.ts b/src/compiler/declarationEmitter.ts
index c24135ba52e..397b135bf23 100644
--- a/src/compiler/declarationEmitter.ts
+++ b/src/compiler/declarationEmitter.ts
@@ -395,6 +395,7 @@ namespace ts {
                 case SyntaxKind.VoidKeyword:
                 case SyntaxKind.UndefinedKeyword:
                 case SyntaxKind.NullKeyword:
+                case SyntaxKind.NeverKeyword:
                 case SyntaxKind.ThisType:
                 case SyntaxKind.StringLiteralType:
                     return writeTextOfNode(currentText, type);
@@ -1129,7 +1130,7 @@ namespace ts {
                     // what we want, namely the name expression enclosed in brackets.
                     writeTextOfNode(currentText, node.name);
                     // If optional property emit ?
-                    if ((node.kind === SyntaxKind.PropertyDeclaration || node.kind === SyntaxKind.PropertySignature) && hasQuestionToken(node)) {
+                    if ((node.kind === SyntaxKind.PropertyDeclaration || node.kind === SyntaxKind.PropertySignature || node.kind === SyntaxKind.Parameter) && hasQuestionToken(node)) {
                         write("?");
                     }
                     if ((node.kind === SyntaxKind.PropertyDeclaration || node.kind === SyntaxKind.PropertySignature) && node.parent.kind === SyntaxKind.TypeLiteral) {
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 899b4292f04..4eb16c5bcb0 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -315,10 +315,6 @@
         "category": "Error",
         "code": 1110
     },
-    "A class member cannot be declared optional.": {
-        "category": "Error",
-        "code": 1112
-    },
     "A 'default' clause cannot appear more than once in a 'switch' statement.": {
         "category": "Error",
         "code": 1113
@@ -1751,6 +1747,10 @@
         "category": "Error",
         "code": 2533
     },
+    "A function returning 'never' cannot have a reachable end point.": {
+        "category": "Error",
+        "code": 2534
+    },
     "JSX element attributes type '{0}' may not be a union type.": {
         "category": "Error",
         "code": 2600
diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index daebf8a930b..b60082969db 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -2368,6 +2368,7 @@ namespace ts {
                 case SyntaxKind.BooleanKeyword:
                 case SyntaxKind.SymbolKeyword:
                 case SyntaxKind.UndefinedKeyword:
+                case SyntaxKind.NeverKeyword:
                     // If these are followed by a dot, then parse these out as a dotted type reference instead.
                     const node = tryParse(parseKeywordAndNoDot);
                     return node || parseTypeReference();
@@ -2410,6 +2411,7 @@ namespace ts {
                 case SyntaxKind.NullKeyword:
                 case SyntaxKind.ThisKeyword:
                 case SyntaxKind.TypeOfKeyword:
+                case SyntaxKind.NeverKeyword:
                 case SyntaxKind.OpenBraceToken:
                 case SyntaxKind.OpenBracketToken:
                 case SyntaxKind.LessThanToken:
diff --git a/src/compiler/scanner.ts b/src/compiler/scanner.ts
index 8979814a7a2..1a2748b38b1 100644
--- a/src/compiler/scanner.ts
+++ b/src/compiler/scanner.ts
@@ -91,6 +91,7 @@ namespace ts {
         "let": SyntaxKind.LetKeyword,
         "module": SyntaxKind.ModuleKeyword,
         "namespace": SyntaxKind.NamespaceKeyword,
+        "never": SyntaxKind.NeverKeyword,
         "new": SyntaxKind.NewKeyword,
         "null": SyntaxKind.NullKeyword,
         "number": SyntaxKind.NumberKeyword,
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 877a1a330fe..81f88098464 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -164,6 +164,7 @@ namespace ts {
         IsKeyword,
         ModuleKeyword,
         NamespaceKeyword,
+        NeverKeyword,
         ReadonlyKeyword,
         RequireKeyword,
         NumberKeyword,
@@ -1772,6 +1773,7 @@ namespace ts {
         getIndexTypeOfType(type: Type, kind: IndexKind): Type;
         getBaseTypes(type: InterfaceType): ObjectType[];
         getReturnTypeOfSignature(signature: Signature): Type;
+        getNonNullableType(type: Type): Type;
 
         getSymbolsInScope(location: Node, meaning: SymbolFlags): Symbol[];
         getSymbolAtLocation(node: Node): Symbol;
@@ -2170,11 +2172,12 @@ namespace ts {
         ESSymbol                = 0x01000000,  // Type of symbol primitive introduced in ES6
         ThisType                = 0x02000000,  // This type
         ObjectLiteralPatternWithComputedProperties = 0x04000000,  // Object literal type implied by binding pattern has computed properties
+        Never                   = 0x08000000,  // Never type
 
         /* @internal */
         Nullable = Undefined | Null,
         /* @internal */
-        Intrinsic = Any | String | Number | Boolean | ESSymbol | Void | Undefined | Null,
+        Intrinsic = Any | String | Number | Boolean | ESSymbol | Void | Undefined | Null | Never,
         /* @internal */
         Primitive = String | Number | Boolean | ESSymbol | Void | Undefined | Null | StringLiteral | Enum,
         StringLike = String | StringLiteral,
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 42ba6b9d0fc..9a86e223ebd 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -613,6 +613,7 @@ namespace ts {
             case SyntaxKind.BooleanKeyword:
             case SyntaxKind.SymbolKeyword:
             case SyntaxKind.UndefinedKeyword:
+            case SyntaxKind.NeverKeyword:
                 return true;
             case SyntaxKind.VoidKeyword:
                 return node.parent.kind !== SyntaxKind.VoidExpression;
diff --git a/src/harness/fourslash.ts b/src/harness/fourslash.ts
index d6035aaa026..c237734e1df 100644
--- a/src/harness/fourslash.ts
+++ b/src/harness/fourslash.ts
@@ -1968,12 +1968,12 @@ namespace FourSlash {
             }
         }
 
-        public verifyGetScriptLexicalStructureListCount(expected: number) {
+        public verifyNavigationBarCount(expected: number) {
             const items = this.languageService.getNavigationBarItems(this.activeFile.fileName);
             const actual = this.getNavigationBarItemsCount(items);
 
             if (expected !== actual) {
-                this.raiseError(`verifyGetScriptLexicalStructureListCount failed - found: ${actual} navigation items, expected: ${expected}.`);
+                this.raiseError(`verifyNavigationBarCount failed - found: ${actual} navigation items, expected: ${expected}.`);
             }
         }
 
@@ -1989,19 +1989,19 @@ namespace FourSlash {
             return result;
         }
 
-        public verifyGetScriptLexicalStructureListContains(name: string, kind: string) {
+        public verifyNavigationBarContains(name: string, kind: string) {
             const items = this.languageService.getNavigationBarItems(this.activeFile.fileName);
 
             if (!items || items.length === 0) {
-                this.raiseError("verifyGetScriptLexicalStructureListContains failed - found 0 navigation items, expected at least one.");
+                this.raiseError("verifyNavigationBarContains failed - found 0 navigation items, expected at least one.");
             }
 
             if (this.navigationBarItemsContains(items, name, kind)) {
                 return;
             }
 
-            const missingItem = { name: name, kind: kind };
-            this.raiseError(`verifyGetScriptLexicalStructureListContains failed - could not find the item: ${JSON.stringify(missingItem, undefined, 2)} in the returned list: (${JSON.stringify(items, undefined, 2)})`);
+            const missingItem = { name, kind };
+            this.raiseError(`verifyNavigationBarContains failed - could not find the item: ${JSON.stringify(missingItem, undefined, 2)} in the returned list: (${JSON.stringify(items, undefined, 2)})`);
         }
 
         private navigationBarItemsContains(items: ts.NavigationBarItem[], name: string, kind: string) {
@@ -2021,6 +2021,20 @@ namespace FourSlash {
             return false;
         }
 
+        public verifyNavigationBarChildItem(parent: string, name: string, kind: string) {
+            const items = this.languageService.getNavigationBarItems(this.activeFile.fileName);
+
+            for (let i = 0; i < items.length; i++) {
+                const item = items[i];
+                if (item.text === parent) {
+                    if (this.navigationBarItemsContains(item.childItems, name, kind))
+                        return;
+                    const missingItem = { name, kind };
+                    this.raiseError(`verifyNavigationBarChildItem failed - could not find the item: ${JSON.stringify(missingItem)} in the children list: (${JSON.stringify(item.childItems, undefined, 2)})`);
+                }
+            }
+        }
+
         public printNavigationItems(searchValue: string) {
             const items = this.languageService.getNavigateToItems(searchValue);
             const length = items && items.length;
@@ -2033,11 +2047,11 @@ namespace FourSlash {
             }
         }
 
-        public printScriptLexicalStructureItems() {
+        public printNavigationBar() {
             const items = this.languageService.getNavigationBarItems(this.activeFile.fileName);
             const length = items && items.length;
 
-            Harness.IO.log(`NavigationItems list (${length} items)`);
+            Harness.IO.log(`Navigation bar (${length} items)`);
 
             for (let i = 0; i < length; i++) {
                 const item = items[i];
@@ -3029,19 +3043,23 @@ namespace FourSlashInterface {
             this.DocCommentTemplate(/*expectedText*/ undefined, /*expectedOffset*/ undefined, /*empty*/ true);
         }
 
-        public getScriptLexicalStructureListCount(count: number) {
-            this.state.verifyGetScriptLexicalStructureListCount(count);
+        public navigationBarCount(count: number) {
+            this.state.verifyNavigationBarCount(count);
         }
 
         // TODO: figure out what to do with the unused arguments.
-        public getScriptLexicalStructureListContains(
+        public navigationBarContains(
             name: string,
             kind: string,
             fileName?: string,
             parentName?: string,
             isAdditionalSpan?: boolean,
             markerPosition?: number) {
-            this.state.verifyGetScriptLexicalStructureListContains(name, kind);
+            this.state.verifyNavigationBarContains(name, kind);
+        }
+
+        public navigationBarChildItem(parent: string, name: string, kind: string) {
+            this.state.verifyNavigationBarChildItem(parent, name, kind);
         }
 
         public navigationItemsListCount(count: number, searchValue: string, matchKind?: string) {
@@ -3234,8 +3252,8 @@ namespace FourSlashInterface {
             this.state.printNavigationItems(searchValue);
         }
 
-        public printScriptLexicalStructureItems() {
-            this.state.printScriptLexicalStructureItems();
+        public printNavigationBar() {
+            this.state.printNavigationBar();
         }
 
         public printReferences() {
diff --git a/src/services/breakpoints.ts b/src/services/breakpoints.ts
index c79a34b1bfa..02456f5add1 100644
--- a/src/services/breakpoints.ts
+++ b/src/services/breakpoints.ts
@@ -24,7 +24,7 @@ namespace ts.BreakpointResolver {
             // token on same line if trailing trivia (comments or white spaces on same line) part of the last token on that line
             tokenAtLocation = findPrecedingToken(tokenAtLocation.pos, sourceFile);
 
-            // Its a blank line
+            // It's a blank line
             if (!tokenAtLocation || sourceFile.getLineAndCharacterOfPosition(tokenAtLocation.getEnd()).line !== lineOfPosition) {
                 return undefined;
             }
@@ -312,7 +312,7 @@ namespace ts.BreakpointResolver {
 
                                 case SyntaxKind.BinaryExpression:
                                     if ((<BinaryExpression>node.parent).operatorToken.kind === SyntaxKind.CommaToken) {
-                                        // if this is comma expression, the breakpoint is possible in this expression
+                                        // If this is a comma expression, the breakpoint is possible in this expression
                                         return textSpan(node);
                                     }
                                     break;
@@ -387,7 +387,7 @@ namespace ts.BreakpointResolver {
                     return spanInNode(variableDeclaration.parent.parent);
                 }
                 
-                // If this is a destructuring pattern set breakpoint in binding pattern
+                // If this is a destructuring pattern, set breakpoint in binding pattern
                 if (isBindingPattern(variableDeclaration.name)) {
                     return spanInBindingPattern(<BindingPattern>variableDeclaration.name);
                 }
@@ -402,9 +402,9 @@ namespace ts.BreakpointResolver {
 
                 let declarations = variableDeclaration.parent.declarations;
                 if (declarations && declarations[0] !== variableDeclaration) {
-                    // If we cant set breakpoint on this declaration, set it on previous one
+                    // If we cannot set breakpoint on this declaration, set it on previous one
                     // Because the variable declaration may be binding pattern and 
-                    // we would like to set breakpoint in last binding element if thats the case,
+                    // we would like to set breakpoint in last binding element if that's the case,
                     // use preceding token instead
                     return spanInNode(findPrecedingToken(variableDeclaration.pos, sourceFile, variableDeclaration.parent));
                 }
@@ -418,7 +418,7 @@ namespace ts.BreakpointResolver {
 
             function spanInParameterDeclaration(parameter: ParameterDeclaration): TextSpan {
                 if (isBindingPattern(parameter.name)) {
-                    // set breakpoint in binding pattern
+                    // Set breakpoint in binding pattern
                     return spanInBindingPattern(<BindingPattern>parameter.name);
                 }
                 else if (canHaveSpanInParameterDeclaration(parameter)) {
@@ -492,7 +492,7 @@ namespace ts.BreakpointResolver {
 
             function spanInInitializerOfForLike(forLikeStatement: ForStatement | ForOfStatement | ForInStatement): TextSpan {
                 if (forLikeStatement.initializer.kind === SyntaxKind.VariableDeclarationList) {
-                    // declaration list, set breakpoint in first declaration
+                    // Declaration list - set breakpoint in first declaration
                     let variableDeclarationList = <VariableDeclarationList>forLikeStatement.initializer;
                     if (variableDeclarationList.declarations.length > 0) {
                         return spanInNode(variableDeclarationList.declarations[0]);
@@ -578,7 +578,7 @@ namespace ts.BreakpointResolver {
             function spanInCloseBraceToken(node: Node): TextSpan {
                 switch (node.parent.kind) {
                     case SyntaxKind.ModuleBlock:
-                        // If this is not instantiated module block no bp span
+                        // If this is not an instantiated module block, no bp span
                         if (getModuleInstanceState(node.parent.parent) !== ModuleInstanceState.Instantiated) {
                             return undefined;
                         }
@@ -593,7 +593,7 @@ namespace ts.BreakpointResolver {
                             // Span on close brace token
                             return textSpan(node);
                         }
-                        // fall through.
+                        // fall through
 
                     case SyntaxKind.CatchClause:
                         return spanInNode(lastOrUndefined((<Block>node.parent).statements));
@@ -714,7 +714,7 @@ namespace ts.BreakpointResolver {
 
             function spanInOfKeyword(node: Node): TextSpan {
                 if (node.parent.kind === SyntaxKind.ForOfStatement) {
-                    // set using next token
+                    // Set using next token
                     return spanInNextNode(node);
                 }
 
@@ -723,4 +723,4 @@ namespace ts.BreakpointResolver {
             }
         }
    }
-}
\ No newline at end of file
+}
diff --git a/src/services/formatting/formatting.ts b/src/services/formatting/formatting.ts
index 7931f1ea733..20b20356907 100644
--- a/src/services/formatting/formatting.ts
+++ b/src/services/formatting/formatting.ts
@@ -20,13 +20,13 @@ namespace ts.formatting {
         Unknown = -1
     }
 
-    /* 
+    /*
      * Indentation for the scope that can be dynamically recomputed.
-     * i.e 
+     * i.e
      * while(true)
      * { let x;
      * }
-     * Normally indentation is applied only to the first token in line so at glance 'var' should not be touched. 
+     * Normally indentation is applied only to the first token in line so at glance 'var' should not be touched.
      * However if some format rule adds new line between '}' and 'var' 'var' will become
      * the first token in line so it should be indented
      */
@@ -48,15 +48,15 @@ namespace ts.formatting {
           * foo(bar({
           *     $
           * }))
-          * Both 'foo', 'bar' introduce new indentation with delta = 4, but total indentation in $ is not 8. 
+          * Both 'foo', 'bar' introduce new indentation with delta = 4, but total indentation in $ is not 8.
           * foo: { indentation: 0, delta: 4 }
           * bar: { indentation: foo.indentation + foo.delta = 4, delta: 4} however 'foo' and 'bar' are on the same line
           * so bar inherits indentation from foo and bar.delta will be 4
-          * 
+          *
           */
         getDelta(child: TextRangeWithKind): number;
         /**
-          * Formatter calls this function when rule adds or deletes new lines from the text 
+          * Formatter calls this function when rule adds or deletes new lines from the text
           * so indentation scope can adjust values of indentation and delta.
           */
         recomputeIndentation(lineAddedByFormatting: boolean): void;
@@ -130,9 +130,9 @@ namespace ts.formatting {
     function findOutermostParent(position: number, expectedTokenKind: SyntaxKind, sourceFile: SourceFile): Node {
         let precedingToken = findPrecedingToken(position, sourceFile);
 
-        // when it is claimed that trigger character was typed at given position 
+        // when it is claimed that trigger character was typed at given position
         // we verify that there is a token with a matching kind whose end is equal to position (because the character was just typed).
-        // If this condition is not hold - then trigger character was typed in some other context, 
+        // If this condition is not hold - then trigger character was typed in some other context,
         // i.e.in comment and thus should not trigger autoformatting
         if (!precedingToken ||
             precedingToken.kind !== expectedTokenKind ||
@@ -142,12 +142,12 @@ namespace ts.formatting {
 
         // walk up and search for the parent node that ends at the same position with precedingToken.
         // for cases like this
-        // 
+        //
         // let x = 1;
         // while (true) {
-        // } 
+        // }
         // after typing close curly in while statement we want to reformat just the while statement.
-        // However if we just walk upwards searching for the parent that has the same end value - 
+        // However if we just walk upwards searching for the parent that has the same end value -
         // we'll end up with the whole source file. isListElement allows to stop on the list element level
         let current = precedingToken;
         while (current &&
@@ -223,7 +223,7 @@ namespace ts.formatting {
             // 'index' tracks the index of the most recent error that was checked.
             while (true) {
                 if (index >= sorted.length) {
-                    // all errors in the range were already checked -> no error in specified range 
+                    // all errors in the range were already checked -> no error in specified range
                     return false;
                 }
 
@@ -249,7 +249,7 @@ namespace ts.formatting {
 
     /**
       * Start of the original range might fall inside the comment - scanner will not yield appropriate results
-      * This function will look for token that is located before the start of target range 
+      * This function will look for token that is located before the start of target range
       * and return its end as start position for the scanner.
       */
     function getScanStartPosition(enclosingNode: Node, originalRange: TextRange, sourceFile: SourceFile): number {
@@ -274,7 +274,7 @@ namespace ts.formatting {
     }
 
     /*
-     * For cases like 
+     * For cases like
      * if (a ||
      *     b ||$
      *     c) {...}
@@ -284,8 +284,8 @@ namespace ts.formatting {
      * Initial indentation for this node will be 0.
      * Binary expressions don't introduce new indentation scopes, however it is possible
      * that some parent node on the same line does - like if statement in this case.
-     * Note that we are considering parents only from the same line with initial node - 
-     * if parent is on the different line - its delta was already contributed 
+     * Note that we are considering parents only from the same line with initial node -
+     * if parent is on the different line - its delta was already contributed
      * to the initial indentation.
      */
     function getOwnOrInheritedDelta(n: Node, options: FormatCodeOptions, sourceFile: SourceFile): number {
@@ -364,10 +364,10 @@ namespace ts.formatting {
         // local functions
 
