diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index a09b63d46fd0a..3517f407f870f 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -2694,7 +2694,11 @@ namespace ts {
                 writePunctuation(writer, SyntaxKind.ColonToken);
                 writeSpace(writer);
 
-                buildTypeDisplay(getTypeOfSymbol(p), writer, enclosingDeclaration, flags, symbolStack);
+                let type = getTypeOfSymbol(p);
+                if (isRequiredInitializedParameter(parameterNode)) {
+                    type = includeFalsyTypes(type, TypeFlags.Undefined);
+                }
+                buildTypeDisplay(type, writer, enclosingDeclaration, flags, symbolStack);
             }
 
             function buildBindingPatternDisplay(bindingPattern: BindingPattern, writer: SymbolWriter, enclosingDeclaration?: Node, flags?: TypeFormatFlags, symbolStack?: Symbol[]) {
@@ -3275,6 +3279,16 @@ namespace ts {
             return strictNullChecks && optional ? includeFalsyTypes(type, TypeFlags.Undefined) : type;
         }
 
+        /** remove undefined from the annotated type of a parameter when there is an initializer (that doesn't include undefined) */
+        function removeOptionalityFromAnnotation(annotatedType: Type, declaration: VariableLikeDeclaration): Type {
+            const annotationIncludesUndefined = strictNullChecks &&
+                declaration.kind === SyntaxKind.Parameter &&
+                declaration.initializer &&
+                getFalsyFlags(annotatedType) & TypeFlags.Undefined &&
+                !(getFalsyFlags(checkExpression(declaration.initializer)) & TypeFlags.Undefined);
+            return annotationIncludesUndefined ? getNonNullableType(annotatedType) : annotatedType;
+        }
+
         // Return the inferred type for a variable, parameter, or property declaration
         function getTypeForVariableLikeDeclaration(declaration: VariableLikeDeclaration, includeOptionality: boolean): Type {
             if (declaration.flags & NodeFlags.JavaScriptFile) {
@@ -3308,7 +3322,8 @@ namespace ts {
 
             // Use type from type annotation if one is present
             if (declaration.type) {
-                return addOptionality(getTypeFromTypeNode(declaration.type), /*optional*/ declaration.questionToken && includeOptionality);
+                const declaredType = removeOptionalityFromAnnotation(getTypeFromTypeNode(declaration.type), declaration);
+                return addOptionality(declaredType, /*optional*/ declaration.questionToken && includeOptionality);
             }
 
             if ((compilerOptions.noImplicitAny || declaration.flags & NodeFlags.JavaScriptFile) &&
@@ -5133,6 +5148,12 @@ namespace ts {
                 Debug.assert(parameterIndex >= 0);
                 return parameterIndex >= signature.minArgumentCount;
             }
+            const iife = getImmediatelyInvokedFunctionExpression(node.parent);
+            if (iife) {
+                return !node.type &&
+                    !node.dotDotDotToken &&
+                    indexOf((node.parent as SignatureDeclaration).parameters, node) >= iife.arguments.length;
+            }
 
             return false;
         }
@@ -20526,6 +20547,13 @@ namespace ts {
             return false;
         }
 
+        function isRequiredInitializedParameter(parameter: ParameterDeclaration) {
+            return strictNullChecks &&
+                !isOptionalParameter(parameter) &&
+                parameter.initializer &&
+                !(getModifierFlags(parameter) & ModifierFlags.ParameterPropertyModifier);
+        }
+
         function getNodeCheckFlags(node: Node): NodeCheckFlags {
             node = getParseTreeNode(node);
