diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 4d854925364..44266402607 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -91,7 +91,8 @@ namespace ts {
             getExportsOfModule: getExportsOfModuleAsArray,
 
             getJsxElementAttributesType,
-            getJsxIntrinsicTagNames
+            getJsxIntrinsicTagNames,
+            isOptionalParameter
         };
 
         let unknownSymbol = createSymbol(SymbolFlags.Property | SymbolFlags.Transient, "unknown");
@@ -1998,15 +1999,15 @@ namespace ts {
             }
 
             return _displayBuilder || (_displayBuilder = {
-                buildSymbolDisplay: buildSymbolDisplay,
-                buildTypeDisplay: buildTypeDisplay,
-                buildTypeParameterDisplay: buildTypeParameterDisplay,
-                buildParameterDisplay: buildParameterDisplay,
-                buildDisplayForParametersAndDelimiters: buildDisplayForParametersAndDelimiters,
-                buildDisplayForTypeParametersAndDelimiters: buildDisplayForTypeParametersAndDelimiters,
-                buildTypeParameterDisplayFromSymbol: buildTypeParameterDisplayFromSymbol,
-                buildSignatureDisplay: buildSignatureDisplay,
-                buildReturnTypeDisplay: buildReturnTypeDisplay
+                buildSymbolDisplay,
+                buildTypeDisplay,
+                buildTypeParameterDisplay,
+                buildParameterDisplay,
+                buildDisplayForParametersAndDelimiters,
+                buildDisplayForTypeParametersAndDelimiters,
+                buildTypeParameterDisplayFromSymbol,
+                buildSignatureDisplay,
+                buildReturnTypeDisplay
             });
         }
 
@@ -3521,7 +3522,19 @@ namespace ts {
         }
 
         function isOptionalParameter(node: ParameterDeclaration) {
-            return hasQuestionToken(node) || !!node.initializer;
+            if (hasQuestionToken(node)) {
+                return true;
+            }
+
+            if (node.initializer) {
+                let signatureDeclaration = <SignatureDeclaration>node.parent;
+                let signature = getSignatureFromDeclaration(signatureDeclaration);
+                let parameterIndex = signatureDeclaration.parameters.indexOf(node);
+                Debug.assert(parameterIndex >= 0);
+                return parameterIndex >= signature.minArgumentCount;
+            }
+
+            return false;
         }
 
         function getSignatureFromDeclaration(declaration: SignatureDeclaration): Signature {
@@ -3539,11 +3552,16 @@ namespace ts {
                     if (param.type && param.type.kind === SyntaxKind.StringLiteral) {
                         hasStringLiterals = true;
                     }
-                    if (minArgumentCount < 0) {
-                        if (param.initializer || param.questionToken || param.dotDotDotToken) {
+
+                    if (param.initializer || param.questionToken || param.dotDotDotToken) {
+                        if (minArgumentCount < 0) {
                             minArgumentCount = i;
                         }
                     }
+                    else {
+                        // If we see any required parameters, it means the prior ones were not in fact optional.
+                        minArgumentCount = -1;
+                    }
                 }
 
                 if (minArgumentCount < 0) {
@@ -14413,6 +14431,7 @@ namespace ts {
                 getBlockScopedVariableId,
                 getReferencedValueDeclaration,
                 getTypeReferenceSerializationKind,
+                isOptionalParameter
             };
         }
 
@@ -14808,17 +14827,15 @@ namespace ts {
                         return grammarErrorOnNode(parameter.name, Diagnostics.A_rest_parameter_cannot_have_an_initializer);
                     }
                 }
-                else if (parameter.questionToken || parameter.initializer) {
+                else if (parameter.questionToken) {
                     seenOptionalParameter = true;
 