         /** Tries to compute the indentation for a list element.
-          * If list element is not in range then 
-          * function will pick its actual indentation 
+          * If list element is not in range then
+          * function will pick its actual indentation
           * so it can be pushed downstream as inherited indentation.
-          * If list element is in the range - its indentation will be equal 
+          * If list element is in the range - its indentation will be equal
           * to inherited indentation from its predecessors.
           */
         function tryComputeIndentationForListItem(startPos: number,
@@ -378,7 +378,7 @@ namespace ts.formatting {
 
             if (rangeOverlapsWithStartEnd(range, startPos, endPos) ||
                 rangeContainsStartEnd(range, startPos, endPos) /* Not to miss zero-range nodes e.g. JsxText */) {
-                
+
                 if (inheritedIndentation !== Constants.Unknown) {
                     return inheritedIndentation;
                 }
@@ -529,12 +529,12 @@ namespace ts.formatting {
             // a useful observations when tracking context node
             //        /
             //      [a]
-            //   /   |   \ 
+            //   /   |   \
             //  [b] [c] [d]
-            // node 'a' is a context node for nodes 'b', 'c', 'd' 
+            // node 'a' is a context node for nodes 'b', 'c', 'd'
             // except for the leftmost leaf token in [b] - in this case context node ('e') is located somewhere above 'a'
             // this rule can be applied recursively to child nodes of 'a'.
-            // 
+            //
             // context node is set to parent node value after processing every child node
             // context node is set to parent of the token after processing every token
 
@@ -567,7 +567,8 @@ namespace ts.formatting {
                 parentDynamicIndentation: DynamicIndentation,
                 parentStartLine: number,
                 undecoratedParentStartLine: number,
-                isListItem: boolean): number {
+                isListItem: boolean,
+                isFirstListItem?: boolean): number {
 
                 let childStartPos = child.getStart(sourceFile);
 
@@ -626,6 +627,10 @@ namespace ts.formatting {
 
                 childContextNode = node;
 
+                if (isFirstListItem && parent.kind === SyntaxKind.ArrayLiteralExpression && inheritedIndentation === Constants.Unknown) {
+                    inheritedIndentation = childIndentation.indentation;
+                }
+
                 return inheritedIndentation;
             }
 
@@ -665,8 +670,9 @@ namespace ts.formatting {
                 }
 
                 let inheritedIndentation = Constants.Unknown;
-                for (let child of nodes) {
-                    inheritedIndentation = processChildNode(child, inheritedIndentation, node, listDynamicIndentation, startLine, startLine, /*isListElement*/ true)
+                for (let i = 0; i < nodes.length; i++) {
+                    const child = nodes[i];
+                    inheritedIndentation = processChildNode(child, inheritedIndentation, node, listDynamicIndentation, startLine, startLine, /*isListElement*/ true, /*isFirstListItem*/ i === 0);
                 }
 
                 if (listEndToken !== SyntaxKind.Unknown) {
@@ -674,7 +680,7 @@ namespace ts.formatting {
                         let tokenInfo = formattingScanner.readTokenInfo(parent);
                         // consume the list end token only if it is still belong to the parent
                         // there might be the case when current token matches end token but does not considered as one
-                        // function (x: function) <-- 
+                        // function (x: function) <--
                         // without this check close paren will be interpreted as list end token for function expression which is wrong
                         if (tokenInfo.token.kind === listEndToken && rangeContainsRange(parent, tokenInfo.token)) {
                             // consume list end token
@@ -733,7 +739,7 @@ namespace ts.formatting {
                         let commentIndentation = dynamicIndentation.getIndentationForComment(currentTokenInfo.token.kind, tokenIndentation, container);
 
                         for (let triviaItem of currentTokenInfo.leadingTrivia) {
-                            const triviaInRange = rangeContainsRange(originalRange, triviaItem); 
+                            const triviaInRange = rangeContainsRange(originalRange, triviaItem);
                             switch (triviaItem.kind) {
                                 case SyntaxKind.MultiLineCommentTrivia:
                                     if (triviaInRange) {
@@ -826,7 +832,7 @@ namespace ts.formatting {
 
                 if (rule.Operation.Action & (RuleAction.Space | RuleAction.Delete) && currentStartLine !== previousStartLine) {
                     lineAdded = false;
-                    // Handle the case where the next line is moved to be the end of this line. 
+                    // Handle the case where the next line is moved to be the end of this line.
                     // In this case we don't indent the next line in the next pass.
                     if (currentParent.getStart(sourceFile) === currentItem.pos) {
                         dynamicIndentation.recomputeIndentation(/*lineAdded*/ false);
@@ -834,7 +840,7 @@ namespace ts.formatting {
                 }
                 else if (rule.Operation.Action & RuleAction.NewLine && currentStartLine === previousStartLine) {
                     lineAdded = true;
-                    // Handle the case where token2 is moved to the new line. 
+                    // Handle the case where token2 is moved to the new line.
                     // In this case we indent token2 in the next pass but we set
                     // sameLineIndent flag to notify the indenter that the indentation is within the line.
                     if (currentParent.getStart(sourceFile) === currentItem.pos) {
diff --git a/src/services/navigationBar.ts b/src/services/navigationBar.ts
index d83d0738ec8..2cd93d3856c 100644
--- a/src/services/navigationBar.ts
+++ b/src/services/navigationBar.ts
@@ -46,7 +46,7 @@ namespace ts.NavigationBar {
         }
 
         function getChildNodes(nodes: Node[]): Node[] {
-            let childNodes: Node[] = [];
+            const childNodes: Node[] = [];
 
             function visit(node: Node) {
                 switch (node.kind) {
@@ -109,7 +109,7 @@ namespace ts.NavigationBar {
                 }
             }
 
-            //for (let i = 0, n = nodes.length; i < n; i++) {
+            // for (let i = 0, n = nodes.length; i < n; i++) {
             //    let node = nodes[i];
 
             //    if (node.kind === SyntaxKind.ClassDeclaration ||
@@ -123,13 +123,13 @@ namespace ts.NavigationBar {
             //    else if (node.kind === SyntaxKind.VariableStatement) {
             //        childNodes.push.apply(childNodes, (<VariableStatement>node).declarations);
             //    }
-            //}
+            // }
             forEach(nodes, visit);
             return sortNodes(childNodes);
         }
 
         function getTopLevelNodes(node: SourceFile): Node[] {
-            let topLevelNodes: Node[] = [];
+            const topLevelNodes: Node[] = [];
             topLevelNodes.push(node);
 
             addTopLevelNodes(node.statements, topLevelNodes);
@@ -157,7 +157,7 @@ namespace ts.NavigationBar {
         function addTopLevelNodes(nodes: Node[], topLevelNodes: Node[]): void {
             nodes = sortNodes(nodes);
 
-            for (let node of nodes) {
+            for (const node of nodes) {
                 switch (node.kind) {
                     case SyntaxKind.ClassDeclaration:
                         topLevelNodes.push(node);
@@ -176,6 +176,7 @@ namespace ts.NavigationBar {
                         break;
                     case SyntaxKind.EnumDeclaration:
                     case SyntaxKind.InterfaceDeclaration:
+                    case SyntaxKind.TypeAliasDeclaration:
                         topLevelNodes.push(node);
                         break;
 
@@ -197,7 +198,7 @@ namespace ts.NavigationBar {
         }
 
         function hasNamedFunctionDeclarations(nodes: NodeArray<Statement>): boolean {
-            for (let s of nodes) {
+            for (const s of nodes) {
                 if (s.kind === SyntaxKind.FunctionDeclaration && !isEmpty((<FunctionDeclaration>s).name.text)) {
                     return true;
                 }
@@ -237,17 +238,17 @@ namespace ts.NavigationBar {
         }
 
         function getItemsWorker(nodes: Node[], createItem: (n: Node) => ts.NavigationBarItem): ts.NavigationBarItem[] {
-            let items: ts.NavigationBarItem[] = [];
+            const items: ts.NavigationBarItem[] = [];
 
-            let keyToItem: Map<NavigationBarItem> = {};
+            const keyToItem: Map<NavigationBarItem> = {};
 
-            for (let child of nodes) {
-                let item = createItem(child);
+            for (const child of nodes) {
+                const item = createItem(child);
                 if (item !== undefined) {
                     if (item.text.length > 0) {
-                        let key = item.text + "-" + item.kind + "-" + item.indent;
+                        const key = item.text + "-" + item.kind + "-" + item.indent;
 
-                        let itemWithSameName = keyToItem[key];
+                        const itemWithSameName = keyToItem[key];
                         if (itemWithSameName) {
                             // We had an item with the same name.  Merge these items together.
                             merge(itemWithSameName, item);
@@ -274,8 +275,8 @@ namespace ts.NavigationBar {
 
                 // Next, recursively merge or add any children in the source as appropriate.
                 outer:
-                for (let sourceChild of source.childItems) {
-                    for (let targetChild of target.childItems) {
+                for (const sourceChild of source.childItems) {
+                    for (const targetChild of target.childItems) {
                         if (targetChild.text === sourceChild.text && targetChild.kind === sourceChild.kind) {
                             // Found a match.  merge them.
                             merge(targetChild, sourceChild);
@@ -382,7 +383,7 @@ namespace ts.NavigationBar {
             return !text || text.trim() === "";
         }
 
-        function getNavigationBarItem(text: string, kind: string, kindModifiers: string, spans: TextSpan[], childItems: NavigationBarItem[] = [], indent: number = 0): NavigationBarItem {
+        function getNavigationBarItem(text: string, kind: string, kindModifiers: string, spans: TextSpan[], childItems: NavigationBarItem[] = [], indent = 0): NavigationBarItem {
             if (isEmpty(text)) {
                 return undefined;
             }
@@ -422,6 +423,9 @@ namespace ts.NavigationBar {
 
                 case SyntaxKind.FunctionDeclaration:
                     return createFunctionItem(<FunctionDeclaration>node);
+
+                case SyntaxKind.TypeAliasDeclaration:
+                    return createTypeAliasItem(<TypeAliasDeclaration>node);
             }
 
             return undefined;
@@ -433,7 +437,7 @@ namespace ts.NavigationBar {
                 }
 
                 // Otherwise, we need to aggregate each identifier to build up the qualified name.
-                let result: string[] = [];
+                const result: string[] = [];
 
                 result.push(moduleDeclaration.name.text);
 
@@ -447,9 +451,9 @@ namespace ts.NavigationBar {
             }
 
             function createModuleItem(node: ModuleDeclaration): NavigationBarItem {
-                let moduleName = getModuleName(node);
+                const moduleName = getModuleName(node);
 
-                let childItems = getItemsWorker(getChildNodes((<Block>getInnermostModule(node).body).statements), createChildItem);
+                const childItems = getItemsWorker(getChildNodes((<Block>getInnermostModule(node).body).statements), createChildItem);
 
                 return getNavigationBarItem(moduleName,
                     ts.ScriptElementKind.moduleElement,
@@ -461,9 +465,9 @@ namespace ts.NavigationBar {
 
             function createFunctionItem(node: FunctionDeclaration): ts.NavigationBarItem  {
                 if (node.body && node.body.kind === SyntaxKind.Block) {
-                    let childItems = getItemsWorker(sortNodes((<Block>node.body).statements), createChildItem);
+                    const childItems = getItemsWorker(sortNodes((<Block>node.body).statements), createChildItem);
 
-                    return getNavigationBarItem(!node.name ? "default": node.name.text,
+                    return getNavigationBarItem(!node.name ? "default" : node.name.text,
                         ts.ScriptElementKind.functionElement,
                         getNodeModifiers(node),
                         [getNodeSpan(node)],
@@ -474,9 +478,18 @@ namespace ts.NavigationBar {
                 return undefined;
             }
 
+            function createTypeAliasItem(node: TypeAliasDeclaration): ts.NavigationBarItem {
+                return getNavigationBarItem(node.name.text,
+                    ts.ScriptElementKind.typeElement,
+                    getNodeModifiers(node),
+                    [getNodeSpan(node)],
+                    [],
+                    getIndent(node));
+            }
+
             function createMemberFunctionLikeItem(node: MethodDeclaration | ConstructorDeclaration): ts.NavigationBarItem  {
                 if (node.body && node.body.kind === SyntaxKind.Block) {
-                    let childItems = getItemsWorker(sortNodes((<Block>node.body).statements), createChildItem);
+                    const childItems = getItemsWorker(sortNodes((<Block>node.body).statements), createChildItem);
                     let scriptElementKind: string;
                     let memberFunctionName: string;
                     if (node.kind === SyntaxKind.MethodDeclaration) {
@@ -500,16 +513,16 @@ namespace ts.NavigationBar {
             }
 
             function createSourceFileItem(node: SourceFile): ts.NavigationBarItem {
-                let childItems = getItemsWorker(getChildNodes(node.statements), createChildItem);
+                const childItems = getItemsWorker(getChildNodes(node.statements), createChildItem);
 
                 if (childItems === undefined || childItems.length === 0) {
                     return undefined;
                 }
 
                 hasGlobalNode = true;
-                let rootName = isExternalModule(node)
+                const rootName = isExternalModule(node)
                     ? "\"" + escapeString(getBaseFileName(removeFileExtension(normalizePath(node.fileName)))) + "\""
-                    : "<global>"
+                    : "<global>";
 
                 return getNavigationBarItem(rootName,
                     ts.ScriptElementKind.moduleElement,
@@ -522,14 +535,14 @@ namespace ts.NavigationBar {
                 let childItems: NavigationBarItem[];
 
                 if (node.members) {
-                    let constructor = <ConstructorDeclaration>forEach(node.members, member => {
+                    const constructor = <ConstructorDeclaration>forEach(node.members, member => {
                         return member.kind === SyntaxKind.Constructor && member;
                     });
 
                     // Add the constructor parameters in as children of the class (for property parameters).
                     // Note that *all non-binding pattern named* parameters will be added to the nodes array, but parameters that
                     // are not properties will be filtered out later by createChildItem.
-                    let nodes: Node[] = removeDynamicallyNamedProperties(node);
+                    const nodes: Node[] = removeDynamicallyNamedProperties(node);
                     if (constructor) {
                         addRange(nodes, filter(constructor.parameters, p => !isBindingPattern(p.name)));
                     }
@@ -537,7 +550,7 @@ namespace ts.NavigationBar {
                     childItems = getItemsWorker(sortNodes(nodes), createChildItem);
                 }
 
-                var nodeName = !node.name ? "default" : node.name.text;
+                const nodeName = !node.name ? "default" : node.name.text;
 
                 return getNavigationBarItem(
                     nodeName,
@@ -549,7 +562,7 @@ namespace ts.NavigationBar {
             }
 
             function createEnumItem(node: EnumDeclaration): ts.NavigationBarItem {
-                let childItems = getItemsWorker(sortNodes(removeComputedProperties(node)), createChildItem);
+                const childItems = getItemsWorker(sortNodes(removeComputedProperties(node)), createChildItem);
                 return getNavigationBarItem(
                     node.name.text,
                     ts.ScriptElementKind.enumElement,
@@ -560,7 +573,7 @@ namespace ts.NavigationBar {
             }
 
             function createInterfaceItem(node: InterfaceDeclaration): ts.NavigationBarItem {
-                let childItems = getItemsWorker(sortNodes(removeDynamicallyNamedProperties(node)), createChildItem);
+                const childItems = getItemsWorker(sortNodes(removeDynamicallyNamedProperties(node)), createChildItem);
                 return getNavigationBarItem(
                     node.name.text,
                     ts.ScriptElementKind.interfaceElement,
@@ -578,7 +591,7 @@ namespace ts.NavigationBar {
         /**
          * Like removeComputedProperties, but retains the properties with well known symbol names
          */
-        function removeDynamicallyNamedProperties(node: ClassDeclaration | InterfaceDeclaration): Declaration[]{
+        function removeDynamicallyNamedProperties(node: ClassDeclaration | InterfaceDeclaration): Declaration[] {
             return filter<Declaration>(node.members, member => !hasDynamicName(member));
         }
 
@@ -606,11 +619,11 @@ namespace ts.NavigationBar {
         const anonClassText = "<class>";
         let indent = 0;
 
-        let rootName = isExternalModule(sourceFile) ?
+        const rootName = isExternalModule(sourceFile) ?
             "\"" + escapeString(getBaseFileName(removeFileExtension(normalizePath(sourceFile.fileName)))) + "\""
             : "<global>";
 
-        let sourceFileItem = getNavBarItem(rootName, ScriptElementKind.moduleElement, [getNodeSpan(sourceFile)]);
+        const sourceFileItem = getNavBarItem(rootName, ScriptElementKind.moduleElement, [getNodeSpan(sourceFile)]);
         let topItem = sourceFileItem;
 
         // Walk the whole file, because we want to also find function expressions - which may be in variable initializer,
@@ -643,12 +656,12 @@ namespace ts.NavigationBar {
             }
         }
 
-        function createNavBarItem(node: Node) : NavigationBarItem {
+        function createNavBarItem(node: Node): NavigationBarItem {
             switch (node.kind) {
                 case SyntaxKind.VariableDeclaration:
                     // Only add to the navbar if at the top-level of the file
                     // Note: "const" and "let" are also SyntaxKind.VariableDeclarations
-                    if(node.parent/*VariableDeclarationList*/.parent/*VariableStatement*/
+                    if (node.parent/*VariableDeclarationList*/.parent/*VariableStatement*/
                            .parent/*SourceFile*/.kind !== SyntaxKind.SourceFile) {
                         return undefined;
                     }
@@ -711,7 +724,7 @@ namespace ts.NavigationBar {
         function getNavBarItem(text: string, kind: string, spans: TextSpan[], kindModifiers = ScriptElementKindModifier.none): NavigationBarItem {
             return {
                 text, kind, kindModifiers, spans, childItems: [], indent, bolded: false, grayed: false
-            }
+            };
         }
 
         function getDefineModuleItem(node: Node): NavigationBarItem {
@@ -724,7 +737,7 @@ namespace ts.NavigationBar {
                 return undefined;
             }
             const callExpr = node.parent as CallExpression;
-            if (callExpr.expression.kind !== SyntaxKind.Identifier || callExpr.expression.getText() !== 'define') {
+            if (callExpr.expression.kind !== SyntaxKind.Identifier || callExpr.expression.getText() !== "define") {
                 return undefined;
             }
 
diff --git a/src/services/services.ts b/src/services/services.ts
index b2f7eb4bdba..ef25fae7bba 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -50,6 +50,7 @@ namespace ts {
         getStringIndexType(): Type;
         getNumberIndexType(): Type;
         getBaseTypes(): ObjectType[];
+        getNonNullableType(): Type;
     }
 
     export interface Signature {
@@ -735,6 +736,9 @@ namespace ts {
                 ? this.checker.getBaseTypes(<InterfaceType><Type>this)
                 : undefined;
         }
+        getNonNullableType(): Type {
+            return this.checker.getNonNullableType(this);
+        }
     }
 
     class SignatureObject implements Signature {
@@ -904,6 +908,7 @@ namespace ts {
             function visit(node: Node): void {
                 switch (node.kind) {
                     case SyntaxKind.FunctionDeclaration:
+                    case SyntaxKind.FunctionExpression:
                     case SyntaxKind.MethodDeclaration:
                     case SyntaxKind.MethodSignature:
                         const functionDeclaration = <FunctionLikeDeclaration>node;
@@ -930,6 +935,7 @@ namespace ts {
                         break;
 
                     case SyntaxKind.ClassDeclaration:
+                    case SyntaxKind.ClassExpression:
                     case SyntaxKind.InterfaceDeclaration:
                     case SyntaxKind.TypeAliasDeclaration:
                     case SyntaxKind.EnumDeclaration:
@@ -944,22 +950,9 @@ namespace ts {
                     case SyntaxKind.SetAccessor:
                     case SyntaxKind.TypeLiteral:
                         addDeclaration(<Declaration>node);
-                    // fall through
-                    case SyntaxKind.Constructor:
-                    case SyntaxKind.VariableStatement:
-                    case SyntaxKind.VariableDeclarationList:
-                    case SyntaxKind.ObjectBindingPattern:
-                    case SyntaxKind.ArrayBindingPattern:
-                    case SyntaxKind.ModuleBlock:
                         forEachChild(node, visit);
                         break;
 