             return node ? getNodeLinks(node).flags : undefined;
@@ -20617,10 +20645,12 @@ namespace ts {
         function writeTypeOfDeclaration(declaration: AccessorDeclaration | VariableLikeDeclaration, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter) {
             // Get type of the symbol if this is the valid symbol otherwise get type at location
             const symbol = getSymbolOfNode(declaration);
-            const type = symbol && !(symbol.flags & (SymbolFlags.TypeLiteral | SymbolFlags.Signature))
+            let type = symbol && !(symbol.flags & (SymbolFlags.TypeLiteral | SymbolFlags.Signature))
                 ? getWidenedLiteralType(getTypeOfSymbol(symbol))
                 : unknownType;
-
+            if (flags & TypeFormatFlags.AddUndefined) {
+                type = includeFalsyTypes(type, TypeFlags.Undefined);
+            }
             getSymbolDisplayBuilder().buildTypeDisplay(type, writer, enclosingDeclaration, flags);
         }
 
@@ -20719,6 +20749,7 @@ namespace ts {
                 isTopLevelValueImportEqualsWithEntityName,
                 isDeclarationVisible,
                 isImplementationOfOverload,
+                isRequiredInitializedParameter,
                 writeTypeOfDeclaration,
                 writeReturnTypeOfSignatureDeclaration,
                 writeTypeOfExpression,
diff --git a/src/compiler/declarationEmitter.ts b/src/compiler/declarationEmitter.ts
index 46df4fed6896d..de8be5a27e80b 100644
--- a/src/compiler/declarationEmitter.ts
+++ b/src/compiler/declarationEmitter.ts
@@ -322,13 +322,20 @@ namespace ts {
         function writeTypeOfDeclaration(declaration: AccessorDeclaration | VariableLikeDeclaration, type: TypeNode, getSymbolAccessibilityDiagnostic: GetSymbolAccessibilityDiagnostic) {
             writer.getSymbolAccessibilityDiagnostic = getSymbolAccessibilityDiagnostic;
             write(": ");
-            if (type) {
+
+            // use the checker's type, not the declared type,
+            // for non-optional initialized parameters that aren't a parameter property
+            const shouldUseResolverType = declaration.kind === SyntaxKind.Parameter &&
+                resolver.isRequiredInitializedParameter(declaration as ParameterDeclaration);
+            if (type && !shouldUseResolverType) {
                 // Write the type
                 emitType(type);
             }
             else {
                 errorNameNode = declaration.name;
-                resolver.writeTypeOfDeclaration(declaration, enclosingDeclaration, TypeFormatFlags.UseTypeOfFunction | TypeFormatFlags.UseTypeAliasValue, writer);
+                const format = TypeFormatFlags.UseTypeOfFunction | TypeFormatFlags.UseTypeAliasValue |
+                    (shouldUseResolverType ? TypeFormatFlags.AddUndefined : 0);
+                resolver.writeTypeOfDeclaration(declaration, enclosingDeclaration, format, writer);
                 errorNameNode = undefined;
             }
         }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 8b49af402cf6c..48feb6bb0922f 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2466,7 +2466,8 @@
         InFirstTypeArgument             = 0x00000100,  // Writing first type argument of the instantiated type
         InTypeAlias                     = 0x00000200,  // Writing type in type alias declaration
         UseTypeAliasValue               = 0x00000400,  // Serialize the type instead of using type-alias. This is needed when we emit declaration file.
-        SuppressAnyReturnType            = 0x00000800,  // If the return type is any-like, don't offer a return type.
+        SuppressAnyReturnType           = 0x00000800,  // If the return type is any-like, don't offer a return type.