-                    if (parameter.questionToken && parameter.initializer) {
+                    if (parameter.initializer) {
                         return grammarErrorOnNode(parameter.name, Diagnostics.Parameter_cannot_have_question_mark_and_initializer);
                     }
                 }
-                else {
-                    if (seenOptionalParameter) {
-                        return grammarErrorOnNode(parameter.name, Diagnostics.A_required_parameter_cannot_follow_an_optional_parameter);
-                    }
+                else if (seenOptionalParameter && !parameter.initializer) {
+                    return grammarErrorOnNode(parameter.name, Diagnostics.A_required_parameter_cannot_follow_an_optional_parameter);
                 }
             }
         }
diff --git a/src/compiler/declarationEmitter.ts b/src/compiler/declarationEmitter.ts
index 6c0803f8761..63d71b0339e 100644
--- a/src/compiler/declarationEmitter.ts
+++ b/src/compiler/declarationEmitter.ts
@@ -1371,7 +1371,7 @@ namespace ts {
             else {
                 writeTextOfNode(currentSourceFile, node.name);
             }
-            if (node.initializer || hasQuestionToken(node)) {
+            if (resolver.isOptionalParameter(node)) {
                 write("?");
             }
             decreaseIndent();
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 9e80311d0c2..c5d7f879c25 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -1431,6 +1431,7 @@ namespace ts {
 
         getJsxElementAttributesType(elementNode: JsxOpeningLikeElement): Type;
         getJsxIntrinsicTagNames(): Symbol[];
+        isOptionalParameter(node: ParameterDeclaration): boolean;
 
         // Should not be called directly.  Should only be accessed through the Program instance.
         /* @internal */ getDiagnostics(sourceFile?: SourceFile, cancellationToken?: CancellationToken): Diagnostic[];
@@ -1574,7 +1575,8 @@ namespace ts {
         getConstantValue(node: EnumMember | PropertyAccessExpression | ElementAccessExpression): number;
         getBlockScopedVariableId(node: Identifier): number;
         getReferencedValueDeclaration(reference: Identifier): Declaration;
-        getTypeReferenceSerializationKind(node: TypeReferenceNode): TypeReferenceSerializationKind; 
+        getTypeReferenceSerializationKind(node: TypeReferenceNode): TypeReferenceSerializationKind;
+        isOptionalParameter(node: ParameterDeclaration): boolean;
     }
 
     export const enum SymbolFlags {
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 56eb2415cf9..312a3666757 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -988,15 +988,13 @@ namespace ts {
         if (node) {
             switch (node.kind) {
                 case SyntaxKind.Parameter:
-                    return (<ParameterDeclaration>node).questionToken !== undefined;
                 case SyntaxKind.MethodDeclaration:
                 case SyntaxKind.MethodSignature:
-                    return (<MethodDeclaration>node).questionToken !== undefined;
                 case SyntaxKind.ShorthandPropertyAssignment:
                 case SyntaxKind.PropertyAssignment:
                 case SyntaxKind.PropertyDeclaration:
                 case SyntaxKind.PropertySignature:
-                    return (<PropertyDeclaration>node).questionToken !== undefined;
+                    return (<ParameterDeclaration | MethodDeclaration | PropertyDeclaration>node).questionToken !== undefined;
             }
         }
 
diff --git a/src/services/signatureHelp.ts b/src/services/signatureHelp.ts
index f44ebade2a7..df321647e5c 100644
--- a/src/services/signatureHelp.ts
+++ b/src/services/signatureHelp.ts
@@ -611,13 +611,11 @@ namespace ts.SignatureHelp {
                 let displayParts = mapToDisplayParts(writer =>
                     typeChecker.getSymbolDisplayBuilder().buildParameterDisplay(parameter, writer, invocation));
 
-                let isOptional = hasQuestionToken(parameter.valueDeclaration);
-
                 return {
                     name: parameter.name,
                     documentation: parameter.getDocumentationComment(),
                     displayParts,
-                    isOptional
+                    isOptional: typeChecker.isOptionalParameter(<ParameterDeclaration>parameter.valueDeclaration)
                 };
             }
 
diff --git a/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors1.errors.txt b/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors1.errors.txt
index 5934ab52ccf..b32a7b67b61 100644
--- a/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors1.errors.txt
+++ b/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors1.errors.txt
@@ -1,10 +1,9 @@
 tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors1.ts(1,9): error TS1016: A required parameter cannot follow an optional parameter.
 tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors1.ts(2,8): error TS1047: A rest parameter cannot be optional.
 tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors1.ts(4,5): error TS1048: A rest parameter cannot have an initializer.
-tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors1.ts(7,12): error TS1016: A required parameter cannot follow an optional parameter.
 