-                    case SyntaxKind.Block:
-                        if (isFunctionBlock(node)) {
-                            forEachChild(node, visit);
-                        }
-                        break;
-
                     case SyntaxKind.Parameter:
                         // Only consider parameter properties
                         if (!(node.flags & NodeFlags.ParameterPropertyModifier)) {
@@ -967,11 +960,15 @@ namespace ts {
                         }
                     // fall through
                     case SyntaxKind.VariableDeclaration:
-                    case SyntaxKind.BindingElement:
-                        if (isBindingPattern((<VariableDeclaration>node).name)) {
-                            forEachChild((<VariableDeclaration>node).name, visit);
+                    case SyntaxKind.BindingElement: {
+                        const decl = <VariableDeclaration> node;
+                        if (isBindingPattern(decl.name)) {
+                            forEachChild(decl.name, visit);
                             break;
                         }
+                        if (decl.initializer)
+                            visit(decl.initializer);
+                    }
                     case SyntaxKind.EnumMember:
                     case SyntaxKind.PropertyDeclaration:
                     case SyntaxKind.PropertySignature:
@@ -1008,6 +1005,9 @@ namespace ts {
                             }
                         }
                         break;
+
+                    default:
+                        forEachChild(node, visit);
                 }
             }
         }
@@ -2770,7 +2770,9 @@ namespace ts {
     /* @internal */ export function getNodeKind(node: Node): string {
         switch (node.kind) {
             case SyntaxKind.ModuleDeclaration: return ScriptElementKind.moduleElement;
-            case SyntaxKind.ClassDeclaration: return ScriptElementKind.classElement;
+            case SyntaxKind.ClassDeclaration:
+            case SyntaxKind.ClassExpression:
+                return ScriptElementKind.classElement;
             case SyntaxKind.InterfaceDeclaration: return ScriptElementKind.interfaceElement;
             case SyntaxKind.TypeAliasDeclaration: return ScriptElementKind.typeElement;
             case SyntaxKind.EnumDeclaration: return ScriptElementKind.enumElement;
@@ -2780,7 +2782,9 @@ namespace ts {
                     : isLet(node)
                         ? ScriptElementKind.letElement
                         : ScriptElementKind.variableElement;
-            case SyntaxKind.FunctionDeclaration: return ScriptElementKind.functionElement;
+            case SyntaxKind.FunctionDeclaration:
+            case SyntaxKind.FunctionExpression:
+                return ScriptElementKind.functionElement;
             case SyntaxKind.GetAccessor: return ScriptElementKind.memberGetAccessorElement;
             case SyntaxKind.SetAccessor: return ScriptElementKind.memberSetAccessorElement;
             case SyntaxKind.MethodDeclaration:
@@ -4366,7 +4370,7 @@ namespace ts {
                         (location.kind === SyntaxKind.ConstructorKeyword && location.parent.kind === SyntaxKind.Constructor)) { // At constructor keyword of constructor declaration
                         // get the signature from the declaration and write it
                         const functionDeclaration = <FunctionLikeDeclaration>location.parent;
-                        const allSignatures = functionDeclaration.kind === SyntaxKind.Constructor ? type.getConstructSignatures() : type.getCallSignatures();
+                        const allSignatures = functionDeclaration.kind === SyntaxKind.Constructor ? type.getNonNullableType().getConstructSignatures() : type.getNonNullableType().getCallSignatures();
                         if (!typeChecker.isImplementationOfOverload(functionDeclaration)) {
                             signature = typeChecker.getSignatureFromDeclaration(functionDeclaration);
                         }
@@ -4564,7 +4568,7 @@ namespace ts {
                             symbolFlags & SymbolFlags.Signature ||
                             symbolFlags & SymbolFlags.Accessor ||
                             symbolKind === ScriptElementKind.memberFunctionElement) {
-                            const allSignatures = type.getCallSignatures();
+                            const allSignatures = type.getNonNullableType().getCallSignatures();
                             addSignatureDisplayParts(allSignatures[0], allSignatures);
                         }
                     }
@@ -4645,7 +4649,7 @@ namespace ts {
 
             const sourceFile = getValidSourceFile(fileName);
             const node = getTouchingPropertyName(sourceFile, position);
-            if (!node) {
+            if (node === sourceFile) {
                 return undefined;
             }
 
@@ -4802,18 +4806,6 @@ namespace ts {
 
             const sourceFile = getValidSourceFile(fileName);
 
-            const node = getTouchingPropertyName(sourceFile, position);
-            if (!node) {
-                return undefined;
-            }
-
-            // Labels
-            if (isJumpStatementTarget(node)) {
-                const labelName = (<Identifier>node).text;
-                const label = getTargetLabel((<BreakOrContinueStatement>node.parent), (<Identifier>node).text);
-                return label ? [createDefinitionInfo(label, ScriptElementKind.label, labelName, /*containerName*/ undefined)] : undefined;
-            }
-
             /// Triple slash reference comments
             const comment = findReferenceInPosition(sourceFile.referencedFiles, position);
             if (comment) {
@@ -4823,6 +4815,7 @@ namespace ts {
                 }
                 return undefined;
             }
+
             // Type reference directives
             const typeReferenceDirective = findReferenceInPosition(sourceFile.typeReferenceDirectives, position);
             if (typeReferenceDirective) {
@@ -4833,6 +4826,18 @@ namespace ts {
                 return undefined;
             }
 
+            const node = getTouchingPropertyName(sourceFile, position);
+            if (node === sourceFile) {
+                return undefined;
+            }
+
+            // Labels
+            if (isJumpStatementTarget(node)) {
+                const labelName = (<Identifier>node).text;
+                const label = getTargetLabel((<BreakOrContinueStatement>node.parent), (<Identifier>node).text);
+                return label ? [createDefinitionInfo(label, ScriptElementKind.label, labelName, /*containerName*/ undefined)] : undefined;
+            }
+
             const typeChecker = program.getTypeChecker();
             let symbol = typeChecker.getSymbolAtLocation(node);
 
@@ -4891,7 +4896,7 @@ namespace ts {
             const sourceFile = getValidSourceFile(fileName);
 
             const node = getTouchingPropertyName(sourceFile, position);
-            if (!node) {
+            if (node === sourceFile) {
                 return undefined;
             }
 
@@ -5628,7 +5633,7 @@ namespace ts {
             const sourceFile = getValidSourceFile(fileName);
 
             const node = getTouchingPropertyName(sourceFile, position);
-            if (!node) {
+            if (node === sourceFile) {
                 return undefined;
             }
 
@@ -6795,7 +6800,7 @@ namespace ts {
             // Get node at the location
             const node = getTouchingPropertyName(sourceFile, startPos);
 
-            if (!node) {
+            if (node === sourceFile) {
                 return;
             }
 
diff --git a/tests/baselines/reference/classWithOptionalParameter.errors.txt b/tests/baselines/reference/classWithOptionalParameter.errors.txt
deleted file mode 100644
index 6e512c2a02e..00000000000
--- a/tests/baselines/reference/classWithOptionalParameter.errors.txt
+++ /dev/null
@@ -1,26 +0,0 @@
-tests/cases/conformance/types/namedTypes/classWithOptionalParameter.ts(4,6): error TS1112: A class member cannot be declared optional.
-tests/cases/conformance/types/namedTypes/classWithOptionalParameter.ts(5,6): error TS1112: A class member cannot be declared optional.
-tests/cases/conformance/types/namedTypes/classWithOptionalParameter.ts(9,6): error TS1112: A class member cannot be declared optional.
-tests/cases/conformance/types/namedTypes/classWithOptionalParameter.ts(10,6): error TS1112: A class member cannot be declared optional.
-
-
-==== tests/cases/conformance/types/namedTypes/classWithOptionalParameter.ts (4 errors) ====
-    // classes do not permit optional parameters, these are errors
-    
-    class C {
-        x?: string;
-         ~
-!!! error TS1112: A class member cannot be declared optional.
-        f?() {}
-         ~
-!!! error TS1112: A class member cannot be declared optional.
-    }
-    
-    class C2<T> {
-        x?: T;
-         ~
-!!! error TS1112: A class member cannot be declared optional.
-        f?(x: T) {}
-         ~
-!!! error TS1112: A class member cannot be declared optional.
-    }
\ No newline at end of file
diff --git a/tests/baselines/reference/classWithOptionalParameter.symbols b/tests/baselines/reference/classWithOptionalParameter.symbols
new file mode 100644
index 00000000000..b85cd5d7a07
--- /dev/null
+++ b/tests/baselines/reference/classWithOptionalParameter.symbols
@@ -0,0 +1,26 @@
+=== tests/cases/conformance/types/namedTypes/classWithOptionalParameter.ts ===
+// classes do not permit optional parameters, these are errors
+
+class C {
+>C : Symbol(C, Decl(classWithOptionalParameter.ts, 0, 0))
+
+    x?: string;
+>x : Symbol(C.x, Decl(classWithOptionalParameter.ts, 2, 9))
+
+    f?() {}
+>f : Symbol(C.f, Decl(classWithOptionalParameter.ts, 3, 15))
+}
+
+class C2<T> {
+>C2 : Symbol(C2, Decl(classWithOptionalParameter.ts, 5, 1))
+>T : Symbol(T, Decl(classWithOptionalParameter.ts, 7, 9))
+
+    x?: T;
+>x : Symbol(C2.x, Decl(classWithOptionalParameter.ts, 7, 13))
+>T : Symbol(T, Decl(classWithOptionalParameter.ts, 7, 9))
+
+    f?(x: T) {}
+>f : Symbol(C2.f, Decl(classWithOptionalParameter.ts, 8, 10))
+>x : Symbol(x, Decl(classWithOptionalParameter.ts, 9, 7))
+>T : Symbol(T, Decl(classWithOptionalParameter.ts, 7, 9))
+}
diff --git a/tests/baselines/reference/classWithOptionalParameter.types b/tests/baselines/reference/classWithOptionalParameter.types
new file mode 100644
index 00000000000..2c62aee6fab
--- /dev/null
+++ b/tests/baselines/reference/classWithOptionalParameter.types
@@ -0,0 +1,26 @@
+=== tests/cases/conformance/types/namedTypes/classWithOptionalParameter.ts ===
+// classes do not permit optional parameters, these are errors
+
+class C {
+>C : C
+
+    x?: string;
+>x : string
+
+    f?() {}
+>f : () => void
+}
+
+class C2<T> {
+>C2 : C2<T>
+>T : T
+
+    x?: T;
+>x : T
+>T : T
+
+    f?(x: T) {}
+>f : (x: T) => void
+>x : T
+>T : T
+}
diff --git a/tests/baselines/reference/controlFlowIteration.types b/tests/baselines/reference/controlFlowIteration.types
index 52be501d68e..cefb250e01d 100644
--- a/tests/baselines/reference/controlFlowIteration.types
+++ b/tests/baselines/reference/controlFlowIteration.types
@@ -4,7 +4,7 @@ let cond: boolean;
 >cond : boolean
 