+        AddUndefined                    = 0x00001000,  // Add undefined to types of initialized, non-optional parameters
     }
 
     export const enum SymbolFormatFlags {
@@ -2571,6 +2572,7 @@
         isDeclarationVisible(node: Declaration): boolean;
         collectLinkedAliases(node: Identifier): Node[];
         isImplementationOfOverload(node: FunctionLikeDeclaration): boolean;
+        isRequiredInitializedParameter(node: ParameterDeclaration): boolean;
         writeTypeOfDeclaration(declaration: AccessorDeclaration | VariableLikeDeclaration, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter): void;
         writeReturnTypeOfSignatureDeclaration(signatureDeclaration: SignatureDeclaration, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter): void;
         writeTypeOfExpression(expr: Expression, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter): void;
diff --git a/tests/baselines/reference/contextuallyTypedIife.types b/tests/baselines/reference/contextuallyTypedIife.types
index 15038e7dd263a..676755fb34175 100644
--- a/tests/baselines/reference/contextuallyTypedIife.types
+++ b/tests/baselines/reference/contextuallyTypedIife.types
@@ -253,8 +253,8 @@ let eleven = (o => o.a(11))({ a: function(n) { return n; } });
 // missing arguments
 (function(x, undefined) { return x; })(42);
 >(function(x, undefined) { return x; })(42) : number
->(function(x, undefined) { return x; }) : (x: number, undefined: any) => number
->function(x, undefined) { return x; } : (x: number, undefined: any) => number
+>(function(x, undefined) { return x; }) : (x: number, undefined?: any) => number
+>function(x, undefined) { return x; } : (x: number, undefined?: any) => number
 >x : number
 >undefined : any
 >x : number
@@ -262,8 +262,8 @@ let eleven = (o => o.a(11))({ a: function(n) { return n; } });
 
 ((x, y, z) => 42)();
 >((x, y, z) => 42)() : number
->((x, y, z) => 42) : (x: any, y: any, z: any) => number
->(x, y, z) => 42 : (x: any, y: any, z: any) => number
+>((x, y, z) => 42) : (x?: any, y?: any, z?: any) => number
+>(x, y, z) => 42 : (x?: any, y?: any, z?: any) => number
 >x : any
 >y : any
 >z : any
diff --git a/tests/baselines/reference/defaultParameterAddsUndefinedWithStrictNullChecks.js b/tests/baselines/reference/defaultParameterAddsUndefinedWithStrictNullChecks.js
new file mode 100644
index 0000000000000..329c2fb19ce01
--- /dev/null
+++ b/tests/baselines/reference/defaultParameterAddsUndefinedWithStrictNullChecks.js
@@ -0,0 +1,112 @@
+//// [defaultParameterAddsUndefinedWithStrictNullChecks.ts]
+function f(addUndefined1 = "J", addUndefined2?: number) {
+    return addUndefined1.length + (addUndefined2 || 0);
+}
+function g(addUndefined = "J", addDefined: number) {
+    return addUndefined.length + addDefined;
+}
+let total = f() + f('a', 1) + f('b') + f(undefined, 2);
+total = g('c', 3) + g(undefined, 4);
+
+function foo1(x: string = "string", b: number) {
+    x.length;
+}
+
+function foo2(x = "string", b: number) {
+    x.length; // ok, should be string
+}
+
+function foo3(x: string | undefined = "string", b: number) {
+    x.length; // ok, should be string
+}
+
+function foo4(x: string | undefined = undefined, b: number) {
+    x; // should be string | undefined
+}
+
+
+
+// .d.ts should have `string | undefined` for foo1, foo2, foo3 and foo4
+foo1(undefined, 1);
+foo2(undefined, 1);
+foo3(undefined, 1);
+foo4(undefined, 1);
+
+
+function removeUndefinedButNotFalse(x = true) {
+    if (x === false) {
+        return x;
+    }
+}
+
+declare const cond: boolean;
+function removeNothing(y = cond ? true : undefined) {
+    if (y !== undefined) {
+        if (y === false) {
+            return y;
+        }
+    }
+    return true;
+}
+
+
+//// [defaultParameterAddsUndefinedWithStrictNullChecks.js]
+function f(addUndefined1, addUndefined2) {