 
-==== tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors1.ts (4 errors) ====
+==== tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors1.ts (3 errors) ====
     (arg1?, arg2) => 101;
             ~~~~
 !!! error TS1016: A required parameter cannot follow an optional parameter.
@@ -16,7 +15,5 @@ tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors1.ts(7,12): error TS1016
         ~~~
 !!! error TS1048: A rest parameter cannot have an initializer.
     
-    // Non optional parameter following an optional one
-    (arg1 = 1, arg2) => 1; 
-               ~~~~
-!!! error TS1016: A required parameter cannot follow an optional parameter.
\ No newline at end of file
+    // Uninitialized parameter makes the initialized one required
+    (arg1 = 1, arg2) => 1; 
\ No newline at end of file
diff --git a/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors1.js b/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors1.js
index e746b24a066..4d27f8230eb 100644
--- a/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors1.js
+++ b/tests/baselines/reference/fatarrowfunctionsOptionalArgsErrors1.js
@@ -4,7 +4,7 @@
 (...arg) => 103;
 (...arg:number [] = []) => 104;
 
-// Non optional parameter following an optional one
+// Uninitialized parameter makes the initialized one required
 (arg1 = 1, arg2) => 1; 
 
 //// [fatarrowfunctionsOptionalArgsErrors1.js]
@@ -30,7 +30,7 @@
     }
     return 104;
 });
-// Non optional parameter following an optional one
+// Uninitialized parameter makes the initialized one required
 (function (arg1, arg2) {
     if (arg1 === void 0) { arg1 = 1; }
     return 1;
diff --git a/tests/baselines/reference/optionalParamArgsTest.errors.txt b/tests/baselines/reference/optionalParamArgsTest.errors.txt
index 3c4d2e37be9..05fefa85065 100644
--- a/tests/baselines/reference/optionalParamArgsTest.errors.txt
+++ b/tests/baselines/reference/optionalParamArgsTest.errors.txt
@@ -1,6 +1,6 @@
 tests/cases/compiler/optionalParamArgsTest.ts(31,12): error TS2393: Duplicate function implementation.
-tests/cases/compiler/optionalParamArgsTest.ts(35,12): error TS2393: Duplicate function implementation.
-tests/cases/compiler/optionalParamArgsTest.ts(35,47): error TS1016: A required parameter cannot follow an optional parameter.
+tests/cases/compiler/optionalParamArgsTest.ts(34,12): error TS2393: Duplicate function implementation.
+tests/cases/compiler/optionalParamArgsTest.ts(98,1): error TS2346: Supplied parameters do not match any signature of call target.
 tests/cases/compiler/optionalParamArgsTest.ts(99,1): error TS2346: Supplied parameters do not match any signature of call target.
 tests/cases/compiler/optionalParamArgsTest.ts(100,1): error TS2346: Supplied parameters do not match any signature of call target.
 tests/cases/compiler/optionalParamArgsTest.ts(101,1): error TS2346: Supplied parameters do not match any signature of call target.
@@ -20,10 +20,9 @@ tests/cases/compiler/optionalParamArgsTest.ts(114,1): error TS2346: Supplied par
 tests/cases/compiler/optionalParamArgsTest.ts(115,1): error TS2346: Supplied parameters do not match any signature of call target.
 tests/cases/compiler/optionalParamArgsTest.ts(116,1): error TS2346: Supplied parameters do not match any signature of call target.
 tests/cases/compiler/optionalParamArgsTest.ts(117,1): error TS2346: Supplied parameters do not match any signature of call target.
-tests/cases/compiler/optionalParamArgsTest.ts(118,1): error TS2346: Supplied parameters do not match any signature of call target.
 
 
-==== tests/cases/compiler/optionalParamArgsTest.ts (23 errors) ====
+==== tests/cases/compiler/optionalParamArgsTest.ts (22 errors) ====
     // Optional parameter and default argument tests
     
     // Key:
@@ -58,13 +57,10 @@ tests/cases/compiler/optionalParamArgsTest.ts(118,1): error TS2346: Supplied par
                ~~~~
 !!! error TS2393: Duplicate function implementation.
     
-        // Negative test
-        // "Optional parameters may only be followed by other optional parameters"
+        // Uninitialized parameter makes the initialized one required
         public C1M5(C1M5A1:number,C1M5A2:number=0,C1M5A3:number) { return C1M5A1 + C1M5A2; }
                ~~~~
 !!! error TS2393: Duplicate function implementation.
-                                                  ~~~~~~
-!!! error TS1016: A required parameter cannot follow an optional parameter.
     }
     
