Add undefined to default-initialised parameters

src/compiler/checker.ts

@@ -3206,7 +3206,8 @@ namespace ts {
             // Use the type of the initializer expression if one is present
             if (declaration.initializer) {
                 const type = checkDeclarationInitializer(declaration);
-                return addOptionality(type, /*optional*/ declaration.questionToken && includeOptionality);
+                const isOptional = declaration.questionToken || (declaration.initializer && declaration.kind === SyntaxKind.Parameter);
+                return addOptionality(type, isOptional && includeOptionality);
             }
 
             // If it is a short-hand property assignment, use the type of the identifier

tests/baselines/reference/defaultParameterAddsUndefinedWithStrictNullChecks.js

@@ -0,0 +1,9 @@
+//// [defaultParameterAddsUndefinedWithStrictNullChecks.ts]
+function f(addUndefined1 = "J", addUndefined2?: number) {
+}
+
+
+//// [defaultParameterAddsUndefinedWithStrictNullChecks.js]
+function f(addUndefined1, addUndefined2) {
+    if (addUndefined1 === void 0) { addUndefined1 = "J"; }
+}

tests/baselines/reference/defaultParameterAddsUndefinedWithStrictNullChecks.symbols

@@ -0,0 +1,7 @@
+=== 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))
+}
+

tests/baselines/reference/defaultParameterAddsUndefinedWithStrictNullChecks.types

@@ -0,0 +1,8 @@
+=== tests/cases/compiler/defaultParameterAddsUndefinedWithStrictNullChecks.ts ===
+function f(addUndefined1 = "J", addUndefined2?: number) {
+>f : (addUndefined1?: string | undefined, addUndefined2?: number | undefined) => void
+>addUndefined1 : string | undefined
+>"J" : "J"
+>addUndefined2 : number | undefined
+}
+

tests/baselines/reference/optionalMethods.js

@@ -128,11 +128,11 @@ interface Foo {
 declare function test1(x: Foo): void;
 declare class Bar {
     d: number;
-    e: number;
+    e: number | undefined;
     a: number;
     b?: number;
     c?: number | undefined;
-    constructor(d?: number, e?: number);
+    constructor(d?: number, e?: number | undefined);
     f(): number;
     g?(): number;
     h?(): number;

tests/baselines/reference/optionalMethods.types

@@ -87,7 +87,7 @@ class Bar {
 
     constructor(public d?: number, public e = 10) {}
 >d : number | undefined
->e : number
+>e : number | undefined
 >10 : 10
 
     f() {
@@ -133,9 +133,9 @@ function test2(x: Bar) {
 >d : number | undefined
 
     x.e;
->x.e : number
+>x.e : number | undefined
 >x : Bar
->e : number
+>e : number | undefined
 
     x.f;
 >x.f : () => number

tests/cases/compiler/defaultParameterAddsUndefinedWithStrictNullChecks.ts

@@ -0,0 +1,3 @@
+// @strictNullChecks: true
+function f(addUndefined1 = "J", addUndefined2?: number) {
+}