+    if (addUndefined1 === void 0) { addUndefined1 = "J"; }
+    return addUndefined1.length + (addUndefined2 || 0);
+}
+function g(addUndefined, addDefined) {
+    if (addUndefined === void 0) { addUndefined = "J"; }
+    return addUndefined.length + addDefined;
+}
+var total = f() + f('a', 1) + f('b') + f(undefined, 2);
+total = g('c', 3) + g(undefined, 4);
+function foo1(x, b) {
+    if (x === void 0) { x = "string"; }
+    x.length;
+}
+function foo2(x, b) {
+    if (x === void 0) { x = "string"; }
+    x.length; // ok, should be string
+}
+function foo3(x, b) {
+    if (x === void 0) { x = "string"; }
+    x.length; // ok, should be string
+}
+function foo4(x, b) {
+    if (x === void 0) { x = undefined; }
+    x; // should be string | undefined
+}
+// .d.ts should have `string | undefined` for foo1, foo2, foo3 and foo4
+foo1(undefined, 1);
+foo2(undefined, 1);
+foo3(undefined, 1);
+foo4(undefined, 1);
+function removeUndefinedButNotFalse(x) {
+    if (x === void 0) { x = true; }
+    if (x === false) {
+        return x;
+    }
+}
+function removeNothing(y) {
+    if (y === void 0) { y = cond ? true : undefined; }
+    if (y !== undefined) {
+        if (y === false) {
+            return y;
+        }
+    }
+    return true;
+}
+
+
+//// [defaultParameterAddsUndefinedWithStrictNullChecks.d.ts]
+declare function f(addUndefined1?: string, addUndefined2?: number): number;
+declare function g(addUndefined: string | undefined, addDefined: number): number;
+declare let total: number;
+declare function foo1(x: string | undefined, b: number): void;
+declare function foo2(x: string | undefined, b: number): void;
+declare function foo3(x: string | undefined, b: number): void;
+declare function foo4(x: string | undefined, b: number): void;
+declare function removeUndefinedButNotFalse(x?: boolean): false | undefined;
+declare const cond: boolean;
+declare function removeNothing(y?: boolean | undefined): boolean;
diff --git a/tests/baselines/reference/defaultParameterAddsUndefinedWithStrictNullChecks.symbols b/tests/baselines/reference/defaultParameterAddsUndefinedWithStrictNullChecks.symbols
new file mode 100644
index 0000000000000..fb0fce7dc7f1a
--- /dev/null
+++ b/tests/baselines/reference/defaultParameterAddsUndefinedWithStrictNullChecks.symbols
@@ -0,0 +1,135 @@
+=== tests/cases/compiler/defaultParameterAddsUndefinedWithStrictNullChecks.ts ===
+function f(addUndefined1 = "J", addUndefined2?: number) {
+>f : Symbol(f, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 0, 0))
+>addUndefined1 : Symbol(addUndefined1, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 0, 11))
+>addUndefined2 : Symbol(addUndefined2, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 0, 31))
+
+    return addUndefined1.length + (addUndefined2 || 0);
+>addUndefined1.length : Symbol(String.length, Decl(lib.d.ts, --, --))
+>addUndefined1 : Symbol(addUndefined1, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 0, 11))
+>length : Symbol(String.length, Decl(lib.d.ts, --, --))
+>addUndefined2 : Symbol(addUndefined2, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 0, 31))
+}
+function g(addUndefined = "J", addDefined: number) {
+>g : Symbol(g, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 2, 1))
+>addUndefined : Symbol(addUndefined, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 3, 11))
+>addDefined : Symbol(addDefined, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 3, 30))
+
+    return addUndefined.length + addDefined;
+>addUndefined.length : Symbol(String.length, Decl(lib.d.ts, --, --))
+>addUndefined : Symbol(addUndefined, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 3, 11))
+>length : Symbol(String.length, Decl(lib.d.ts, --, --))
+>addDefined : Symbol(addDefined, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 3, 30))
+}
+let total = f() + f('a', 1) + f('b') + f(undefined, 2);