 function ff() {
->ff : () => void
+>ff : () => never
 
     let x: string | undefined;
 >x : string | undefined
diff --git a/tests/baselines/reference/declFileConstructors.js b/tests/baselines/reference/declFileConstructors.js
index 44e816712fb..8e92b1a36ae 100644
--- a/tests/baselines/reference/declFileConstructors.js
+++ b/tests/baselines/reference/declFileConstructors.js
@@ -247,7 +247,7 @@ export declare class ConstructorWithPrivateParameterProperty {
     constructor(x: string);
 }
 export declare class ConstructorWithOptionalParameterProperty {
-    x: string;
+    x?: string;
     constructor(x?: string);
 }
 export declare class ConstructorWithParameterInitializer {
@@ -281,7 +281,7 @@ declare class GlobalConstructorWithPrivateParameterProperty {
     constructor(x: string);
 }
 declare class GlobalConstructorWithOptionalParameterProperty {
-    x: string;
+    x?: string;
     constructor(x?: string);
 }
 declare class GlobalConstructorWithParameterInitializer {
diff --git a/tests/baselines/reference/doubleUnderStringLiteralAssignability.errors.txt b/tests/baselines/reference/doubleUnderStringLiteralAssignability.errors.txt
new file mode 100644
index 00000000000..c4226a83bfc
--- /dev/null
+++ b/tests/baselines/reference/doubleUnderStringLiteralAssignability.errors.txt
@@ -0,0 +1,11 @@
+tests/cases/compiler/doubleUnderStringLiteralAssignability.ts(2,5): error TS2322: Type '"no_dunder"' is not assignable to type '"__dunder"'.
+
+
+==== tests/cases/compiler/doubleUnderStringLiteralAssignability.ts (1 errors) ====
+    var shouldBeOk: '__dunder' = '__dunder';
+    var bad: '__dunder' = 'no_dunder';
+        ~~~
+!!! error TS2322: Type '"no_dunder"' is not assignable to type '"__dunder"'.
+    var okok: '___thunder' = '___thunder';
+    var alsoOk: '_sunder' = '_sunder';
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/doubleUnderStringLiteralAssignability.js b/tests/baselines/reference/doubleUnderStringLiteralAssignability.js
new file mode 100644
index 00000000000..baf2b521948
--- /dev/null
+++ b/tests/baselines/reference/doubleUnderStringLiteralAssignability.js
@@ -0,0 +1,12 @@
+//// [doubleUnderStringLiteralAssignability.ts]
+var shouldBeOk: '__dunder' = '__dunder';
+var bad: '__dunder' = 'no_dunder';
+var okok: '___thunder' = '___thunder';
+var alsoOk: '_sunder' = '_sunder';
+
+
+//// [doubleUnderStringLiteralAssignability.js]
+var shouldBeOk = '__dunder';
+var bad = 'no_dunder';
+var okok = '___thunder';
+var alsoOk = '_sunder';
diff --git a/tests/baselines/reference/duplicateLabel3.types b/tests/baselines/reference/duplicateLabel3.types
index 920a077aa9e..bcbe11beee3 100644
--- a/tests/baselines/reference/duplicateLabel3.types
+++ b/tests/baselines/reference/duplicateLabel3.types
@@ -7,7 +7,7 @@ while (true) {
 >true : boolean
 
   function f() {
->f : () => void
+>f : () => never
 
     target:
 >target : any
diff --git a/tests/baselines/reference/fallbackToBindingPatternForTypeInference.js b/tests/baselines/reference/fallbackToBindingPatternForTypeInference.js
new file mode 100644
index 00000000000..1c02ae83982
--- /dev/null
+++ b/tests/baselines/reference/fallbackToBindingPatternForTypeInference.js
@@ -0,0 +1,30 @@
+//// [fallbackToBindingPatternForTypeInference.ts]
+declare function trans<T>(f: (x: T) => string): number;
+trans(({a}) => a);
+trans(([b,c]) => 'foo');
+trans(({d: [e,f]}) => 'foo');
+trans(([{g},{h}]) => 'foo');
+trans(({a, b = 10}) => a);
+
+
+//// [fallbackToBindingPatternForTypeInference.js]
+trans(function (_a) {
+    var a = _a.a;
+    return a;
+});
+trans(function (_a) {
+    var b = _a[0], c = _a[1];
+    return 'foo';
+});
+trans(function (_a) {
+    var _b = _a.d, e = _b[0], f = _b[1];
+    return 'foo';
+});
+trans(function (_a) {
+    var g = _a[0].g, h = _a[1].h;
+    return 'foo';
+});
+trans(function (_a) {
+    var a = _a.a, _b = _a.b, b = _b === void 0 ? 10 : _b;
+    return a;
+});
diff --git a/tests/baselines/reference/fallbackToBindingPatternForTypeInference.symbols b/tests/baselines/reference/fallbackToBindingPatternForTypeInference.symbols
new file mode 100644
index 00000000000..f57b234f39f
--- /dev/null
+++ b/tests/baselines/reference/fallbackToBindingPatternForTypeInference.symbols
@@ -0,0 +1,34 @@
+=== tests/cases/compiler/fallbackToBindingPatternForTypeInference.ts ===
+declare function trans<T>(f: (x: T) => string): number;
+>trans : Symbol(trans, Decl(fallbackToBindingPatternForTypeInference.ts, 0, 0))
+>T : Symbol(T, Decl(fallbackToBindingPatternForTypeInference.ts, 0, 23))
+>f : Symbol(f, Decl(fallbackToBindingPatternForTypeInference.ts, 0, 26))
+>x : Symbol(x, Decl(fallbackToBindingPatternForTypeInference.ts, 0, 30))
+>T : Symbol(T, Decl(fallbackToBindingPatternForTypeInference.ts, 0, 23))
+
+trans(({a}) => a);
+>trans : Symbol(trans, Decl(fallbackToBindingPatternForTypeInference.ts, 0, 0))
+>a : Symbol(a, Decl(fallbackToBindingPatternForTypeInference.ts, 1, 8))
+>a : Symbol(a, Decl(fallbackToBindingPatternForTypeInference.ts, 1, 8))
+
+trans(([b,c]) => 'foo');
+>trans : Symbol(trans, Decl(fallbackToBindingPatternForTypeInference.ts, 0, 0))
+>b : Symbol(b, Decl(fallbackToBindingPatternForTypeInference.ts, 2, 8))
+>c : Symbol(c, Decl(fallbackToBindingPatternForTypeInference.ts, 2, 10))
+
+trans(({d: [e,f]}) => 'foo');
+>trans : Symbol(trans, Decl(fallbackToBindingPatternForTypeInference.ts, 0, 0))
+>e : Symbol(e, Decl(fallbackToBindingPatternForTypeInference.ts, 3, 12))
+>f : Symbol(f, Decl(fallbackToBindingPatternForTypeInference.ts, 3, 14))
+
+trans(([{g},{h}]) => 'foo');
+>trans : Symbol(trans, Decl(fallbackToBindingPatternForTypeInference.ts, 0, 0))
+>g : Symbol(g, Decl(fallbackToBindingPatternForTypeInference.ts, 4, 9))
+>h : Symbol(h, Decl(fallbackToBindingPatternForTypeInference.ts, 4, 13))
+
+trans(({a, b = 10}) => a);
+>trans : Symbol(trans, Decl(fallbackToBindingPatternForTypeInference.ts, 0, 0))
+>a : Symbol(a, Decl(fallbackToBindingPatternForTypeInference.ts, 5, 8))
+>b : Symbol(b, Decl(fallbackToBindingPatternForTypeInference.ts, 5, 10))
+>a : Symbol(a, Decl(fallbackToBindingPatternForTypeInference.ts, 5, 8))
+
diff --git a/tests/baselines/reference/fallbackToBindingPatternForTypeInference.types b/tests/baselines/reference/fallbackToBindingPatternForTypeInference.types
new file mode 100644
index 00000000000..fc41085098a
--- /dev/null
+++ b/tests/baselines/reference/fallbackToBindingPatternForTypeInference.types
@@ -0,0 +1,49 @@
+=== tests/cases/compiler/fallbackToBindingPatternForTypeInference.ts ===
+declare function trans<T>(f: (x: T) => string): number;
+>trans : <T>(f: (x: T) => string) => number
+>T : T
+>f : (x: T) => string
+>x : T
+>T : T
+
+trans(({a}) => a);
+>trans(({a}) => a) : number
+>trans : <T>(f: (x: T) => string) => number
+>({a}) => a : ({a}: { a: any; }) => any
+>a : any
+>a : any
+
+trans(([b,c]) => 'foo');
+>trans(([b,c]) => 'foo') : number
+>trans : <T>(f: (x: T) => string) => number
+>([b,c]) => 'foo' : ([b, c]: [any, any]) => string
+>b : any
+>c : any
+>'foo' : string
+
+trans(({d: [e,f]}) => 'foo');
+>trans(({d: [e,f]}) => 'foo') : number
+>trans : <T>(f: (x: T) => string) => number
+>({d: [e,f]}) => 'foo' : ({d: [e, f]}: { d: [any, any]; }) => string
+>d : any
+>e : any
+>f : any
+>'foo' : string
+
+trans(([{g},{h}]) => 'foo');
+>trans(([{g},{h}]) => 'foo') : number
+>trans : <T>(f: (x: T) => string) => number
+>([{g},{h}]) => 'foo' : ([{g}, {h}]: [{ g: any; }, { h: any; }]) => string
+>g : any
+>h : any
+>'foo' : string
+
+trans(({a, b = 10}) => a);
+>trans(({a, b = 10}) => a) : number
+>trans : <T>(f: (x: T) => string) => number
+>({a, b = 10}) => a : ({a, b}: { a: any; b?: number; }) => any
+>a : any
+>b : number
+>10 : number
+>a : any
+
diff --git a/tests/baselines/reference/forStatementsMultipleValidDecl.types b/tests/baselines/reference/forStatementsMultipleValidDecl.types
index acb26f17312..ae5959f1bd3 100644
--- a/tests/baselines/reference/forStatementsMultipleValidDecl.types
+++ b/tests/baselines/reference/forStatementsMultipleValidDecl.types
@@ -16,7 +16,7 @@ for (var x = <number>undefined; ;) { }
 
 // new declaration space, making redeclaring x as a string valid
 function declSpace() {
->declSpace : () => void
+>declSpace : () => never
 
     for (var x = 'this is a string'; ;) { }
 >x : string
diff --git a/tests/baselines/reference/instanceOfAssignability.types b/tests/baselines/reference/instanceOfAssignability.types
index 0a9e1a3e996..70d068fb0cc 100644
--- a/tests/baselines/reference/instanceOfAssignability.types
+++ b/tests/baselines/reference/instanceOfAssignability.types
@@ -133,8 +133,8 @@ function fn5(x: Derived1) {
 		// 1.5: y: Derived1
 		// Want: ???
 		let y = x;
->y : nothing
->x : nothing
+>y : never
+>x : never
 	}
 }
 
diff --git a/tests/baselines/reference/narrowingOfDottedNames.types b/tests/baselines/reference/narrowingOfDottedNames.types
index 697e080b661..9ab15afbd08 100644
--- a/tests/baselines/reference/narrowingOfDottedNames.types
+++ b/tests/baselines/reference/narrowingOfDottedNames.types
@@ -42,7 +42,7 @@ function isB(x: any): x is B {
 }
 
 function f1(x: A | B) {
->f1 : (x: A | B) => void
+>f1 : (x: A | B) => never
 >x : A | B
 >A : A
 >B : B
@@ -78,7 +78,7 @@ function f1(x: A | B) {
 }
 
 function f2(x: A | B) {
->f2 : (x: A | B) => void
+>f2 : (x: A | B) => never
 >x : A | B
 >A : A
 >B : B
diff --git a/tests/baselines/reference/nestedBlockScopedBindings3.types b/tests/baselines/reference/nestedBlockScopedBindings3.types
index 37b89242f3e..f153a9a7cea 100644
--- a/tests/baselines/reference/nestedBlockScopedBindings3.types
+++ b/tests/baselines/reference/nestedBlockScopedBindings3.types
@@ -1,6 +1,6 @@
 === tests/cases/compiler/nestedBlockScopedBindings3.ts ===
 function a0() {
->a0 : () => void
+>a0 : () => never
     {
         for (let x = 0; x < 1; ) {
 >x : number
@@ -26,7 +26,7 @@ function a0() {
 }
 
 function a1() {
->a1 : () => void
+>a1 : () => never
 
     for (let x; x < 1;) {
 >x : any
@@ -48,7 +48,7 @@ function a1() {
 }
 
 function a2() {
->a2 : () => void
+>a2 : () => never
 
     for (let x; x < 1;) {
 >x : any
diff --git a/tests/baselines/reference/nestedBlockScopedBindings4.types b/tests/baselines/reference/nestedBlockScopedBindings4.types
index b38d61f3f44..5d0ed2a1d91 100644
--- a/tests/baselines/reference/nestedBlockScopedBindings4.types
+++ b/tests/baselines/reference/nestedBlockScopedBindings4.types
@@ -1,6 +1,6 @@
 === tests/cases/compiler/nestedBlockScopedBindings4.ts ===
 function a0() {
->a0 : () => void
+>a0 : () => never
 
     for (let x; x < 1;) {
 >x : any
@@ -28,7 +28,7 @@ function a0() {
 }
 
 function a1() {
->a1 : () => void
+>a1 : () => never
 
     for (let x; x < 1;) {
 >x : any
@@ -60,7 +60,7 @@ function a1() {
 }
 
 function a2() {
->a2 : () => void
+>a2 : () => never
 
     for (let x; x < 1;) {
 >x : any
@@ -93,7 +93,7 @@ function a2() {
 
 
 function a3() {
->a3 : () => void
+>a3 : () => never
 
     for (let x; x < 1;) {
 >x : any
diff --git a/tests/baselines/reference/nestedBlockScopedBindings6.types b/tests/baselines/reference/nestedBlockScopedBindings6.types
index 5c8c1fa68fa..f6c918766ae 100644
--- a/tests/baselines/reference/nestedBlockScopedBindings6.types
+++ b/tests/baselines/reference/nestedBlockScopedBindings6.types
@@ -1,6 +1,6 @@
 === tests/cases/compiler/nestedBlockScopedBindings6.ts ===
 function a0() {
->a0 : () => void
+>a0 : () => never
 
     for (let x of [1]) {
 >x : number
@@ -27,7 +27,7 @@ function a0() {
 }
 
 function a1() {
->a1 : () => void
+>a1 : () => never
 
     for (let x of [1]) {
 >x : number
@@ -58,7 +58,7 @@ function a1() {
 }
 
 function a2() {
->a2 : () => void
+>a2 : () => never
 
     for (let x of [1]) {
 >x : number
@@ -89,7 +89,7 @@ function a2() {
 }
 
 function a3() {
->a3 : () => void
+>a3 : () => never
 
     for (let x of [1]) {
 >x : number
diff --git a/tests/baselines/reference/neverType.js b/tests/baselines/reference/neverType.js
new file mode 100644
index 00000000000..8c2c0293c21
--- /dev/null
+++ b/tests/baselines/reference/neverType.js
@@ -0,0 +1,138 @@
+//// [neverType.ts]
+
+function error(message: string) {
+    throw new Error(message);
+}
+
+function fail() {
+    return error("Something failed");
+}
+
+function infiniteLoop() {
+    while (true) {
+    }
+}
+
+function move1(direction: "up" | "down") {
+    switch (direction) {
+        case "up":
+            return 1;
+        case "down":
+            return -1; 
+    }
+    return error("Should never get here");
+}
+
+function move2(direction: "up" | "down") {
+    return direction === "up" ? 1 :
+        direction === "down" ? -1 :
+        error("Should never get here");
+}
+
+function check<T>(x: T | undefined) {
+    return x || error("Undefined value");
+}
+
+function f1(x: string | number) {
+    if (typeof x === "boolean") {
+        x;  // never
+    }
+}
+
+function f2(x: string | number) {
+    while (true) {
+        if (typeof x === "boolean") {
+            return x;  // never
+        }
+    }
+}
+
+function failOrThrow(shouldFail: boolean) {
+    if (shouldFail) {
+        return fail();
+    }
+    throw new Error();
+}
+
+function test(cb: () => string) {
+    let s = cb();
+    return s;
+}
+
+let errorCallback = () => error("Error callback");
+
+test(() => "hello");
+test(() => fail());
+test(() => { throw new Error(); })
+test(errorCallback);
+
+
+//// [neverType.js]
+function error(message) {
+    throw new Error(message);
+}
+function fail() {
+    return error("Something failed");
+}
+function infiniteLoop() {
+    while (true) {
+    }
+}
+function move1(direction) {
+    switch (direction) {
+        case "up":
+            return 1;
+        case "down":
+            return -1;
+    }
+    return error("Should never get here");
+}
+function move2(direction) {
+    return direction === "up" ? 1 :
+        direction === "down" ? -1 :
+            error("Should never get here");
+}
+function check(x) {
+    return x || error("Undefined value");
+}
+function f1(x) {
+    if (typeof x === "boolean") {
+        x; // never
+    }
+}
+function f2(x) {
+    while (true) {
+        if (typeof x === "boolean") {
+            return x; // never
+        }
+    }
+}
+function failOrThrow(shouldFail) {
+    if (shouldFail) {
+        return fail();
+    }
+    throw new Error();
+}
+function test(cb) {
+    var s = cb();
+    return s;
+}
+var errorCallback = function () { return error("Error callback"); };
+test(function () { return "hello"; });
+test(function () { return fail(); });
+test(function () { throw new Error(); });
+test(errorCallback);
+
+
+//// [neverType.d.ts]
+declare function error(message: string): never;
+declare function fail(): never;
+declare function infiniteLoop(): never;
+declare function move1(direction: "up" | "down"): number;
+declare function move2(direction: "up" | "down"): number;
+declare function check<T>(x: T | undefined): T;
+declare function f1(x: string | number): void;
+declare function f2(x: string | number): never;
+declare function failOrThrow(shouldFail: boolean): never;
+declare function test(cb: () => string): string;
+declare let errorCallback: () => never;
diff --git a/tests/baselines/reference/neverType.symbols b/tests/baselines/reference/neverType.symbols
new file mode 100644
index 00000000000..e1c4e98c24c
--- /dev/null
+++ b/tests/baselines/reference/neverType.symbols
@@ -0,0 +1,137 @@
+=== tests/cases/conformance/types/never/neverType.ts ===
+
+function error(message: string) {
+>error : Symbol(error, Decl(neverType.ts, 0, 0))
+>message : Symbol(message, Decl(neverType.ts, 1, 15))
+
+    throw new Error(message);
+>Error : Symbol(Error, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+>message : Symbol(message, Decl(neverType.ts, 1, 15))
+}
+
+function fail() {
+>fail : Symbol(fail, Decl(neverType.ts, 3, 1))
+
+    return error("Something failed");
+>error : Symbol(error, Decl(neverType.ts, 0, 0))
+}
+
+function infiniteLoop() {
+>infiniteLoop : Symbol(infiniteLoop, Decl(neverType.ts, 7, 1))
+
+    while (true) {
+    }
+}
+
+function move1(direction: "up" | "down") {
+>move1 : Symbol(move1, Decl(neverType.ts, 12, 1))
+>direction : Symbol(direction, Decl(neverType.ts, 14, 15))
+
+    switch (direction) {
+>direction : Symbol(direction, Decl(neverType.ts, 14, 15))
+
+        case "up":
+            return 1;
+        case "down":
+            return -1; 
+    }
+    return error("Should never get here");
+>error : Symbol(error, Decl(neverType.ts, 0, 0))
+}
+
+function move2(direction: "up" | "down") {
+>move2 : Symbol(move2, Decl(neverType.ts, 22, 1))
+>direction : Symbol(direction, Decl(neverType.ts, 24, 15))
+
+    return direction === "up" ? 1 :
+>direction : Symbol(direction, Decl(neverType.ts, 24, 15))
+
+        direction === "down" ? -1 :
+>direction : Symbol(direction, Decl(neverType.ts, 24, 15))
+
+        error("Should never get here");
+>error : Symbol(error, Decl(neverType.ts, 0, 0))
+}
+
+function check<T>(x: T | undefined) {
+>check : Symbol(check, Decl(neverType.ts, 28, 1))
+>T : Symbol(T, Decl(neverType.ts, 30, 15))
+>x : Symbol(x, Decl(neverType.ts, 30, 18))
+>T : Symbol(T, Decl(neverType.ts, 30, 15))
+
+    return x || error("Undefined value");
+>x : Symbol(x, Decl(neverType.ts, 30, 18))
+>error : Symbol(error, Decl(neverType.ts, 0, 0))
+}
+
+function f1(x: string | number) {
+>f1 : Symbol(f1, Decl(neverType.ts, 32, 1))
+>x : Symbol(x, Decl(neverType.ts, 34, 12))
+
+    if (typeof x === "boolean") {
+>x : Symbol(x, Decl(neverType.ts, 34, 12))
+
+        x;  // never
+>x : Symbol(x, Decl(neverType.ts, 34, 12))
+    }
+}
+
+function f2(x: string | number) {
+>f2 : Symbol(f2, Decl(neverType.ts, 38, 1))
+>x : Symbol(x, Decl(neverType.ts, 40, 12))
+
+    while (true) {
+        if (typeof x === "boolean") {
+>x : Symbol(x, Decl(neverType.ts, 40, 12))
+
+            return x;  // never
