diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 71c19e1214a..0f057e61667 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -3216,7 +3216,9 @@ 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;
         }
@@ -9921,7 +9923,8 @@ 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);
@@ -13396,7 +13399,7 @@ 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);
             }
 
@@ -18290,7 +18293,7 @@ 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) {
@@ -18299,9 +18302,6 @@ 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,
@@ -18523,8 +18523,7 @@ 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/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 899b4292f04..cb347234ec7 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