+>total : Symbol(total, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 6, 3))
+>f : Symbol(f, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 0, 0))
+>f : Symbol(f, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 0, 0))
+>f : Symbol(f, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 0, 0))
+>f : Symbol(f, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 0, 0))
+>undefined : Symbol(undefined)
+
+total = g('c', 3) + g(undefined, 4);
+>total : Symbol(total, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 6, 3))
+>g : Symbol(g, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 2, 1))
+>g : Symbol(g, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 2, 1))
+>undefined : Symbol(undefined)
+
+function foo1(x: string = "string", b: number) {
+>foo1 : Symbol(foo1, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 7, 36))
+>x : Symbol(x, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 9, 14))
+>b : Symbol(b, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 9, 35))
+
+    x.length;
+>x.length : Symbol(String.length, Decl(lib.d.ts, --, --))
+>x : Symbol(x, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 9, 14))
+>length : Symbol(String.length, Decl(lib.d.ts, --, --))
+}
+
+function foo2(x = "string", b: number) {
+>foo2 : Symbol(foo2, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 11, 1))
+>x : Symbol(x, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 13, 14))
+>b : Symbol(b, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 13, 27))
+
+    x.length; // ok, should be string
+>x.length : Symbol(String.length, Decl(lib.d.ts, --, --))
+>x : Symbol(x, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 13, 14))
+>length : Symbol(String.length, Decl(lib.d.ts, --, --))
+}
+
+function foo3(x: string | undefined = "string", b: number) {
+>foo3 : Symbol(foo3, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 15, 1))
+>x : Symbol(x, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 17, 14))
+>b : Symbol(b, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 17, 47))
+
+    x.length; // ok, should be string
+>x.length : Symbol(String.length, Decl(lib.d.ts, --, --))
+>x : Symbol(x, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 17, 14))
+>length : Symbol(String.length, Decl(lib.d.ts, --, --))
+}
+
+function foo4(x: string | undefined = undefined, b: number) {
+>foo4 : Symbol(foo4, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 19, 1))
+>x : Symbol(x, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 21, 14))
+>undefined : Symbol(undefined)
+>b : Symbol(b, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 21, 48))
+
+    x; // should be string | undefined
+>x : Symbol(x, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 21, 14))
+}
+
+
+
+// .d.ts should have `string | undefined` for foo1, foo2, foo3 and foo4
+foo1(undefined, 1);
+>foo1 : Symbol(foo1, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 7, 36))
+>undefined : Symbol(undefined)
+
+foo2(undefined, 1);
+>foo2 : Symbol(foo2, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 11, 1))
+>undefined : Symbol(undefined)
+
+foo3(undefined, 1);
+>foo3 : Symbol(foo3, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 15, 1))
+>undefined : Symbol(undefined)
+
+foo4(undefined, 1);
+>foo4 : Symbol(foo4, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 19, 1))
+>undefined : Symbol(undefined)
+
+
+function removeUndefinedButNotFalse(x = true) {
+>removeUndefinedButNotFalse : Symbol(removeUndefinedButNotFalse, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 31, 19))
+>x : Symbol(x, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 34, 36))
+
+    if (x === false) {
+>x : Symbol(x, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 34, 36))
+
+        return x;
+>x : Symbol(x, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 34, 36))