+>x : Symbol(x, Decl(neverType.ts, 40, 12))
+        }
+    }
+}
+
+function failOrThrow(shouldFail: boolean) {
+>failOrThrow : Symbol(failOrThrow, Decl(neverType.ts, 46, 1))
+>shouldFail : Symbol(shouldFail, Decl(neverType.ts, 48, 21))
+
+    if (shouldFail) {
+>shouldFail : Symbol(shouldFail, Decl(neverType.ts, 48, 21))
+
+        return fail();
+>fail : Symbol(fail, Decl(neverType.ts, 3, 1))
+    }
+    throw new Error();
+>Error : Symbol(Error, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+}
+
+function test(cb: () => string) {
+>test : Symbol(test, Decl(neverType.ts, 53, 1))
+>cb : Symbol(cb, Decl(neverType.ts, 55, 14))
+
+    let s = cb();
+>s : Symbol(s, Decl(neverType.ts, 56, 7))
+>cb : Symbol(cb, Decl(neverType.ts, 55, 14))
+
+    return s;
+>s : Symbol(s, Decl(neverType.ts, 56, 7))
+}
+
+let errorCallback = () => error("Error callback");
+>errorCallback : Symbol(errorCallback, Decl(neverType.ts, 60, 3))
+>error : Symbol(error, Decl(neverType.ts, 0, 0))
+
+test(() => "hello");
+>test : Symbol(test, Decl(neverType.ts, 53, 1))
+
+test(() => fail());
+>test : Symbol(test, Decl(neverType.ts, 53, 1))
+>fail : Symbol(fail, Decl(neverType.ts, 3, 1))
+
+test(() => { throw new Error(); })
+>test : Symbol(test, Decl(neverType.ts, 53, 1))
+>Error : Symbol(Error, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+
+test(errorCallback);
+>test : Symbol(test, Decl(neverType.ts, 53, 1))
+>errorCallback : Symbol(errorCallback, Decl(neverType.ts, 60, 3))
+
diff --git a/tests/baselines/reference/neverType.types b/tests/baselines/reference/neverType.types
new file mode 100644
index 00000000000..e6b36bcbace
--- /dev/null
+++ b/tests/baselines/reference/neverType.types
@@ -0,0 +1,189 @@
+=== tests/cases/conformance/types/never/neverType.ts ===
+
+function error(message: string) {
+>error : (message: string) => never
+>message : string
+
+    throw new Error(message);
+>new Error(message) : Error
+>Error : ErrorConstructor
+>message : string
+}
+
+function fail() {
+>fail : () => never
+
+    return error("Something failed");
+>error("Something failed") : never
+>error : (message: string) => never
+>"Something failed" : string
+}
+
+function infiniteLoop() {
+>infiniteLoop : () => never
+
+    while (true) {
+>true : boolean
+    }
+}
+
+function move1(direction: "up" | "down") {
+>move1 : (direction: "up" | "down") => number
+>direction : "up" | "down"
+
+    switch (direction) {
+>direction : "up" | "down"
+
+        case "up":
+>"up" : string
+
+            return 1;
+>1 : number
+
+        case "down":
+>"down" : string
+
+            return -1; 
+>-1 : number
+>1 : number
+    }
+    return error("Should never get here");
+>error("Should never get here") : never
+>error : (message: string) => never
+>"Should never get here" : string
+}
+
+function move2(direction: "up" | "down") {
+>move2 : (direction: "up" | "down") => number
+>direction : "up" | "down"
+
+    return direction === "up" ? 1 :
+>direction === "up" ? 1 :        direction === "down" ? -1 :        error("Should never get here") : number
+>direction === "up" : boolean
+>direction : "up" | "down"
+>"up" : string
+>1 : number
+
+        direction === "down" ? -1 :
+>direction === "down" ? -1 :        error("Should never get here") : number
+>direction === "down" : boolean
+>direction : "up" | "down"
+>"down" : string
+>-1 : number
+>1 : number
+
+        error("Should never get here");
+>error("Should never get here") : never
+>error : (message: string) => never
+>"Should never get here" : string
+}
+
+function check<T>(x: T | undefined) {
+>check : <T>(x: T | undefined) => T
+>T : T
+>x : T | undefined
+>T : T
+
+    return x || error("Undefined value");
+>x || error("Undefined value") : T
+>x : T | undefined
+>error("Undefined value") : never
+>error : (message: string) => never
+>"Undefined value" : string
+}
+
+function f1(x: string | number) {
+>f1 : (x: string | number) => void
+>x : string | number
+
+    if (typeof x === "boolean") {
+>typeof x === "boolean" : boolean
+>typeof x : string
+>x : string | number
+>"boolean" : string
+
+        x;  // never
+>x : never
+    }
+}
+
+function f2(x: string | number) {
+>f2 : (x: string | number) => never
+>x : string | number
+
+    while (true) {
+>true : boolean
+
+        if (typeof x === "boolean") {
+>typeof x === "boolean" : boolean
+>typeof x : string
+>x : string | number
+>"boolean" : string
+
+            return x;  // never
+>x : never
+        }
+    }
+}
+
+function failOrThrow(shouldFail: boolean) {
+>failOrThrow : (shouldFail: boolean) => never
+>shouldFail : boolean
+
+    if (shouldFail) {
+>shouldFail : boolean
+
+        return fail();
+>fail() : never
+>fail : () => never
+    }
+    throw new Error();
+>new Error() : Error
+>Error : ErrorConstructor
+}
+
+function test(cb: () => string) {
+>test : (cb: () => string) => string
+>cb : () => string
+
+    let s = cb();
+>s : string
+>cb() : string
+>cb : () => string
+
+    return s;
+>s : string
+}
+
+let errorCallback = () => error("Error callback");
+>errorCallback : () => never
+>() => error("Error callback") : () => never
+>error("Error callback") : never
+>error : (message: string) => never
+>"Error callback" : string
+
+test(() => "hello");
+>test(() => "hello") : string
+>test : (cb: () => string) => string
+>() => "hello" : () => string
+>"hello" : string
+
+test(() => fail());
+>test(() => fail()) : string
+>test : (cb: () => string) => string
+>() => fail() : () => never
+>fail() : never
+>fail : () => never
+
+test(() => { throw new Error(); })
+>test(() => { throw new Error(); }) : string
+>test : (cb: () => string) => string
+>() => { throw new Error(); } : () => never
+>new Error() : Error
+>Error : ErrorConstructor
+
+test(errorCallback);
+>test(errorCallback) : string
+>test : (cb: () => string) => string
+>errorCallback : () => never
+
diff --git a/tests/baselines/reference/neverTypeErrors1.errors.txt b/tests/baselines/reference/neverTypeErrors1.errors.txt
new file mode 100644
index 00000000000..8fd6f9c924e
--- /dev/null
+++ b/tests/baselines/reference/neverTypeErrors1.errors.txt
@@ -0,0 +1,50 @@
+tests/cases/conformance/types/never/neverTypeErrors1.ts(3,5): error TS2322: Type 'number' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors1.ts(4,5): error TS2322: Type 'string' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors1.ts(5,5): error TS2322: Type 'boolean' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors1.ts(6,5): error TS2322: Type 'undefined' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors1.ts(7,5): error TS2322: Type 'null' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors1.ts(8,5): error TS2322: Type '{}' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors1.ts(12,5): error TS2322: Type 'undefined' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors1.ts(16,12): error TS2322: Type 'number' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors1.ts(19,16): error TS2534: A function returning 'never' cannot have a reachable end point.
+
+
+==== tests/cases/conformance/types/never/neverTypeErrors1.ts (9 errors) ====
+    function f1() {
+        let x: never;
+        x = 1;
+        ~
+!!! error TS2322: Type 'number' is not assignable to type 'never'.
+        x = "abc";
+        ~
+!!! error TS2322: Type 'string' is not assignable to type 'never'.
+        x = false;
+        ~
+!!! error TS2322: Type 'boolean' is not assignable to type 'never'.
+        x = undefined;
+        ~
+!!! error TS2322: Type 'undefined' is not assignable to type 'never'.
+        x = null;
+        ~
+!!! error TS2322: Type 'null' is not assignable to type 'never'.
+        x = {};
+        ~
+!!! error TS2322: Type '{}' is not assignable to type 'never'.
+    }
+    
+    function f2(): never {
+        return;
+        ~~~~~~~
+!!! error TS2322: Type 'undefined' is not assignable to type 'never'.
+    }
+    
+    function f3(): never {
+        return 1;
+               ~
+!!! error TS2322: Type 'number' is not assignable to type 'never'.
+    }
+    
+    function f4(): never {
+                   ~~~~~
+!!! error TS2534: A function returning 'never' cannot have a reachable end point.
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/neverTypeErrors1.js b/tests/baselines/reference/neverTypeErrors1.js
new file mode 100644
index 00000000000..81b3f8f9cdb
--- /dev/null
+++ b/tests/baselines/reference/neverTypeErrors1.js
@@ -0,0 +1,40 @@
+//// [neverTypeErrors1.ts]
+function f1() {
+    let x: never;
+    x = 1;
+    x = "abc";
+    x = false;
+    x = undefined;
+    x = null;
+    x = {};
+}
+
+function f2(): never {
+    return;
+}
+
+function f3(): never {
+    return 1;
+}
+
+function f4(): never {
+}
+
+//// [neverTypeErrors1.js]
+function f1() {
+    var x;
+    x = 1;
+    x = "abc";
+    x = false;
+    x = undefined;
+    x = null;
+    x = {};
+}
+function f2() {
+    return;
+}
+function f3() {
+    return 1;
+}
+function f4() {
+}
diff --git a/tests/baselines/reference/neverTypeErrors2.errors.txt b/tests/baselines/reference/neverTypeErrors2.errors.txt
new file mode 100644
index 00000000000..c6292657ad1
--- /dev/null
+++ b/tests/baselines/reference/neverTypeErrors2.errors.txt
@@ -0,0 +1,51 @@
+tests/cases/conformance/types/never/neverTypeErrors2.ts(4,5): error TS2322: Type 'number' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors2.ts(5,5): error TS2322: Type 'string' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors2.ts(6,5): error TS2322: Type 'boolean' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors2.ts(7,5): error TS2322: Type 'undefined' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors2.ts(8,5): error TS2322: Type 'null' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors2.ts(9,5): error TS2322: Type '{}' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors2.ts(13,5): error TS2322: Type 'undefined' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors2.ts(17,12): error TS2322: Type 'number' is not assignable to type 'never'.
+tests/cases/conformance/types/never/neverTypeErrors2.ts(20,16): error TS2534: A function returning 'never' cannot have a reachable end point.
+
+
+==== tests/cases/conformance/types/never/neverTypeErrors2.ts (9 errors) ====
+    
+    function f1() {
+        let x: never;
+        x = 1;
+        ~
+!!! error TS2322: Type 'number' is not assignable to type 'never'.
+        x = "abc";
+        ~
+!!! error TS2322: Type 'string' is not assignable to type 'never'.
+        x = false;
+        ~
+!!! error TS2322: Type 'boolean' is not assignable to type 'never'.
+        x = undefined;
+        ~
+!!! error TS2322: Type 'undefined' is not assignable to type 'never'.
+        x = null;
+        ~
+!!! error TS2322: Type 'null' is not assignable to type 'never'.
+        x = {};
+        ~
+!!! error TS2322: Type '{}' is not assignable to type 'never'.
+    }
+    
+    function f2(): never {
+        return;
+        ~~~~~~~
+!!! error TS2322: Type 'undefined' is not assignable to type 'never'.
+    }
+    
+    function f3(): never {
+        return 1;
+               ~
+!!! error TS2322: Type 'number' is not assignable to type 'never'.
+    }
+    
+    function f4(): never {
+                   ~~~~~
+!!! error TS2534: A function returning 'never' cannot have a reachable end point.
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/neverTypeErrors2.js b/tests/baselines/reference/neverTypeErrors2.js
new file mode 100644
index 00000000000..18c92922bc1
--- /dev/null
+++ b/tests/baselines/reference/neverTypeErrors2.js
@@ -0,0 +1,41 @@
+//// [neverTypeErrors2.ts]
+
+function f1() {
+    let x: never;
+    x = 1;
+    x = "abc";
+    x = false;
+    x = undefined;
+    x = null;
+    x = {};
+}
+
+function f2(): never {
+    return;
+}
+
+function f3(): never {
+    return 1;
+}
+
+function f4(): never {
+}
+
+//// [neverTypeErrors2.js]
+function f1() {
+    var x;
+    x = 1;
+    x = "abc";
+    x = false;
+    x = undefined;
+    x = null;
+    x = {};
+}
+function f2() {
+    return;
+}
+function f3() {
+    return 1;
+}
+function f4() {
+}
diff --git a/tests/baselines/reference/objectLiteralMemberWithQuestionMark1.errors.txt b/tests/baselines/reference/objectLiteralMemberWithQuestionMark1.errors.txt
index b10ba701143..f77bad8664c 100644
--- a/tests/baselines/reference/objectLiteralMemberWithQuestionMark1.errors.txt
+++ b/tests/baselines/reference/objectLiteralMemberWithQuestionMark1.errors.txt
@@ -1,7 +1,7 @@
-tests/cases/compiler/objectLiteralMemberWithQuestionMark1.ts(1,14): error TS1112: A class member cannot be declared optional.
+tests/cases/compiler/objectLiteralMemberWithQuestionMark1.ts(1,14): error TS1162: An object member cannot be declared optional.
 
 
 ==== tests/cases/compiler/objectLiteralMemberWithQuestionMark1.ts (1 errors) ====
     var v = { foo?() { } }
                  ~
-!!! error TS1112: A class member cannot be declared optional.
\ No newline at end of file
+!!! error TS1162: An object member cannot be declared optional.
\ No newline at end of file
diff --git a/tests/baselines/reference/objectTypesWithOptionalProperties.errors.txt b/tests/baselines/reference/objectTypesWithOptionalProperties.errors.txt
index a2269c29d11..74057195c64 100644
--- a/tests/baselines/reference/objectTypesWithOptionalProperties.errors.txt
+++ b/tests/baselines/reference/objectTypesWithOptionalProperties.errors.txt
@@ -1,9 +1,7 @@
-tests/cases/conformance/types/objectTypeLiteral/methodSignatures/objectTypesWithOptionalProperties.ts(12,6): error TS1112: A class member cannot be declared optional.
-tests/cases/conformance/types/objectTypeLiteral/methodSignatures/objectTypesWithOptionalProperties.ts(20,6): error TS1112: A class member cannot be declared optional.
 tests/cases/conformance/types/objectTypeLiteral/methodSignatures/objectTypesWithOptionalProperties.ts(24,6): error TS1162: An object member cannot be declared optional.
 
 
-==== tests/cases/conformance/types/objectTypeLiteral/methodSignatures/objectTypesWithOptionalProperties.ts (3 errors) ====
+==== tests/cases/conformance/types/objectTypeLiteral/methodSignatures/objectTypesWithOptionalProperties.ts (1 errors) ====
     // Basic uses of optional properties
     
     var a: {
@@ -15,9 +13,7 @@ tests/cases/conformance/types/objectTypeLiteral/methodSignatures/objectTypesWith
     }
     
     class C {
-        x?: number; // error
-         ~
-!!! error TS1112: A class member cannot be declared optional.
+        x?: number; // ok
     }
     
     interface I2<T> {
@@ -25,9 +21,7 @@ tests/cases/conformance/types/objectTypeLiteral/methodSignatures/objectTypesWith
     }
     
     class C2<T> {
-        x?: T; // error
-         ~
-!!! error TS1112: A class member cannot be declared optional.
+        x?: T; // ok
     }
     
     var b = {
diff --git a/tests/baselines/reference/objectTypesWithOptionalProperties.js b/tests/baselines/reference/objectTypesWithOptionalProperties.js
index 98141c048dd..9b2ada010d9 100644
--- a/tests/baselines/reference/objectTypesWithOptionalProperties.js
+++ b/tests/baselines/reference/objectTypesWithOptionalProperties.js
@@ -10,7 +10,7 @@ interface I {
 }
 
 class C {
-    x?: number; // error
+    x?: number; // ok
 }
 
 interface I2<T> {
@@ -18,7 +18,7 @@ interface I2<T> {
 }
 
 class C2<T> {
-    x?: T; // error
+    x?: T; // ok
 }
 
 var b = {
diff --git a/tests/baselines/reference/optionalMethods.js b/tests/baselines/reference/optionalMethods.js
new file mode 100644
index 00000000000..42decf512e4
--- /dev/null
+++ b/tests/baselines/reference/optionalMethods.js
@@ -0,0 +1,147 @@
+//// [optionalMethods.ts]
+
+interface Foo {
+    a: number;
+    b?: number;
+    f(): number;
+    g?(): number;
+}
+
+function test1(x: Foo) {
+    x.a;
+    x.b;
+    x.f;
+    x.g;
+    let f1 = x.f();
+    let g1 = x.g && x.g();
+    let g2 = x.g ? x.g() : 0;
+}
+
+class Bar {
+    a: number;
+    b?: number;
+    c? = 2;
+    constructor(public d?: number, public e = 10) {}
+    f() {
+        return 1;
+    }
+    g?(): number;  // Body of optional method can be omitted
+    h?() {
+        return 2;
+    }
+}
+
+function test2(x: Bar) {
+    x.a;
+    x.b;
+    x.c;
+    x.d;
+    x.e;
+    x.f;
+    x.g;
+    let f1 = x.f();
+    let g1 = x.g && x.g();
+    let g2 = x.g ? x.g() : 0;
+    let h1 = x.h && x.h();
+    let h2 = x.h ? x.h() : 0;
+}
+
+class Base {
+    a?: number;
+    f?(): number;
+}
+
+class Derived extends Base {
+    a = 1;
+    f(): number { return 1; }
+}
+
+
+//// [optionalMethods.js]
+var __extends = (this && this.__extends) || function (d, b) {
+    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
+    function __() { this.constructor = d; }
+    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+};
+function test1(x) {
+    x.a;
+    x.b;
+    x.f;
+    x.g;
+    var f1 = x.f();
+    var g1 = x.g && x.g();
+    var g2 = x.g ? x.g() : 0;
+}
+var Bar = (function () {
+    function Bar(d, e) {
+        if (e === void 0) { e = 10; }
+        this.d = d;
+        this.e = e;
+        this.c = 2;
+    }
+    Bar.prototype.f = function () {
+        return 1;
+    };
+    Bar.prototype.h = function () {
+        return 2;
+    };
+    return Bar;
+}());
+function test2(x) {
+    x.a;
+    x.b;
+    x.c;
+    x.d;
+    x.e;
+    x.f;
+    x.g;
+    var f1 = x.f();
+    var g1 = x.g && x.g();
+    var g2 = x.g ? x.g() : 0;
+    var h1 = x.h && x.h();
+    var h2 = x.h ? x.h() : 0;
+}
+var Base = (function () {
+    function Base() {
+    }
+    return Base;
+}());
+var Derived = (function (_super) {
+    __extends(Derived, _super);
+    function Derived() {
+        _super.apply(this, arguments);
+        this.a = 1;
+    }
+    Derived.prototype.f = function () { return 1; };
+    return Derived;
+}(Base));
+
+
+//// [optionalMethods.d.ts]
+interface Foo {
+    a: number;
+    b?: number;
+    f(): number;