     class C2 extends C1 {
diff --git a/tests/baselines/reference/optionalParamArgsTest.js b/tests/baselines/reference/optionalParamArgsTest.js
index 414f474598d..0c5a5d82814 100644
--- a/tests/baselines/reference/optionalParamArgsTest.js
+++ b/tests/baselines/reference/optionalParamArgsTest.js
@@ -31,8 +31,7 @@ class C1 {
 
     public C1M5(C1M5A1:number,C1M5A2:number=0,C1M5A3?:number) { return C1M5A1 + C1M5A2; }
 
-    // Negative test
-    // "Optional parameters may only be followed by other optional parameters"
+    // Uninitialized parameter makes the initialized one required
     public C1M5(C1M5A1:number,C1M5A2:number=0,C1M5A3:number) { return C1M5A1 + C1M5A2; }
 }
 
@@ -152,8 +151,7 @@ var C1 = (function () {
         if (C1M5A2 === void 0) { C1M5A2 = 0; }
         return C1M5A1 + C1M5A2;
     };
-    // Negative test
-    // "Optional parameters may only be followed by other optional parameters"
+    // Uninitialized parameter makes the initialized one required
     C1.prototype.C1M5 = function (C1M5A1, C1M5A2, C1M5A3) {
         if (C1M5A2 === void 0) { C1M5A2 = 0; }
         return C1M5A1 + C1M5A2;
diff --git a/tests/baselines/reference/requiredInitializedParameter1.errors.txt b/tests/baselines/reference/requiredInitializedParameter1.errors.txt
new file mode 100644
index 00000000000..ef958dbdec9
--- /dev/null
+++ b/tests/baselines/reference/requiredInitializedParameter1.errors.txt
@@ -0,0 +1,28 @@
+tests/cases/compiler/requiredInitializedParameter1.ts(11,1): error TS2346: Supplied parameters do not match any signature of call target.
+tests/cases/compiler/requiredInitializedParameter1.ts(16,1): error TS2346: Supplied parameters do not match any signature of call target.
+
+
+==== tests/cases/compiler/requiredInitializedParameter1.ts (2 errors) ====
+    function f1(a, b = 0, c) { }
+    function f2(a, b = 0, c = 0) { }
+    function f3(a, b = 0, c?) { }
+    function f4(a, b = 0, ...c) { }
+    
+    f1(0, 1, 2);
+    f2(0, 1, 2);
+    f3(0, 1, 2);
+    f4(0, 1, 2);
+    
+    f1(0, 1);
+    ~~~~~~~~
+!!! error TS2346: Supplied parameters do not match any signature of call target.
+    f2(0, 1);
+    f3(0, 1);
+    f4(0, 1);
+    
+    f1(0);
+    ~~~~~
+!!! error TS2346: Supplied parameters do not match any signature of call target.
+    f2(0);
+    f3(0);
+    f4(0);
\ No newline at end of file
diff --git a/tests/baselines/reference/requiredInitializedParameter1.js b/tests/baselines/reference/requiredInitializedParameter1.js
new file mode 100644
index 00000000000..0fe07503056
--- /dev/null
+++ b/tests/baselines/reference/requiredInitializedParameter1.js
@@ -0,0 +1,51 @@
+//// [requiredInitializedParameter1.ts]
+function f1(a, b = 0, c) { }
+function f2(a, b = 0, c = 0) { }
+function f3(a, b = 0, c?) { }
+function f4(a, b = 0, ...c) { }
+
+f1(0, 1, 2);
+f2(0, 1, 2);
+f3(0, 1, 2);
+f4(0, 1, 2);
+
+f1(0, 1);
+f2(0, 1);
+f3(0, 1);
+f4(0, 1);
+
+f1(0);
+f2(0);
+f3(0);
+f4(0);
+
+//// [requiredInitializedParameter1.js]
+function f1(a, b, c) {
+    if (b === void 0) { b = 0; }
+}
+function f2(a, b, c) {
+    if (b === void 0) { b = 0; }
+    if (c === void 0) { c = 0; }
+}
+function f3(a, b, c) {
+    if (b === void 0) { b = 0; }
+}