+    }
+}
+
+declare const cond: boolean;
+>cond : Symbol(cond, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 40, 13))
+
+function removeNothing(y = cond ? true : undefined) {
+>removeNothing : Symbol(removeNothing, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 40, 28))
+>y : Symbol(y, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 41, 23))
+>cond : Symbol(cond, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 40, 13))
+>undefined : Symbol(undefined)
+
+    if (y !== undefined) {
+>y : Symbol(y, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 41, 23))
+>undefined : Symbol(undefined)
+
+        if (y === false) {
+>y : Symbol(y, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 41, 23))
+
+            return y;
+>y : Symbol(y, Decl(defaultParameterAddsUndefinedWithStrictNullChecks.ts, 41, 23))
+        }
+    }
+    return true;
+}
+
diff --git a/tests/baselines/reference/defaultParameterAddsUndefinedWithStrictNullChecks.types b/tests/baselines/reference/defaultParameterAddsUndefinedWithStrictNullChecks.types
new file mode 100644
index 0000000000000..d95f925989173
--- /dev/null
+++ b/tests/baselines/reference/defaultParameterAddsUndefinedWithStrictNullChecks.types
@@ -0,0 +1,180 @@
+=== tests/cases/compiler/defaultParameterAddsUndefinedWithStrictNullChecks.ts ===
+function f(addUndefined1 = "J", addUndefined2?: number) {
+>f : (addUndefined1?: string, addUndefined2?: number | undefined) => number
+>addUndefined1 : string
+>"J" : "J"
+>addUndefined2 : number | undefined
+
+    return addUndefined1.length + (addUndefined2 || 0);
+>addUndefined1.length + (addUndefined2 || 0) : number
+>addUndefined1.length : number
+>addUndefined1 : string
+>length : number
+>(addUndefined2 || 0) : number
+>addUndefined2 || 0 : number
+>addUndefined2 : number | undefined
+>0 : 0
+}
+function g(addUndefined = "J", addDefined: number) {
+>g : (addUndefined: string | undefined, addDefined: number) => number
+>addUndefined : string
+>"J" : "J"
+>addDefined : number
+
+    return addUndefined.length + addDefined;
+>addUndefined.length + addDefined : number
+>addUndefined.length : number
+>addUndefined : string
+>length : number
+>addDefined : number
+}
+let total = f() + f('a', 1) + f('b') + f(undefined, 2);
+>total : number
+>f() + f('a', 1) + f('b') + f(undefined, 2) : number
+>f() + f('a', 1) + f('b') : number
+>f() + f('a', 1) : number
+>f() : number
+>f : (addUndefined1?: string, addUndefined2?: number | undefined) => number
+>f('a', 1) : number
+>f : (addUndefined1?: string, addUndefined2?: number | undefined) => number
+>'a' : "a"
+>1 : 1
+>f('b') : number
+>f : (addUndefined1?: string, addUndefined2?: number | undefined) => number
+>'b' : "b"
+>f(undefined, 2) : number
+>f : (addUndefined1?: string, addUndefined2?: number | undefined) => number
+>undefined : undefined
+>2 : 2
+
+total = g('c', 3) + g(undefined, 4);
+>total = g('c', 3) + g(undefined, 4) : number
+>total : number
+>g('c', 3) + g(undefined, 4) : number
+>g('c', 3) : number
+>g : (addUndefined: string | undefined, addDefined: number) => number
+>'c' : "c"
+>3 : 3
+>g(undefined, 4) : number
+>g : (addUndefined: string | undefined, addDefined: number) => number
+>undefined : undefined
+>4 : 4
+
+function foo1(x: string = "string", b: number) {
+>foo1 : (x: string | undefined, b: number) => void
+>x : string
+>"string" : "string"
+>b : number
+
+    x.length;
+>x.length : number
+>x : string
+>length : number
+}
+
+function foo2(x = "string", b: number) {
+>foo2 : (x: string | undefined, b: number) => void
+>x : string
+>"string" : "string"
+>b : number
+
+    x.length; // ok, should be string
+>x.length : number
+>x : string
+>length : number
+}
+
+function foo3(x: string | undefined = "string", b: number) {
+>foo3 : (x: string | undefined, b: number) => void
+>x : string