+    g?(): number;
+}
+declare function test1(x: Foo): void;
+declare class Bar {
+    d?: number;
+    e: number;
+    a: number;
+    b?: number;
+    c?: number | undefined;
+    constructor(d?: number, e?: number);
+    f(): number;
+    g?(): number;
+    h?(): number;
+}
+declare function test2(x: Bar): void;
+declare class Base {
+    a?: number;
+    f?(): number;
+}
+declare class Derived extends Base {
+    a: number;
+    f(): number;
+}
diff --git a/tests/baselines/reference/optionalMethods.symbols b/tests/baselines/reference/optionalMethods.symbols
new file mode 100644
index 00000000000..55c5ec958da
--- /dev/null
+++ b/tests/baselines/reference/optionalMethods.symbols
@@ -0,0 +1,203 @@
+=== tests/cases/conformance/types/namedTypes/optionalMethods.ts ===
+
+interface Foo {
+>Foo : Symbol(Foo, Decl(optionalMethods.ts, 0, 0))
+
+    a: number;
+>a : Symbol(Foo.a, Decl(optionalMethods.ts, 1, 15))
+
+    b?: number;
+>b : Symbol(Foo.b, Decl(optionalMethods.ts, 2, 14))
+
+    f(): number;
+>f : Symbol(Foo.f, Decl(optionalMethods.ts, 3, 15))
+
+    g?(): number;
+>g : Symbol(Foo.g, Decl(optionalMethods.ts, 4, 16))
+}
+
+function test1(x: Foo) {
+>test1 : Symbol(test1, Decl(optionalMethods.ts, 6, 1))
+>x : Symbol(x, Decl(optionalMethods.ts, 8, 15))
+>Foo : Symbol(Foo, Decl(optionalMethods.ts, 0, 0))
+
+    x.a;
+>x.a : Symbol(Foo.a, Decl(optionalMethods.ts, 1, 15))
+>x : Symbol(x, Decl(optionalMethods.ts, 8, 15))
+>a : Symbol(Foo.a, Decl(optionalMethods.ts, 1, 15))
+
+    x.b;
+>x.b : Symbol(Foo.b, Decl(optionalMethods.ts, 2, 14))
+>x : Symbol(x, Decl(optionalMethods.ts, 8, 15))
+>b : Symbol(Foo.b, Decl(optionalMethods.ts, 2, 14))
+
+    x.f;
+>x.f : Symbol(Foo.f, Decl(optionalMethods.ts, 3, 15))
+>x : Symbol(x, Decl(optionalMethods.ts, 8, 15))
+>f : Symbol(Foo.f, Decl(optionalMethods.ts, 3, 15))
+
+    x.g;
+>x.g : Symbol(Foo.g, Decl(optionalMethods.ts, 4, 16))
+>x : Symbol(x, Decl(optionalMethods.ts, 8, 15))
+>g : Symbol(Foo.g, Decl(optionalMethods.ts, 4, 16))
+
+    let f1 = x.f();
+>f1 : Symbol(f1, Decl(optionalMethods.ts, 13, 7))
+>x.f : Symbol(Foo.f, Decl(optionalMethods.ts, 3, 15))
+>x : Symbol(x, Decl(optionalMethods.ts, 8, 15))
+>f : Symbol(Foo.f, Decl(optionalMethods.ts, 3, 15))
+
+    let g1 = x.g && x.g();
+>g1 : Symbol(g1, Decl(optionalMethods.ts, 14, 7))
+>x.g : Symbol(Foo.g, Decl(optionalMethods.ts, 4, 16))
+>x : Symbol(x, Decl(optionalMethods.ts, 8, 15))
+>g : Symbol(Foo.g, Decl(optionalMethods.ts, 4, 16))
+>x.g : Symbol(Foo.g, Decl(optionalMethods.ts, 4, 16))
+>x : Symbol(x, Decl(optionalMethods.ts, 8, 15))
+>g : Symbol(Foo.g, Decl(optionalMethods.ts, 4, 16))
+
+    let g2 = x.g ? x.g() : 0;
+>g2 : Symbol(g2, Decl(optionalMethods.ts, 15, 7))
+>x.g : Symbol(Foo.g, Decl(optionalMethods.ts, 4, 16))
+>x : Symbol(x, Decl(optionalMethods.ts, 8, 15))
+>g : Symbol(Foo.g, Decl(optionalMethods.ts, 4, 16))
+>x.g : Symbol(Foo.g, Decl(optionalMethods.ts, 4, 16))
+>x : Symbol(x, Decl(optionalMethods.ts, 8, 15))
+>g : Symbol(Foo.g, Decl(optionalMethods.ts, 4, 16))
+}
+
+class Bar {
+>Bar : Symbol(Bar, Decl(optionalMethods.ts, 16, 1))
+
+    a: number;
+>a : Symbol(Bar.a, Decl(optionalMethods.ts, 18, 11))
+
+    b?: number;
+>b : Symbol(Bar.b, Decl(optionalMethods.ts, 19, 14))
+
+    c? = 2;
+>c : Symbol(Bar.c, Decl(optionalMethods.ts, 20, 15))
+
+    constructor(public d?: number, public e = 10) {}
+>d : Symbol(Bar.d, Decl(optionalMethods.ts, 22, 16))
+>e : Symbol(Bar.e, Decl(optionalMethods.ts, 22, 34))
+
+    f() {
+>f : Symbol(Bar.f, Decl(optionalMethods.ts, 22, 52))
+
+        return 1;
+    }
+    g?(): number;  // Body of optional method can be omitted
+>g : Symbol(Bar.g, Decl(optionalMethods.ts, 25, 5))
+
+    h?() {
+>h : Symbol(Bar.h, Decl(optionalMethods.ts, 26, 17))
+
+        return 2;
+    }
+}
+
+function test2(x: Bar) {
+>test2 : Symbol(test2, Decl(optionalMethods.ts, 30, 1))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>Bar : Symbol(Bar, Decl(optionalMethods.ts, 16, 1))
+
+    x.a;
+>x.a : Symbol(Bar.a, Decl(optionalMethods.ts, 18, 11))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>a : Symbol(Bar.a, Decl(optionalMethods.ts, 18, 11))
+
+    x.b;
+>x.b : Symbol(Bar.b, Decl(optionalMethods.ts, 19, 14))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>b : Symbol(Bar.b, Decl(optionalMethods.ts, 19, 14))
+
+    x.c;
+>x.c : Symbol(Bar.c, Decl(optionalMethods.ts, 20, 15))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>c : Symbol(Bar.c, Decl(optionalMethods.ts, 20, 15))
+
+    x.d;
+>x.d : Symbol(Bar.d, Decl(optionalMethods.ts, 22, 16))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>d : Symbol(Bar.d, Decl(optionalMethods.ts, 22, 16))
+
+    x.e;
+>x.e : Symbol(Bar.e, Decl(optionalMethods.ts, 22, 34))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>e : Symbol(Bar.e, Decl(optionalMethods.ts, 22, 34))
+
+    x.f;
+>x.f : Symbol(Bar.f, Decl(optionalMethods.ts, 22, 52))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>f : Symbol(Bar.f, Decl(optionalMethods.ts, 22, 52))
+
+    x.g;
+>x.g : Symbol(Bar.g, Decl(optionalMethods.ts, 25, 5))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>g : Symbol(Bar.g, Decl(optionalMethods.ts, 25, 5))
+
+    let f1 = x.f();
+>f1 : Symbol(f1, Decl(optionalMethods.ts, 40, 7))
+>x.f : Symbol(Bar.f, Decl(optionalMethods.ts, 22, 52))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>f : Symbol(Bar.f, Decl(optionalMethods.ts, 22, 52))
+
+    let g1 = x.g && x.g();
+>g1 : Symbol(g1, Decl(optionalMethods.ts, 41, 7))
+>x.g : Symbol(Bar.g, Decl(optionalMethods.ts, 25, 5))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>g : Symbol(Bar.g, Decl(optionalMethods.ts, 25, 5))
+>x.g : Symbol(Bar.g, Decl(optionalMethods.ts, 25, 5))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>g : Symbol(Bar.g, Decl(optionalMethods.ts, 25, 5))
+
+    let g2 = x.g ? x.g() : 0;
+>g2 : Symbol(g2, Decl(optionalMethods.ts, 42, 7))
+>x.g : Symbol(Bar.g, Decl(optionalMethods.ts, 25, 5))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>g : Symbol(Bar.g, Decl(optionalMethods.ts, 25, 5))
+>x.g : Symbol(Bar.g, Decl(optionalMethods.ts, 25, 5))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>g : Symbol(Bar.g, Decl(optionalMethods.ts, 25, 5))
+
+    let h1 = x.h && x.h();
+>h1 : Symbol(h1, Decl(optionalMethods.ts, 43, 7))
+>x.h : Symbol(Bar.h, Decl(optionalMethods.ts, 26, 17))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>h : Symbol(Bar.h, Decl(optionalMethods.ts, 26, 17))
+>x.h : Symbol(Bar.h, Decl(optionalMethods.ts, 26, 17))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>h : Symbol(Bar.h, Decl(optionalMethods.ts, 26, 17))
+
+    let h2 = x.h ? x.h() : 0;
+>h2 : Symbol(h2, Decl(optionalMethods.ts, 44, 7))
+>x.h : Symbol(Bar.h, Decl(optionalMethods.ts, 26, 17))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>h : Symbol(Bar.h, Decl(optionalMethods.ts, 26, 17))
+>x.h : Symbol(Bar.h, Decl(optionalMethods.ts, 26, 17))
+>x : Symbol(x, Decl(optionalMethods.ts, 32, 15))
+>h : Symbol(Bar.h, Decl(optionalMethods.ts, 26, 17))
+}
+
+class Base {
+>Base : Symbol(Base, Decl(optionalMethods.ts, 45, 1))
+
+    a?: number;
+>a : Symbol(Base.a, Decl(optionalMethods.ts, 47, 12))
+
+    f?(): number;
+>f : Symbol(Base.f, Decl(optionalMethods.ts, 48, 15))
+}
+
+class Derived extends Base {
+>Derived : Symbol(Derived, Decl(optionalMethods.ts, 50, 1))
+>Base : Symbol(Base, Decl(optionalMethods.ts, 45, 1))
+
+    a = 1;
+>a : Symbol(Derived.a, Decl(optionalMethods.ts, 52, 28))
+
+    f(): number { return 1; }
+>f : Symbol(Derived.f, Decl(optionalMethods.ts, 53, 10))
+}
+
diff --git a/tests/baselines/reference/optionalMethods.types b/tests/baselines/reference/optionalMethods.types
new file mode 100644
index 00000000000..70fda27c012
--- /dev/null
+++ b/tests/baselines/reference/optionalMethods.types
@@ -0,0 +1,226 @@
+=== tests/cases/conformance/types/namedTypes/optionalMethods.ts ===
+
+interface Foo {
+>Foo : Foo
+
+    a: number;
+>a : number
+
+    b?: number;
+>b : number | undefined
+
+    f(): number;
+>f : () => number
+
+    g?(): number;
+>g : (() => number) | undefined
+}
+
+function test1(x: Foo) {
+>test1 : (x: Foo) => void
+>x : Foo
+>Foo : Foo
+
+    x.a;
+>x.a : number
+>x : Foo
+>a : number
+
+    x.b;
+>x.b : number | undefined
+>x : Foo
+>b : number | undefined
+
+    x.f;
+>x.f : () => number
+>x : Foo
+>f : () => number
+
+    x.g;
+>x.g : (() => number) | undefined
+>x : Foo
+>g : (() => number) | undefined
+
+    let f1 = x.f();
+>f1 : number
+>x.f() : number
+>x.f : () => number
+>x : Foo
+>f : () => number
+
+    let g1 = x.g && x.g();
+>g1 : number | undefined
+>x.g && x.g() : number | undefined
+>x.g : (() => number) | undefined
+>x : Foo
+>g : (() => number) | undefined
+>x.g() : number
+>x.g : () => number
+>x : Foo
+>g : () => number
+
+    let g2 = x.g ? x.g() : 0;
+>g2 : number
+>x.g ? x.g() : 0 : number
+>x.g : (() => number) | undefined
+>x : Foo
+>g : (() => number) | undefined
+>x.g() : number
+>x.g : () => number
+>x : Foo
+>g : () => number
+>0 : number
+}
+
+class Bar {
+>Bar : Bar
+
+    a: number;
+>a : number
+
+    b?: number;
+>b : number | undefined
+
+    c? = 2;
+>c : number | undefined
+>2 : number
+
+    constructor(public d?: number, public e = 10) {}
+>d : number | undefined
+>e : number
+>10 : number
+
+    f() {
+>f : () => number
+
+        return 1;
+>1 : number
+    }
+    g?(): number;  // Body of optional method can be omitted
+>g : (() => number) | undefined
+
+    h?() {
+>h : (() => number) | undefined
+
+        return 2;
+>2 : number
+    }
+}
+
+function test2(x: Bar) {
+>test2 : (x: Bar) => void
+>x : Bar
+>Bar : Bar
+
+    x.a;
+>x.a : number
+>x : Bar
+>a : number
+
+    x.b;
+>x.b : number | undefined
+>x : Bar
+>b : number | undefined
+
+    x.c;
+>x.c : number | undefined
+>x : Bar
+>c : number | undefined
+
+    x.d;
+>x.d : number | undefined
+>x : Bar
+>d : number | undefined
+
+    x.e;
+>x.e : number
+>x : Bar
+>e : number
+
+    x.f;
+>x.f : () => number
+>x : Bar
+>f : () => number
+
+    x.g;
+>x.g : (() => number) | undefined
+>x : Bar
+>g : (() => number) | undefined
+
+    let f1 = x.f();
+>f1 : number
+>x.f() : number
+>x.f : () => number
+>x : Bar
+>f : () => number
+
+    let g1 = x.g && x.g();
+>g1 : number | undefined
+>x.g && x.g() : number | undefined
+>x.g : (() => number) | undefined
+>x : Bar
+>g : (() => number) | undefined
+>x.g() : number
+>x.g : () => number
+>x : Bar
+>g : () => number
+
+    let g2 = x.g ? x.g() : 0;
+>g2 : number
+>x.g ? x.g() : 0 : number
+>x.g : (() => number) | undefined
+>x : Bar
+>g : (() => number) | undefined
+>x.g() : number
+>x.g : () => number
+>x : Bar
+>g : () => number
+>0 : number
+
+    let h1 = x.h && x.h();
+>h1 : number | undefined
+>x.h && x.h() : number | undefined
+>x.h : (() => number) | undefined
+>x : Bar
+>h : (() => number) | undefined
+>x.h() : number
+>x.h : () => number
+>x : Bar
+>h : () => number
+
+    let h2 = x.h ? x.h() : 0;
+>h2 : number
+>x.h ? x.h() : 0 : number
+>x.h : (() => number) | undefined
+>x : Bar
+>h : (() => number) | undefined
+>x.h() : number
+>x.h : () => number
+>x : Bar
+>h : () => number
+>0 : number
+}
+
+class Base {
+>Base : Base
+
+    a?: number;
+>a : number | undefined
+
+    f?(): number;
+>f : (() => number) | undefined
+}
+
+class Derived extends Base {
+>Derived : Derived
+>Base : Base
+
+    a = 1;
+>a : number
+>1 : number
+
+    f(): number { return 1; }
+>f : () => number
+>1 : number
+}
+
diff --git a/tests/baselines/reference/parser_duplicateLabel3.types b/tests/baselines/reference/parser_duplicateLabel3.types
index 88ea435bf3e..cc420085ef0 100644
--- a/tests/baselines/reference/parser_duplicateLabel3.types
+++ b/tests/baselines/reference/parser_duplicateLabel3.types
@@ -7,7 +7,7 @@ while (true) {
 >true : boolean
 
   function f() {
->f : () => void
+>f : () => never
 
     target:
 >target : any
diff --git a/tests/baselines/reference/promiseVoidErrorCallback.types b/tests/baselines/reference/promiseVoidErrorCallback.types
index 82b248f64bd..b536ab089bd 100644
--- a/tests/baselines/reference/promiseVoidErrorCallback.types
+++ b/tests/baselines/reference/promiseVoidErrorCallback.types
@@ -63,7 +63,7 @@ var x3 = f1()
     .then(f2, (e: Error) => {
 >then : { <TResult>(onfulfilled?: (value: T1) => TResult | PromiseLike<TResult>, onrejected?: (reason: any) => TResult | PromiseLike<TResult>): Promise<TResult>; <TResult>(onfulfilled?: (value: T1) => TResult | PromiseLike<TResult>, onrejected?: (reason: any) => void): Promise<TResult>; }
 >f2 : (x: T1) => T2
->(e: Error) => {    throw e;} : (e: Error) => void
+>(e: Error) => {    throw e;} : (e: Error) => never
 >e : Error
 >Error : Error
 
diff --git a/tests/baselines/reference/stringLiteralTypesAsTags01.types b/tests/baselines/reference/stringLiteralTypesAsTags01.types
index 9f273cf93a9..7b8ac0a8c02 100644
--- a/tests/baselines/reference/stringLiteralTypesAsTags01.types
+++ b/tests/baselines/reference/stringLiteralTypesAsTags01.types
@@ -116,6 +116,6 @@ if (!hasKind(x, "B")) {
 }
 else {
     let d = x;
->d : nothing
->x : nothing
+>d : never
+>x : never
 }
diff --git a/tests/baselines/reference/stringLiteralTypesAsTags02.types b/tests/baselines/reference/stringLiteralTypesAsTags02.types
index bcfa74ef55f..290402cd299 100644
--- a/tests/baselines/reference/stringLiteralTypesAsTags02.types
+++ b/tests/baselines/reference/stringLiteralTypesAsTags02.types
@@ -110,6 +110,6 @@ if (!hasKind(x, "B")) {
 }
 else {
     let d = x;
->d : nothing
->x : nothing
+>d : never
+>x : never
 }
diff --git a/tests/baselines/reference/stringLiteralTypesAsTags03.types b/tests/baselines/reference/stringLiteralTypesAsTags03.types
index c7fdfc7152e..9d004982dda 100644
--- a/tests/baselines/reference/stringLiteralTypesAsTags03.types
+++ b/tests/baselines/reference/stringLiteralTypesAsTags03.types
@@ -113,6 +113,6 @@ if (!hasKind(x, "B")) {
 }
 else {
     let d = x;
->d : nothing
->x : nothing
+>d : never
+>x : never
 }
diff --git a/tests/baselines/reference/throwInEnclosingStatements.types b/tests/baselines/reference/throwInEnclosingStatements.types
index 32f0fb093bc..26866ebb240 100644
--- a/tests/baselines/reference/throwInEnclosingStatements.types
+++ b/tests/baselines/reference/throwInEnclosingStatements.types
@@ -1,7 +1,7 @@
 === tests/cases/conformance/statements/throwStatements/throwInEnclosingStatements.ts ===
 
 function fn(x) {
->fn : (x: any) => void
+>fn : (x: any) => never
 >x : any
 
     throw x;
@@ -9,7 +9,7 @@ function fn(x) {
 }
 
 <T>(x: T) => { throw x; }
-><T>(x: T) => { throw x; } : <T>(x: T) => void
+><T>(x: T) => { throw x; } : <T>(x: T) => never
 >T : T
 >x : T
 >T : T
@@ -78,7 +78,7 @@ class C<T> {
 >T : T
 
     biz() {
->biz : () => void
+>biz : () => never
 
         throw this.value;
 >this.value : T
@@ -93,15 +93,15 @@ class C<T> {
 }
 
 var aa = {
->aa : { id: number; biz(): void; }
->{    id:12,    biz() {        throw this;    }} : { id: number; biz(): void; }
+>aa : { id: number; biz(): never; }
+>{    id:12,    biz() {        throw this;    }} : { id: number; biz(): never; }
 
     id:12,
 >id : number
 >12 : number
 
     biz() {
->biz : () => void
+>biz : () => never
 
         throw this;
 >this : any
diff --git a/tests/baselines/reference/typeGuardOfFormTypeOfBoolean.types b/tests/baselines/reference/typeGuardOfFormTypeOfBoolean.types
index 564c6ceaa72..7a4b279b572 100644
--- a/tests/baselines/reference/typeGuardOfFormTypeOfBoolean.types
+++ b/tests/baselines/reference/typeGuardOfFormTypeOfBoolean.types
@@ -121,7 +121,7 @@ if (typeof strOrNum === "boolean") {
 
     let z1: {} = strOrNum; // {}
 >z1 : {}
->strOrNum : nothing
+>strOrNum : never
 }
 else {
     let z2: string | number = strOrNum; // string | number
@@ -215,6 +215,6 @@ if (typeof strOrNum !== "boolean") {
 else {
     let z2: {} = strOrNum; // {}
 >z2 : {}
->strOrNum : nothing
+>strOrNum : never
 }
 
diff --git a/tests/baselines/reference/typeGuardOfFormTypeOfNumber.types b/tests/baselines/reference/typeGuardOfFormTypeOfNumber.types
index 51326c68cd9..c6a5615e75a 100644
--- a/tests/baselines/reference/typeGuardOfFormTypeOfNumber.types
+++ b/tests/baselines/reference/typeGuardOfFormTypeOfNumber.types
@@ -120,7 +120,7 @@ if (typeof strOrBool === "number") {
 
     let y1: {} = strOrBool; // {}
 >y1 : {}
->strOrBool : nothing
+>strOrBool : never
 }
 else {
     let y2: string | boolean = strOrBool; // string | boolean
@@ -212,6 +212,6 @@ if (typeof strOrBool !== "number") {
 else {
     let y2: {} = strOrBool; // {}
 >y2 : {}
->strOrBool : nothing
+>strOrBool : never
 }
 
diff --git a/tests/baselines/reference/typeGuardOfFormTypeOfOther.types b/tests/baselines/reference/typeGuardOfFormTypeOfOther.types
index c8c150b5a3b..8e42a8e0d8e 100644
--- a/tests/baselines/reference/typeGuardOfFormTypeOfOther.types
+++ b/tests/baselines/reference/typeGuardOfFormTypeOfOther.types
@@ -105,7 +105,7 @@ if (typeof strOrNumOrBool === "Object") {
 
     let q1: {} = strOrNumOrBool; // {}
 >q1 : {}
->strOrNumOrBool : nothing
+>strOrNumOrBool : never
 }
 else {
     let q2: string | number | boolean = strOrNumOrBool; // string | number | boolean
@@ -178,6 +178,6 @@ if (typeof strOrNumOrBool !== "Object") {
 else {
     let q2: {} = strOrNumOrBool; // {}
 >q2 : {}
->strOrNumOrBool : nothing
+>strOrNumOrBool : never
 }
 
diff --git a/tests/baselines/reference/typeGuardOfFormTypeOfString.types b/tests/baselines/reference/typeGuardOfFormTypeOfString.types
index 23c789fb987..971109215f7 100644
--- a/tests/baselines/reference/typeGuardOfFormTypeOfString.types
+++ b/tests/baselines/reference/typeGuardOfFormTypeOfString.types
@@ -121,7 +121,7 @@ if (typeof numOrBool === "string") {
 
     let x1: {} = numOrBool; // {}
 >x1 : {}
->numOrBool : nothing
+>numOrBool : never
 }
 else {
     let x2: number | boolean = numOrBool; // number | boolean