+function f4(a, b) {
+    if (b === void 0) { b = 0; }
+    var c = [];
+    for (var _i = 2; _i < arguments.length; _i++) {
+        c[_i - 2] = arguments[_i];
+    }
+}
+f1(0, 1, 2);
+f2(0, 1, 2);
+f3(0, 1, 2);
+f4(0, 1, 2);
+f1(0, 1);
+f2(0, 1);
+f3(0, 1);
+f4(0, 1);
+f1(0);
+f2(0);
+f3(0);
+f4(0);
diff --git a/tests/baselines/reference/requiredInitializedParameter2.errors.txt b/tests/baselines/reference/requiredInitializedParameter2.errors.txt
new file mode 100644
index 00000000000..5dcec536e1b
--- /dev/null
+++ b/tests/baselines/reference/requiredInitializedParameter2.errors.txt
@@ -0,0 +1,17 @@
+tests/cases/compiler/requiredInitializedParameter2.ts(5,7): error TS2420: Class 'C1' incorrectly implements interface 'I1'.
+  Types of property 'method' are incompatible.
+    Type '(a: number, b: any) => void' is not assignable to type '() => any'.
+
+
+==== tests/cases/compiler/requiredInitializedParameter2.ts (1 errors) ====
+    interface I1 {
+        method();
+    }
+    
+    class C1 implements I1 {
+          ~~
+!!! error TS2420: Class 'C1' incorrectly implements interface 'I1'.
+!!! error TS2420:   Types of property 'method' are incompatible.
+!!! error TS2420:     Type '(a: number, b: any) => void' is not assignable to type '() => any'.
+        method(a = 0, b) { }
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/requiredInitializedParameter2.js b/tests/baselines/reference/requiredInitializedParameter2.js
new file mode 100644
index 00000000000..3c69d54e1d9
--- /dev/null
+++ b/tests/baselines/reference/requiredInitializedParameter2.js
@@ -0,0 +1,18 @@
+//// [requiredInitializedParameter2.ts]
+interface I1 {
+    method();
+}
+
+class C1 implements I1 {
+    method(a = 0, b) { }
+}
+
+//// [requiredInitializedParameter2.js]
+var C1 = (function () {
+    function C1() {
+    }
+    C1.prototype.method = function (a, b) {
+        if (a === void 0) { a = 0; }
+    };
+    return C1;
+})();
diff --git a/tests/baselines/reference/requiredInitializedParameter3.js b/tests/baselines/reference/requiredInitializedParameter3.js
new file mode 100644
index 00000000000..68e1aa696ea
--- /dev/null
+++ b/tests/baselines/reference/requiredInitializedParameter3.js
@@ -0,0 +1,27 @@
+//// [requiredInitializedParameter3.ts]
+interface I1 {
+    method();
+}
+
+class C1 implements I1 {
+    method(a = 0, b?) { }
+}
+
+//// [requiredInitializedParameter3.js]
+var C1 = (function () {
+    function C1() {
+    }
+    C1.prototype.method = function (a, b) {
+        if (a === void 0) { a = 0; }
+    };
+    return C1;
+})();
+
+
+//// [requiredInitializedParameter3.d.ts]
+interface I1 {
+    method(): any;
+}
+declare class C1 implements I1 {
+    method(a?: number, b?: any): void;
+}
diff --git a/tests/baselines/reference/requiredInitializedParameter3.symbols b/tests/baselines/reference/requiredInitializedParameter3.symbols
new file mode 100644
index 00000000000..e9f35b21bcf
--- /dev/null
+++ b/tests/baselines/reference/requiredInitializedParameter3.symbols
@@ -0,0 +1,17 @@
+=== tests/cases/compiler/requiredInitializedParameter3.ts ===
+interface I1 {
+>I1 : Symbol(I1, Decl(requiredInitializedParameter3.ts, 0, 0))
+
+    method();
+>method : Symbol(method, Decl(requiredInitializedParameter3.ts, 0, 14))