+>"string" : "string"
+>b : number
+
+    x.length; // ok, should be string
+>x.length : number
+>x : string
+>length : number
+}
+
+function foo4(x: string | undefined = undefined, b: number) {
+>foo4 : (x: string | undefined, b: number) => void
+>x : string | undefined
+>undefined : undefined
+>b : number
+
+    x; // should be string | undefined
+>x : string | undefined
+}
+
+
+
+// .d.ts should have `string | undefined` for foo1, foo2, foo3 and foo4
+foo1(undefined, 1);
+>foo1(undefined, 1) : void
+>foo1 : (x: string | undefined, b: number) => void
+>undefined : undefined
+>1 : 1
+
+foo2(undefined, 1);
+>foo2(undefined, 1) : void
+>foo2 : (x: string | undefined, b: number) => void
+>undefined : undefined
+>1 : 1
+
+foo3(undefined, 1);
+>foo3(undefined, 1) : void
+>foo3 : (x: string | undefined, b: number) => void
+>undefined : undefined
+>1 : 1
+
+foo4(undefined, 1);
+>foo4(undefined, 1) : void
+>foo4 : (x: string | undefined, b: number) => void
+>undefined : undefined
+>1 : 1
+
+
+function removeUndefinedButNotFalse(x = true) {
+>removeUndefinedButNotFalse : (x?: boolean) => false | undefined
+>x : boolean
+>true : true
+
+    if (x === false) {
+>x === false : boolean
+>x : boolean
+>false : false
+
+        return x;
+>x : false
+    }
+}
+
+declare const cond: boolean;
+>cond : boolean
+
+function removeNothing(y = cond ? true : undefined) {
+>removeNothing : (y?: boolean | undefined) => boolean
+>y : boolean | undefined
+>cond ? true : undefined : true | undefined
+>cond : boolean
+>true : true
+>undefined : undefined
+
+    if (y !== undefined) {
+>y !== undefined : boolean
+>y : boolean | undefined
+>undefined : undefined
+
+        if (y === false) {
+>y === false : boolean
+>y : boolean
+>false : false
+
+            return y;
+>y : false
+        }
+    }
+    return true;
+>true : true
+}
+
diff --git a/tests/cases/compiler/defaultParameterAddsUndefinedWithStrictNullChecks.ts b/tests/cases/compiler/defaultParameterAddsUndefinedWithStrictNullChecks.ts
new file mode 100644
index 0000000000000..8abea19f6cd35
--- /dev/null
+++ b/tests/cases/compiler/defaultParameterAddsUndefinedWithStrictNullChecks.ts
@@ -0,0 +1,51 @@
+// @strictNullChecks: true
+// @declaration: true
+function f(addUndefined1 = "J", addUndefined2?: number) {
+    return addUndefined1.length + (addUndefined2 || 0);
+}
+function g(addUndefined = "J", addDefined: number) {
+    return addUndefined.length + addDefined;
+}
+let total = f() + f('a', 1) + f('b') + f(undefined, 2);
+total = g('c', 3) + g(undefined, 4);
+
+function foo1(x: string = "string", b: number) {
+    x.length;
+}
+
+function foo2(x = "string", b: number) {
+    x.length; // ok, should be string
+}
+
+function foo3(x: string | undefined = "string", b: number) {
+    x.length; // ok, should be string
+}
+
+function foo4(x: string | undefined = undefined, b: number) {
+    x; // should be string | undefined
+}
+
+
+
+// .d.ts should have `string | undefined` for foo1, foo2, foo3 and foo4
+foo1(undefined, 1);
+foo2(undefined, 1);
+foo3(undefined, 1);
+foo4(undefined, 1);
+
+
+function removeUndefinedButNotFalse(x = true) {
+    if (x === false) {
+        return x;
+    }
+}
+
+declare const cond: boolean;
+function removeNothing(y = cond ? true : undefined) {
+    if (y !== undefined) {
+        if (y === false) {
+            return y;
+        }
+    }
+    return true;
+}
diff --git a/tests/cases/fourslash/quickInfoDisplayPartsIife.ts b/tests/cases/fourslash/quickInfoDisplayPartsIife.ts
new file mode 100644
index 0000000000000..e4ce2bf96e3ec
--- /dev/null
+++ b/tests/cases/fourslash/quickInfoDisplayPartsIife.ts
@@ -0,0 +1,5 @@
+/// <reference path='fourslash.ts'/>
+// @strictNullChecks: true
+////var iife = (function foo/*1*/(x, y) { return x })(12);
+
+verify.quickInfoAt('1', '(local function) foo(x: number, y?: undefined): number');