@@ -214,6 +214,6 @@ if (typeof numOrBool !== "string") {
 else {
     let x2: {} = numOrBool; // {}
 >x2 : {}
->numOrBool : nothing
+>numOrBool : never
 }
 
diff --git a/tests/baselines/reference/typeGuardsAsAssertions.types b/tests/baselines/reference/typeGuardsAsAssertions.types
index 6ec84ada1f4..82e92a72d8d 100644
--- a/tests/baselines/reference/typeGuardsAsAssertions.types
+++ b/tests/baselines/reference/typeGuardsAsAssertions.types
@@ -259,7 +259,7 @@ function f4() {
 >"boolean" : string
 
         x;  // nothing (boolean not in declared type)
->x : nothing
+>x : never
     }
     x;  // undefined
 >x : undefined
diff --git a/tests/baselines/reference/typeGuardsWithInstanceOf.errors.txt b/tests/baselines/reference/typeGuardsWithInstanceOf.errors.txt
index 662d3697159..dfcb8a598da 100644
--- a/tests/baselines/reference/typeGuardsWithInstanceOf.errors.txt
+++ b/tests/baselines/reference/typeGuardsWithInstanceOf.errors.txt
@@ -1,4 +1,4 @@
-tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOf.ts(7,20): error TS2339: Property 'global' does not exist on type 'nothing'.
+tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOf.ts(7,20): error TS2339: Property 'global' does not exist on type 'never'.
 
 
 ==== tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOf.ts (1 errors) ====
@@ -10,5 +10,5 @@ tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOf.ts(7,20)
         result = result2;
     } else if (!result.global) {
                        ~~~~~~
-!!! error TS2339: Property 'global' does not exist on type 'nothing'.
+!!! error TS2339: Property 'global' does not exist on type 'never'.
     }
\ No newline at end of file
diff --git a/tests/cases/compiler/doubleUnderStringLiteralAssignability.ts b/tests/cases/compiler/doubleUnderStringLiteralAssignability.ts
new file mode 100644
index 00000000000..4a1ae75164c
--- /dev/null
+++ b/tests/cases/compiler/doubleUnderStringLiteralAssignability.ts
@@ -0,0 +1,4 @@
+var shouldBeOk: '__dunder' = '__dunder';
+var bad: '__dunder' = 'no_dunder';
+var okok: '___thunder' = '___thunder';
+var alsoOk: '_sunder' = '_sunder';
diff --git a/tests/cases/compiler/fallbackToBindingPatternForTypeInference.ts b/tests/cases/compiler/fallbackToBindingPatternForTypeInference.ts
new file mode 100644
index 00000000000..9aa979c1020
--- /dev/null
+++ b/tests/cases/compiler/fallbackToBindingPatternForTypeInference.ts
@@ -0,0 +1,6 @@
+declare function trans<T>(f: (x: T) => string): number;
+trans(({a}) => a);
+trans(([b,c]) => 'foo');
+trans(({d: [e,f]}) => 'foo');
+trans(([{g},{h}]) => 'foo');
+trans(({a, b = 10}) => a);
diff --git a/tests/cases/conformance/types/namedTypes/optionalMethods.ts b/tests/cases/conformance/types/namedTypes/optionalMethods.ts
new file mode 100644
index 00000000000..932521425f9
--- /dev/null
+++ b/tests/cases/conformance/types/namedTypes/optionalMethods.ts
@@ -0,0 +1,58 @@
+// @strictNullChecks: true
+// @declaration: true
+
+interface Foo {
+    a: number;
+    b?: number;
+    f(): number;
+    g?(): number;
+}
+
+function test1(x: Foo) {
+    x.a;
+    x.b;
+    x.f;
+    x.g;
+    let f1 = x.f();
+    let g1 = x.g && x.g();
+    let g2 = x.g ? x.g() : 0;
+}
+
+class Bar {
+    a: number;
+    b?: number;
+    c? = 2;
+    constructor(public d?: number, public e = 10) {}
+    f() {
+        return 1;
+    }
+    g?(): number;  // Body of optional method can be omitted
+    h?() {
+        return 2;
+    }
+}
+
+function test2(x: Bar) {
+    x.a;
+    x.b;
+    x.c;
+    x.d;
+    x.e;
+    x.f;
+    x.g;
+    let f1 = x.f();
+    let g1 = x.g && x.g();
+    let g2 = x.g ? x.g() : 0;
+    let h1 = x.h && x.h();
+    let h2 = x.h ? x.h() : 0;
+}
+
+class Base {
+    a?: number;
+    f?(): number;
+}
+
+class Derived extends Base {
+    a = 1;
+    f(): number { return 1; }
+}
diff --git a/tests/cases/conformance/types/never/neverType.ts b/tests/cases/conformance/types/never/neverType.ts
new file mode 100644
index 00000000000..d37528a022d
--- /dev/null
+++ b/tests/cases/conformance/types/never/neverType.ts
@@ -0,0 +1,68 @@
+// @strictNullChecks: true
+// @declaration: true
+
+function error(message: string) {
+    throw new Error(message);
+}
+
+function fail() {
+    return error("Something failed");
+}
+
+function infiniteLoop() {
+    while (true) {
+    }
+}
+
+function move1(direction: "up" | "down") {
+    switch (direction) {
+        case "up":
+            return 1;
+        case "down":
+            return -1; 
+    }
+    return error("Should never get here");
+}
+
+function move2(direction: "up" | "down") {
+    return direction === "up" ? 1 :
+        direction === "down" ? -1 :
+        error("Should never get here");
+}
+
+function check<T>(x: T | undefined) {
+    return x || error("Undefined value");
+}
+
+function f1(x: string | number) {
+    if (typeof x === "boolean") {
+        x;  // never
+    }
+}
+
+function f2(x: string | number) {
+    while (true) {
+        if (typeof x === "boolean") {
+            return x;  // never
+        }
+    }
+}
+
+function failOrThrow(shouldFail: boolean) {
+    if (shouldFail) {
+        return fail();
+    }
+    throw new Error();
+}
+
+function test(cb: () => string) {
+    let s = cb();
+    return s;
+}
+
+let errorCallback = () => error("Error callback");
+
+test(() => "hello");
+test(() => fail());
+test(() => { throw new Error(); })
+test(errorCallback);
diff --git a/tests/cases/conformance/types/never/neverTypeErrors1.ts b/tests/cases/conformance/types/never/neverTypeErrors1.ts
new file mode 100644
index 00000000000..8d78e863098
--- /dev/null
+++ b/tests/cases/conformance/types/never/neverTypeErrors1.ts
@@ -0,0 +1,20 @@
+function f1() {
+    let x: never;
+    x = 1;
+    x = "abc";
+    x = false;
+    x = undefined;
+    x = null;
+    x = {};
+}
+
+function f2(): never {
+    return;
+}
+
+function f3(): never {
+    return 1;
+}
+
+function f4(): never {
+}
\ No newline at end of file
diff --git a/tests/cases/conformance/types/never/neverTypeErrors2.ts b/tests/cases/conformance/types/never/neverTypeErrors2.ts
new file mode 100644
index 00000000000..635d1c9c6ad
--- /dev/null
+++ b/tests/cases/conformance/types/never/neverTypeErrors2.ts
@@ -0,0 +1,22 @@
+// @strictNullChecks: true
+
+function f1() {
+    let x: never;
+    x = 1;
+    x = "abc";
+    x = false;
+    x = undefined;
+    x = null;
+    x = {};
+}
+
+function f2(): never {
+    return;
+}
+
+function f3(): never {
+    return 1;
+}
+
+function f4(): never {
+}
\ No newline at end of file
diff --git a/tests/cases/conformance/types/objectTypeLiteral/methodSignatures/objectTypesWithOptionalProperties.ts b/tests/cases/conformance/types/objectTypeLiteral/methodSignatures/objectTypesWithOptionalProperties.ts
index f80c3a5952d..7c05ff09e56 100644
--- a/tests/cases/conformance/types/objectTypeLiteral/methodSignatures/objectTypesWithOptionalProperties.ts
+++ b/tests/cases/conformance/types/objectTypeLiteral/methodSignatures/objectTypesWithOptionalProperties.ts
@@ -9,7 +9,7 @@ interface I {
 }
 
 class C {
-    x?: number; // error
+    x?: number; // ok
 }
 
 interface I2<T> {
@@ -17,7 +17,7 @@ interface I2<T> {
 }
 
 class C2<T> {
-    x?: T; // error
+    x?: T; // ok
 }
 
 var b = {
diff --git a/tests/cases/fourslash/declarationExpressions.ts b/tests/cases/fourslash/declarationExpressions.ts
new file mode 100644
index 00000000000..7f2a64d8ecf
--- /dev/null
+++ b/tests/cases/fourslash/declarationExpressions.ts
@@ -0,0 +1,30 @@
+/// <reference path="fourslash.ts"/>
+
+////class A {}
+////const B = class C {
+////    public x;
+////};
+////function D() {}
+////const E = function F() {}
+////console.log(function() {}, class {}); // Expression with no name should have no effect.
+////console.log(function inner() {});
+////String(function fun() { class cls { public prop; } }));
+
+function navExact(name: string, kind: string) {
+	verify.navigationItemsListContains(name, kind, name, "exact");
+}
+
+navExact("A", "class");
+navExact("B", "const");
+navExact("C", "class");
+navExact("x", "property");
+
+navExact("D", "function");
+navExact("E", "const");
+navExact("F", "function")
+
+navExact("inner", "function");
+
+navExact("fun", "function");
+navExact("cls", "class");
+navExact("prop", "property");
diff --git a/tests/cases/fourslash/deleteClassWithEnumPresent.ts b/tests/cases/fourslash/deleteClassWithEnumPresent.ts
index 2b7a02ec717..29f7c61e259 100644
--- a/tests/cases/fourslash/deleteClassWithEnumPresent.ts
+++ b/tests/cases/fourslash/deleteClassWithEnumPresent.ts
@@ -5,4 +5,4 @@
 
 goTo.marker();
 edit.deleteAtCaret('class Bar { }'.length);
-verify.getScriptLexicalStructureListContains('Foo', 'enum', 'tests/cases/fourslash/deleteClassWithEnumPresent.ts', '');
\ No newline at end of file
+verify.navigationBarContains('Foo', 'enum', 'tests/cases/fourslash/deleteClassWithEnumPresent.ts', '');
\ No newline at end of file
diff --git a/tests/cases/fourslash/formatArrayLiteralExpression.ts b/tests/cases/fourslash/formatArrayLiteralExpression.ts
new file mode 100644
index 00000000000..9c2224f8414
--- /dev/null
+++ b/tests/cases/fourslash/formatArrayLiteralExpression.ts
@@ -0,0 +1,45 @@
+///<reference path='fourslash.ts' />
+
+////export let Things = [{
+////    Hat: 'hat', /*1*/
+////    Glove: 'glove',
+////    Umbrella: 'umbrella'
+////},{/*2*/
+////        Salad: 'salad', /*3*/
+////        Burrito: 'burrito',
+////        Pie: 'pie'
+////    }];/*4*/
+////
+////export let Things2 = [
+////{
+////    Hat: 'hat', /*5*/
+////    Glove: 'glove',
+////    Umbrella: 'umbrella'
+////}/*6*/,
+////    {
+////        Salad: 'salad', /*7*/
+////        Burrito: ['burrito', 'carne asada', 'tinga de res', 'tinga de pollo'], /*8*/
+////        Pie: 'pie'
+////    }];/*9*/
+
+format.document();
+
+goTo.marker("1");
+verify.currentLineContentIs("    Hat: 'hat',");
+goTo.marker("2");
+verify.currentLineContentIs("}, {");
+goTo.marker("3");
+verify.currentLineContentIs("    Salad: 'salad',");
+goTo.marker("4");
+verify.currentLineContentIs("}];");
+
+goTo.marker("5");
+verify.currentLineContentIs("        Hat: 'hat',");
+goTo.marker("6");
+verify.currentLineContentIs("    },");
+goTo.marker("7");
+verify.currentLineContentIs("        Salad: 'salad',");
+goTo.marker("8");
+verify.currentLineContentIs("        Burrito: ['burrito', 'carne asada', 'tinga de res', 'tinga de pollo'],");
+goTo.marker("9");
+verify.currentLineContentIs("    }];");
diff --git a/tests/cases/fourslash/fourslash.ts b/tests/cases/fourslash/fourslash.ts
index 48d69f0a85e..08883f163f2 100644
--- a/tests/cases/fourslash/fourslash.ts
+++ b/tests/cases/fourslash/fourslash.ts
@@ -175,8 +175,9 @@ declare namespace FourSlashInterface {
         DocCommentTemplate(expectedText: string, expectedOffset: number, empty?: boolean): void;
         noDocCommentTemplate(): void;
 
-        getScriptLexicalStructureListCount(count: number): void;
-        getScriptLexicalStructureListContains(name: string, kind: string, fileName?: string, parentName?: string, isAdditionalSpan?: boolean, markerPosition?: number): void;
+        navigationBarCount(count: number): void;
+        navigationBarContains(name: string, kind: string, fileName?: string, parentName?: string, isAdditionalSpan?: boolean, markerPosition?: number): void;
+        navigationBarChildItem(parent: string, text: string, kind: string): void;
         navigationItemsListCount(count: number, searchValue: string, matchKind?: string): void;
         navigationItemsListContains(name: string, kind: string, searchValue: string, matchKind: string, fileName?: string, parentName?: string): void;
         occurrencesAtPositionContains(range: Range, isWriteAccess?: boolean): void;
@@ -236,6 +237,7 @@ declare namespace FourSlashInterface {
         printBreakpointAtCurrentLocation(): void;
         printNameOrDottedNameSpans(pos: number): void;
         printErrorList(): void;
+        printNavigationBar(): void;
         printNavigationItems(searchValue?: string): void;
         printScriptLexicalStructureItems(): void;
         printReferences(): void;
diff --git a/tests/cases/fourslash/getNavigationBarItems.ts b/tests/cases/fourslash/getNavigationBarItems.ts
new file mode 100644
index 00000000000..8e9b8de5b05
--- /dev/null
+++ b/tests/cases/fourslash/getNavigationBarItems.ts
@@ -0,0 +1,11 @@
+/// <reference path="fourslash.ts" />
+
+////class C {
+////    foo;
+////    ["bar"]: string;
+////}
+
+verify.navigationBarCount(3);
+verify.navigationBarContains("C", "class");
+verify.navigationBarChildItem("C", "[\"bar\"]", "property");
+verify.navigationBarChildItem("C", "foo", "property");
diff --git a/tests/cases/fourslash/hoverOverComment.ts b/tests/cases/fourslash/hoverOverComment.ts
new file mode 100644
index 00000000000..a9549a932f7
--- /dev/null
+++ b/tests/cases/fourslash/hoverOverComment.ts
@@ -0,0 +1,11 @@
+/// <reference path="fourslash.ts" />
+
+////export function f() {}
+//////foo
+/////**///moo
+
+goTo.marker();
+verify.quickInfoIs("");
+verify.verifyDefinitionsName("", "");
+verify.typeDefinitionCountIs(0);
+verify.referencesCountIs(0);
diff --git a/tests/cases/fourslash/navbar_const.ts b/tests/cases/fourslash/navbar_const.ts
index 6c0738747f3..a2cdb80baef 100644
--- a/tests/cases/fourslash/navbar_const.ts
+++ b/tests/cases/fourslash/navbar_const.ts
@@ -3,7 +3,7 @@
 //// {| "itemName": "c", "kind": "const", "parentName": "" |}const c = 0;
 
 test.markers().forEach(marker => {
-    verify.getScriptLexicalStructureListContains(
+    verify.navigationBarContains(
         marker.data.itemName,
         marker.data.kind,
         marker.fileName,
diff --git a/tests/cases/fourslash/navbar_contains-no-duplicates.ts b/tests/cases/fourslash/navbar_contains-no-duplicates.ts
index 49d570ed770..4c612136b55 100644
--- a/tests/cases/fourslash/navbar_contains-no-duplicates.ts
+++ b/tests/cases/fourslash/navbar_contains-no-duplicates.ts
@@ -29,7 +29,7 @@
 
 test.markers().forEach(marker => {
     if (marker.data) {
-        verify.getScriptLexicalStructureListContains(
+        verify.navigationBarContains(
             marker.data.itemName,
             marker.data.kind,
             marker.fileName,
@@ -38,4 +38,4 @@ test.markers().forEach(marker => {
             marker.position);
     }
 });
-verify.getScriptLexicalStructureListCount(12);
\ No newline at end of file
+verify.navigationBarCount(12);
\ No newline at end of file
diff --git a/tests/cases/fourslash/navbar_exportDefault.ts b/tests/cases/fourslash/navbar_exportDefault.ts
index a8fe854fa28..a56eeb8b226 100644
--- a/tests/cases/fourslash/navbar_exportDefault.ts
+++ b/tests/cases/fourslash/navbar_exportDefault.ts
@@ -14,7 +14,7 @@
 
 test.markers().forEach(marker => {
     goTo.file(marker.fileName);
-    verify.getScriptLexicalStructureListContains(
+    verify.navigationBarContains(
         marker.data.itemName,
         marker.data.kind,
         marker.fileName,
diff --git a/tests/cases/fourslash/navbar_let.ts b/tests/cases/fourslash/navbar_let.ts
index 841382abc53..c3b125526ef 100644
--- a/tests/cases/fourslash/navbar_let.ts
+++ b/tests/cases/fourslash/navbar_let.ts
@@ -3,7 +3,7 @@
 //// {| "itemName": "c", "kind": "let", "parentName": "" |}let c = 0;
 
 test.markers().forEach(marker => {
-    verify.getScriptLexicalStructureListContains(
+    verify.navigationBarContains(
         marker.data.itemName,
         marker.data.kind,
         marker.fileName,
diff --git a/tests/cases/fourslash/navigationBarItemsBindingPatterns.ts b/tests/cases/fourslash/navigationBarItemsBindingPatterns.ts
index 2b363bfad3b..afac15daacc 100644
--- a/tests/cases/fourslash/navigationBarItemsBindingPatterns.ts
+++ b/tests/cases/fourslash/navigationBarItemsBindingPatterns.ts
@@ -6,16 +6,16 @@
 ////const bar1, [c, d]
 ////var {e, x: [f, g]} = {a:1, x:[]};
 
-verify.getScriptLexicalStructureListCount(12); // global (1) + variable declarations (4) + binding patterns (7)
-verify.getScriptLexicalStructureListContains("foo", "var");
-verify.getScriptLexicalStructureListContains("bar", "var");
-verify.getScriptLexicalStructureListContains("foo1", "let")
-verify.getScriptLexicalStructureListContains("a", "let");
-verify.getScriptLexicalStructureListContains("b", "let");
-verify.getScriptLexicalStructureListContains("bar1", "const");
-verify.getScriptLexicalStructureListContains("c", "const");
-verify.getScriptLexicalStructureListContains("d", "const");
-verify.getScriptLexicalStructureListContains("e", "var");
-verify.getScriptLexicalStructureListContains("f", "var");
-verify.getScriptLexicalStructureListContains("g", "var");
+verify.navigationBarCount(12); // global (1) + variable declarations (4) + binding patterns (7)
+verify.navigationBarContains("foo", "var");
+verify.navigationBarContains("bar", "var");
+verify.navigationBarContains("foo1", "let")
+verify.navigationBarContains("a", "let");
+verify.navigationBarContains("b", "let");
+verify.navigationBarContains("bar1", "const");
+verify.navigationBarContains("c", "const");
+verify.navigationBarContains("d", "const");
+verify.navigationBarContains("e", "var");
+verify.navigationBarContains("f", "var");
+verify.navigationBarContains("g", "var");
 
diff --git a/tests/cases/fourslash/navigationBarItemsBindingPatternsInConstructor.ts b/tests/cases/fourslash/navigationBarItemsBindingPatternsInConstructor.ts
index c496a0281e3..84c1d09efe7 100644
--- a/tests/cases/fourslash/navigationBarItemsBindingPatternsInConstructor.ts
+++ b/tests/cases/fourslash/navigationBarItemsBindingPatternsInConstructor.ts
@@ -11,4 +11,4 @@
 ////    }
 ////}
 
-verify.getScriptLexicalStructureListCount(6); // 2x(class + field + constructor)
+verify.navigationBarCount(6); // 2x(class + field + constructor)
diff --git a/tests/cases/fourslash/navigationBarItemsEmptyConstructors.ts b/tests/cases/fourslash/navigationBarItemsEmptyConstructors.ts
index 570b2f7518c..2255d29e293 100644