+}
+
+class C1 implements I1 {
+>C1 : Symbol(C1, Decl(requiredInitializedParameter3.ts, 2, 1))
+>I1 : Symbol(I1, Decl(requiredInitializedParameter3.ts, 0, 0))
+
+    method(a = 0, b?) { }
+>method : Symbol(method, Decl(requiredInitializedParameter3.ts, 4, 24))
+>a : Symbol(a, Decl(requiredInitializedParameter3.ts, 5, 11))
+>b : Symbol(b, Decl(requiredInitializedParameter3.ts, 5, 17))
+}
diff --git a/tests/baselines/reference/requiredInitializedParameter3.types b/tests/baselines/reference/requiredInitializedParameter3.types
new file mode 100644
index 00000000000..aa37f5e13dd
--- /dev/null
+++ b/tests/baselines/reference/requiredInitializedParameter3.types
@@ -0,0 +1,18 @@
+=== tests/cases/compiler/requiredInitializedParameter3.ts ===
+interface I1 {
+>I1 : I1
+
+    method();
+>method : () => any
+}
+
+class C1 implements I1 {
+>C1 : C1
+>I1 : I1
+
+    method(a = 0, b?) { }
+>method : (a?: number, b?: any) => void
+>a : number
+>0 : number
+>b : any
+}
diff --git a/tests/baselines/reference/requiredInitializedParameter4.js b/tests/baselines/reference/requiredInitializedParameter4.js
new file mode 100644
index 00000000000..3b23f119391
--- /dev/null
+++ b/tests/baselines/reference/requiredInitializedParameter4.js
@@ -0,0 +1,20 @@
+//// [requiredInitializedParameter4.ts]
+class C1 {
+    method(a = 0, b) { }
+}
+
+//// [requiredInitializedParameter4.js]
+var C1 = (function () {
+    function C1() {
+    }
+    C1.prototype.method = function (a, b) {
+        if (a === void 0) { a = 0; }
+    };
+    return C1;
+})();
+
+
+//// [requiredInitializedParameter4.d.ts]
+declare class C1 {
+    method(a: number, b: any): void;
+}
diff --git a/tests/baselines/reference/requiredInitializedParameter4.symbols b/tests/baselines/reference/requiredInitializedParameter4.symbols
new file mode 100644
index 00000000000..2790b33698c
--- /dev/null
+++ b/tests/baselines/reference/requiredInitializedParameter4.symbols
@@ -0,0 +1,9 @@
+=== tests/cases/compiler/requiredInitializedParameter4.ts ===
+class C1 {
+>C1 : Symbol(C1, Decl(requiredInitializedParameter4.ts, 0, 0))
+
+    method(a = 0, b) { }
+>method : Symbol(method, Decl(requiredInitializedParameter4.ts, 0, 10))
+>a : Symbol(a, Decl(requiredInitializedParameter4.ts, 1, 11))
+>b : Symbol(b, Decl(requiredInitializedParameter4.ts, 1, 17))
+}
diff --git a/tests/baselines/reference/requiredInitializedParameter4.types b/tests/baselines/reference/requiredInitializedParameter4.types
new file mode 100644
index 00000000000..c483146a1ba
--- /dev/null
+++ b/tests/baselines/reference/requiredInitializedParameter4.types
@@ -0,0 +1,10 @@
+=== tests/cases/compiler/requiredInitializedParameter4.ts ===
+class C1 {
+>C1 : C1
+
+    method(a = 0, b) { }
+>method : (a: number, b: any) => void
+>a : number
+>0 : number
+>b : any
+}
diff --git a/tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors1.ts b/tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors1.ts
index d8ddcf4bb8f..814f1b41619 100644
--- a/tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors1.ts
+++ b/tests/cases/compiler/fatarrowfunctionsOptionalArgsErrors1.ts
@@ -3,5 +3,5 @@
 (...arg) => 103;
 (...arg:number [] = []) => 104;
 