--- a/tests/cases/fourslash/navigationBarItemsEmptyConstructors.ts
+++ b/tests/cases/fourslash/navigationBarItemsEmptyConstructors.ts
@@ -5,8 +5,8 @@
 ////    }
 ////}
 
-verify.getScriptLexicalStructureListContains("Test", "class");
-verify.getScriptLexicalStructureListContains("constructor", "constructor");
+verify.navigationBarContains("Test", "class");
+verify.navigationBarContains("constructor", "constructor");
 
 // no other items
-verify.getScriptLexicalStructureListCount(2);
\ No newline at end of file
+verify.navigationBarCount(2);
\ No newline at end of file
diff --git a/tests/cases/fourslash/navigationBarItemsExports.ts b/tests/cases/fourslash/navigationBarItemsExports.ts
index d9db125b9b0..b88b60d0efb 100644
--- a/tests/cases/fourslash/navigationBarItemsExports.ts
+++ b/tests/cases/fourslash/navigationBarItemsExports.ts
@@ -11,8 +11,8 @@
 
 test.markers().forEach((marker) => {
     if (marker.data) {
-        verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+        verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
     }
 });
 
-verify.getScriptLexicalStructureListCount(4);
+verify.navigationBarCount(4);
diff --git a/tests/cases/fourslash/navigationBarItemsFunctions.ts b/tests/cases/fourslash/navigationBarItemsFunctions.ts
index b683790c60d..0948923c046 100644
--- a/tests/cases/fourslash/navigationBarItemsFunctions.ts
+++ b/tests/cases/fourslash/navigationBarItemsFunctions.ts
@@ -17,7 +17,7 @@
 ////}
 
 test.markers().forEach((marker) => {
-    verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+    verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
 });
 
-verify.getScriptLexicalStructureListCount(8); // 4 functions + global.  Note: there are 8 because of the functions show up at the top level and as child items.
+verify.navigationBarCount(8); // 4 functions + global.  Note: there are 8 because of the functions show up at the top level and as child items.
diff --git a/tests/cases/fourslash/navigationBarItemsFunctionsBroken.ts b/tests/cases/fourslash/navigationBarItemsFunctionsBroken.ts
index 7846a9723d1..e8bf1d33fc3 100644
--- a/tests/cases/fourslash/navigationBarItemsFunctionsBroken.ts
+++ b/tests/cases/fourslash/navigationBarItemsFunctionsBroken.ts
@@ -6,7 +6,7 @@
 ////}
 
 test.markers().forEach((marker) => {
-    verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+    verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
 });
 
-verify.getScriptLexicalStructureListCount(3); // <global> and 'f'. 
\ No newline at end of file
+verify.navigationBarCount(3); // <global> and 'f'. 
\ No newline at end of file
diff --git a/tests/cases/fourslash/navigationBarItemsFunctionsBroken2.ts b/tests/cases/fourslash/navigationBarItemsFunctionsBroken2.ts
index 50bf91e02d6..9576d4dd789 100644
--- a/tests/cases/fourslash/navigationBarItemsFunctionsBroken2.ts
+++ b/tests/cases/fourslash/navigationBarItemsFunctionsBroken2.ts
@@ -7,7 +7,7 @@
 ////}
 
 test.markers().forEach((marker) => {
-    verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+    verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
 });
 
-verify.getScriptLexicalStructureListCount(3); // <global> and 'f'
\ No newline at end of file
+verify.navigationBarCount(3); // <global> and 'f'
\ No newline at end of file
diff --git a/tests/cases/fourslash/navigationBarItemsImports.ts b/tests/cases/fourslash/navigationBarItemsImports.ts
index 8b0d02bd9f5..53619da310a 100644
--- a/tests/cases/fourslash/navigationBarItemsImports.ts
+++ b/tests/cases/fourslash/navigationBarItemsImports.ts
@@ -18,8 +18,8 @@
 
 test.markers().forEach((marker) => {
     if (marker.data) {
-        verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+        verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
     }
 });
 
-verify.getScriptLexicalStructureListCount(9);
+verify.navigationBarCount(9);
diff --git a/tests/cases/fourslash/navigationBarItemsInsideMethodsAndConstructors.ts b/tests/cases/fourslash/navigationBarItemsInsideMethodsAndConstructors.ts
index 4dcca43af35..7aaa3d7f5a2 100644
--- a/tests/cases/fourslash/navigationBarItemsInsideMethodsAndConstructors.ts
+++ b/tests/cases/fourslash/navigationBarItemsInsideMethodsAndConstructors.ts
@@ -35,8 +35,8 @@
 ////}
 
 test.markers().forEach((marker) => {
-    verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+    verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
 });
 
 // no other items
-verify.getScriptLexicalStructureListCount(17);
+verify.navigationBarCount(17);
diff --git a/tests/cases/fourslash/navigationBarItemsItems.ts b/tests/cases/fourslash/navigationBarItemsItems.ts
index 67e14fe1009..3804a0dab55 100644
--- a/tests/cases/fourslash/navigationBarItemsItems.ts
+++ b/tests/cases/fourslash/navigationBarItemsItems.ts
@@ -45,8 +45,8 @@
 
 test.markers().forEach((marker) => {
     if (marker.data) {
-        verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+        verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
     }
 });
 
-verify.getScriptLexicalStructureListCount(23);
+verify.navigationBarCount(23);
diff --git a/tests/cases/fourslash/navigationBarItemsItems2.ts b/tests/cases/fourslash/navigationBarItemsItems2.ts
index 3b94260ddbd..d4ffde54051 100644
--- a/tests/cases/fourslash/navigationBarItemsItems2.ts
+++ b/tests/cases/fourslash/navigationBarItemsItems2.ts
@@ -8,5 +8,5 @@ edit.insertLine("module A");
 edit.insert("export class ");
 
 // should not crash
-verify.getScriptLexicalStructureListCount(2);
+verify.navigationBarCount(2);
 
diff --git a/tests/cases/fourslash/navigationBarItemsItemsContainsNoAnonymousFunctions.ts b/tests/cases/fourslash/navigationBarItemsItemsContainsNoAnonymousFunctions.ts
index b7430ad03de..dac6e96ab93 100644
--- a/tests/cases/fourslash/navigationBarItemsItemsContainsNoAnonymousFunctions.ts
+++ b/tests/cases/fourslash/navigationBarItemsItemsContainsNoAnonymousFunctions.ts
@@ -30,15 +30,15 @@
 ////}
 
 goTo.marker("file1");
-verify.getScriptLexicalStructureListCount(0);
+verify.navigationBarCount(0);
 
 goTo.marker("file2");
-verify.getScriptLexicalStructureListContains("<global>", "module");
-verify.getScriptLexicalStructureListContains("x", "var");
-verify.getScriptLexicalStructureListCount(2);
+verify.navigationBarContains("<global>", "module");
+verify.navigationBarContains("x", "var");
+verify.navigationBarCount(2);
 
 goTo.marker("file3");
-verify.getScriptLexicalStructureListContains("<global>", "module");
-verify.getScriptLexicalStructureListContains("foo", "function");
-verify.getScriptLexicalStructureListContains("bar", "function");
-verify.getScriptLexicalStructureListCount(5);
\ No newline at end of file
+verify.navigationBarContains("<global>", "module");
+verify.navigationBarContains("foo", "function");
+verify.navigationBarContains("bar", "function");
+verify.navigationBarCount(5);
\ No newline at end of file
diff --git a/tests/cases/fourslash/navigationBarItemsItemsExternalModules.ts b/tests/cases/fourslash/navigationBarItemsItemsExternalModules.ts
index 550f1aed783..64a595cafb1 100644
--- a/tests/cases/fourslash/navigationBarItemsItemsExternalModules.ts
+++ b/tests/cases/fourslash/navigationBarItemsItemsExternalModules.ts
@@ -5,7 +5,7 @@
 ////}
 
 test.markers().forEach((marker) => {
-    verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+    verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
 });
 
-verify.getScriptLexicalStructureListCount(2); // external module node + class + property
+verify.navigationBarCount(2); // external module node + class + property
diff --git a/tests/cases/fourslash/navigationBarItemsItemsExternalModules2.ts b/tests/cases/fourslash/navigationBarItemsItemsExternalModules2.ts
index b0bf6eb3fa2..7c1c32cff76 100644
--- a/tests/cases/fourslash/navigationBarItemsItemsExternalModules2.ts
+++ b/tests/cases/fourslash/navigationBarItemsItemsExternalModules2.ts
@@ -9,7 +9,7 @@
 ////export var x: number;
 
 test.markers().forEach((marker) => {
-    verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+    verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
 });
 
-verify.getScriptLexicalStructureListCount(4); // external module node + variable in module + class + property
+verify.navigationBarCount(4); // external module node + variable in module + class + property
diff --git a/tests/cases/fourslash/navigationBarItemsItemsExternalModules3.ts b/tests/cases/fourslash/navigationBarItemsItemsExternalModules3.ts
index 9ef49e38776..193da9ff47b 100644
--- a/tests/cases/fourslash/navigationBarItemsItemsExternalModules3.ts
+++ b/tests/cases/fourslash/navigationBarItemsItemsExternalModules3.ts
@@ -9,7 +9,7 @@
 ////export var x: number;
 
 test.markers().forEach((marker) => {
-    verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+    verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
 });
 
-verify.getScriptLexicalStructureListCount(4); // external module node + variable in module + class + property
+verify.navigationBarCount(4); // external module node + variable in module + class + property
diff --git a/tests/cases/fourslash/navigationBarItemsItemsModuleVariables.ts b/tests/cases/fourslash/navigationBarItemsItemsModuleVariables.ts
index 85bf609105c..afc1acfb9cb 100644
--- a/tests/cases/fourslash/navigationBarItemsItemsModuleVariables.ts
+++ b/tests/cases/fourslash/navigationBarItemsItemsModuleVariables.ts
@@ -19,12 +19,12 @@
 ////    export var z = 0;
 ////}
 goTo.marker("file1");
-verify.getScriptLexicalStructureListContains("Module1", "module");
-verify.getScriptLexicalStructureListContains("x", "var");
+verify.navigationBarContains("Module1", "module");
+verify.navigationBarContains("x", "var");
 // nothing else should show up
-verify.getScriptLexicalStructureListCount(2);
+verify.navigationBarCount(2);
 
 goTo.marker("file2");
-verify.getScriptLexicalStructureListContains("Module1.SubModule", "module");
-verify.getScriptLexicalStructureListContains("y", "var");
-verify.getScriptLexicalStructureListCount(2);
+verify.navigationBarContains("Module1.SubModule", "module");
+verify.navigationBarContains("y", "var");
+verify.navigationBarCount(2);
diff --git a/tests/cases/fourslash/navigationBarItemsMissingName1.ts b/tests/cases/fourslash/navigationBarItemsMissingName1.ts
index c5282d696f3..98321039c5a 100644
--- a/tests/cases/fourslash/navigationBarItemsMissingName1.ts
+++ b/tests/cases/fourslash/navigationBarItemsMissingName1.ts
@@ -9,8 +9,8 @@
 
 
 test.markers().forEach((marker) => {
-    verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+    verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
 });
 
 /// Only have two named elements.
-verify.getScriptLexicalStructureListCount(2);
+verify.navigationBarCount(2);
diff --git a/tests/cases/fourslash/navigationBarItemsMissingName2.ts b/tests/cases/fourslash/navigationBarItemsMissingName2.ts
index bf0dc038991..d26aa0b553d 100644
--- a/tests/cases/fourslash/navigationBarItemsMissingName2.ts
+++ b/tests/cases/fourslash/navigationBarItemsMissingName2.ts
@@ -8,4 +8,4 @@
 
 
 // The class is unnamed, so its method is not included either.
-verify.getScriptLexicalStructureListCount(2);
+verify.navigationBarCount(2);
diff --git a/tests/cases/fourslash/navigationBarItemsModules.ts b/tests/cases/fourslash/navigationBarItemsModules.ts
index a5fc9b641f9..2f6e1d7f844 100644
--- a/tests/cases/fourslash/navigationBarItemsModules.ts
+++ b/tests/cases/fourslash/navigationBarItemsModules.ts
@@ -39,10 +39,10 @@
 
 
 test.markers().forEach((marker) => {
-    verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+    verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
 });
 
 /// We have 8 module keywords, and 4 var keywords.
 /// The declarations of A.B.C.x do not get merged, so the 4 vars are independent.
 /// The two 'A' modules, however, do get merged, so in reality we have 7 modules.
-verify.getScriptLexicalStructureListCount(11);
+verify.navigationBarCount(11);
diff --git a/tests/cases/fourslash/navigationBarItemsMultilineStringIdentifiers.ts b/tests/cases/fourslash/navigationBarItemsMultilineStringIdentifiers.ts
index 59eca486b62..8856c4c044a 100644
--- a/tests/cases/fourslash/navigationBarItemsMultilineStringIdentifiers.ts
+++ b/tests/cases/fourslash/navigationBarItemsMultilineStringIdentifiers.ts
@@ -35,7 +35,7 @@
 
 
 test.markers().forEach((marker) => {
-    verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+    verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
 });
 
-verify.getScriptLexicalStructureListCount(9); // interface w/ 2 properties, class w/ 2 properties, 3 modules
\ No newline at end of file
+verify.navigationBarCount(9); // interface w/ 2 properties, class w/ 2 properties, 3 modules
\ No newline at end of file
diff --git a/tests/cases/fourslash/navigationBarItemsPropertiesDefinedInConstructors.ts b/tests/cases/fourslash/navigationBarItemsPropertiesDefinedInConstructors.ts
index adda43066a3..9b3f4aacd9c 100644
--- a/tests/cases/fourslash/navigationBarItemsPropertiesDefinedInConstructors.ts
+++ b/tests/cases/fourslash/navigationBarItemsPropertiesDefinedInConstructors.ts
@@ -6,10 +6,10 @@
 ////    }
 ////}
 
-verify.getScriptLexicalStructureListContains("List", "class");
-verify.getScriptLexicalStructureListContains("constructor", "constructor");
-verify.getScriptLexicalStructureListContains("a", "property");
-verify.getScriptLexicalStructureListContains("b", "property");
+verify.navigationBarContains("List", "class");
+verify.navigationBarContains("constructor", "constructor");
+verify.navigationBarContains("a", "property");
+verify.navigationBarContains("b", "property");
 
 // no other items
-verify.getScriptLexicalStructureListCount(4);
\ No newline at end of file
+verify.navigationBarCount(4);
\ No newline at end of file
diff --git a/tests/cases/fourslash/navigationBarItemsSymbols1.ts b/tests/cases/fourslash/navigationBarItemsSymbols1.ts
index e3dde6738e9..c9df85b3ece 100644
--- a/tests/cases/fourslash/navigationBarItemsSymbols1.ts
+++ b/tests/cases/fourslash/navigationBarItemsSymbols1.ts
@@ -11,7 +11,7 @@
 ////}
 
 test.markers().forEach(marker => {
-    verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+    verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
 });
 
-verify.getScriptLexicalStructureListCount(test.markers().length);
\ No newline at end of file
+verify.navigationBarCount(test.markers().length);
\ No newline at end of file
diff --git a/tests/cases/fourslash/navigationBarItemsSymbols2.ts b/tests/cases/fourslash/navigationBarItemsSymbols2.ts
index d048de895ec..579a6353c13 100644
--- a/tests/cases/fourslash/navigationBarItemsSymbols2.ts
+++ b/tests/cases/fourslash/navigationBarItemsSymbols2.ts
@@ -9,7 +9,7 @@
 ////}
 
 test.markers().forEach(marker => {
-    verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+    verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
 });
 
-verify.getScriptLexicalStructureListCount(test.markers().length);
\ No newline at end of file
+verify.navigationBarCount(test.markers().length);
\ No newline at end of file
diff --git a/tests/cases/fourslash/navigationBarItemsSymbols3.ts b/tests/cases/fourslash/navigationBarItemsSymbols3.ts
index 19f81037559..5677c0b6301 100644
--- a/tests/cases/fourslash/navigationBarItemsSymbols3.ts
+++ b/tests/cases/fourslash/navigationBarItemsSymbols3.ts
@@ -7,7 +7,7 @@
 ////}
 
 test.markers().forEach(marker => {
-    verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+    verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
 });
 
-verify.getScriptLexicalStructureListCount(test.markers().length);
\ No newline at end of file
+verify.navigationBarCount(test.markers().length);
\ No newline at end of file
diff --git a/tests/cases/fourslash/navigationBarItemsTypeAlias.ts b/tests/cases/fourslash/navigationBarItemsTypeAlias.ts
new file mode 100644
index 00000000000..50923256533
--- /dev/null
+++ b/tests/cases/fourslash/navigationBarItemsTypeAlias.ts
@@ -0,0 +1,6 @@
+/// <reference path="fourslash.ts"/>
+
+////type T = number | string;
+
+verify.navigationBarCount(1);
+verify.navigationBarContains("T", "type");
diff --git a/tests/cases/fourslash/server/navbar01.ts b/tests/cases/fourslash/server/navbar01.ts
index 82ded5e6f54..8e76e7c017e 100644
--- a/tests/cases/fourslash/server/navbar01.ts
+++ b/tests/cases/fourslash/server/navbar01.ts
@@ -45,8 +45,8 @@
 
 test.markers().forEach((marker) => {
     if (marker.data) {
-        verify.getScriptLexicalStructureListContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
+        verify.navigationBarContains(marker.data.itemName, marker.data.kind, marker.fileName, marker.data.parentName);
     }
 });
 
-verify.getScriptLexicalStructureListCount(23);
+verify.navigationBarCount(23);
diff --git a/tests/cases/fourslash/shims-pp/getNavigationBarItems.ts b/tests/cases/fourslash/shims-pp/getNavigationBarItems.ts
index 6c0738747f3..a2cdb80baef 100644
--- a/tests/cases/fourslash/shims-pp/getNavigationBarItems.ts
+++ b/tests/cases/fourslash/shims-pp/getNavigationBarItems.ts
@@ -3,7 +3,7 @@
 //// {| "itemName": "c", "kind": "const", "parentName": "" |}const c = 0;
 
 test.markers().forEach(marker => {
-    verify.getScriptLexicalStructureListContains(
+    verify.navigationBarContains(
         marker.data.itemName,
         marker.data.kind,
         marker.fileName,
diff --git a/tests/cases/fourslash/shims/getNavigationBarItems.ts b/tests/cases/fourslash/shims/getNavigationBarItems.ts
index 6c0738747f3..a2cdb80baef 100644
--- a/tests/cases/fourslash/shims/getNavigationBarItems.ts
+++ b/tests/cases/fourslash/shims/getNavigationBarItems.ts
@@ -3,7 +3,7 @@
 //// {| "itemName": "c", "kind": "const", "parentName": "" |}const c = 0;
 
 test.markers().forEach(marker => {
-    verify.getScriptLexicalStructureListContains(
+    verify.navigationBarContains(
         marker.data.itemName,
         marker.data.kind,
         marker.fileName,