-// Non optional parameter following an optional one
+// Uninitialized parameter makes the initialized one required
 (arg1 = 1, arg2) => 1; 
\ No newline at end of file
diff --git a/tests/cases/compiler/optionalParamArgsTest.ts b/tests/cases/compiler/optionalParamArgsTest.ts
index ce34acae869..609cbfe4734 100644
--- a/tests/cases/compiler/optionalParamArgsTest.ts
+++ b/tests/cases/compiler/optionalParamArgsTest.ts
@@ -30,8 +30,7 @@ class C1 {
 
     public C1M5(C1M5A1:number,C1M5A2:number=0,C1M5A3?:number) { return C1M5A1 + C1M5A2; }
 
-    // Negative test
-    // "Optional parameters may only be followed by other optional parameters"
+    // Uninitialized parameter makes the initialized one required
     public C1M5(C1M5A1:number,C1M5A2:number=0,C1M5A3:number) { return C1M5A1 + C1M5A2; }
 }
 
diff --git a/tests/cases/compiler/requiredInitializedParameter1.ts b/tests/cases/compiler/requiredInitializedParameter1.ts
new file mode 100644
index 00000000000..35d606426f0
--- /dev/null
+++ b/tests/cases/compiler/requiredInitializedParameter1.ts
@@ -0,0 +1,19 @@
+function f1(a, b = 0, c) { }
+function f2(a, b = 0, c = 0) { }
+function f3(a, b = 0, c?) { }
+function f4(a, b = 0, ...c) { }
+
+f1(0, 1, 2);
+f2(0, 1, 2);
+f3(0, 1, 2);
+f4(0, 1, 2);
+
+f1(0, 1);
+f2(0, 1);
+f3(0, 1);
+f4(0, 1);
+
+f1(0);
+f2(0);
+f3(0);
+f4(0);
\ No newline at end of file
diff --git a/tests/cases/compiler/requiredInitializedParameter2.ts b/tests/cases/compiler/requiredInitializedParameter2.ts
new file mode 100644
index 00000000000..48d42869ee7
--- /dev/null
+++ b/tests/cases/compiler/requiredInitializedParameter2.ts
@@ -0,0 +1,7 @@
+interface I1 {
+    method();
+}
+
+class C1 implements I1 {
+    method(a = 0, b) { }
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/requiredInitializedParameter3.ts b/tests/cases/compiler/requiredInitializedParameter3.ts
new file mode 100644
index 00000000000..803eefb6342
--- /dev/null
+++ b/tests/cases/compiler/requiredInitializedParameter3.ts
@@ -0,0 +1,8 @@
+//@declaration: true
+interface I1 {
+    method();
+}
+
+class C1 implements I1 {
+    method(a = 0, b?) { }
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/requiredInitializedParameter4.ts b/tests/cases/compiler/requiredInitializedParameter4.ts
new file mode 100644
index 00000000000..4ba15878e43
--- /dev/null
+++ b/tests/cases/compiler/requiredInitializedParameter4.ts
@@ -0,0 +1,4 @@
+//@declaration: true
+class C1 {
+    method(a = 0, b) { }
+}
\ No newline at end of file