Cherry-pick PR #42846 into release-4.2 (#42852)

Component commits:
0edae12 Reduce void | undefined only in conjunction with subtype reduction

6b487a6 Accept new baselines

e7b6601 Add regression test

Co-authored-by: Anders Hejlsberg <andersh@microsoft.com>

Author: TypeScript Bot

src/compiler/checker.ts

@@ -13358,7 +13358,7 @@ namespace ts {
             return true;
         }
 
-        function removeRedundantLiteralTypes(types: Type[], includes: TypeFlags) {
+        function removeRedundantLiteralTypes(types: Type[], includes: TypeFlags, reduceVoidUndefined: boolean) {
             let i = types.length;
             while (i > 0) {
                 i--;
@@ -13369,7 +13369,7 @@ namespace ts {
                     flags & TypeFlags.NumberLiteral && includes & TypeFlags.Number ||
                     flags & TypeFlags.BigIntLiteral && includes & TypeFlags.BigInt ||
                     flags & TypeFlags.UniqueESSymbol && includes & TypeFlags.ESSymbol ||
-                    flags & TypeFlags.Undefined && includes & TypeFlags.Void ||
+                    reduceVoidUndefined && flags & TypeFlags.Undefined && includes & TypeFlags.Void ||
                     isFreshLiteralType(t) && containsType(types, (<LiteralType>t).regularType);
                 if (remove) {
                     orderedRemoveItemAt(types, i);
@@ -13437,7 +13437,7 @@ namespace ts {
                 }
                 if (unionReduction & (UnionReduction.Literal | UnionReduction.Subtype)) {
                     if (includes & (TypeFlags.Literal | TypeFlags.UniqueESSymbol) || includes & TypeFlags.Void && includes & TypeFlags.Undefined) {
-                        removeRedundantLiteralTypes(typeSet, includes);
+                        removeRedundantLiteralTypes(typeSet, includes, !!(unionReduction & UnionReduction.Subtype));
                     }
                     if (includes & TypeFlags.StringLiteral && includes & TypeFlags.TemplateLiteral) {
                         removeStringLiteralsMatchedByTemplateLiterals(typeSet);

tests/baselines/reference/callChain.types

@@ -260,7 +260,7 @@ declare const o5: <T>() => undefined | (() => void);
 >o5 : <T>() => undefined | (() => void)
 
 o5<number>()?.();
->o5<number>()?.() : void
+>o5<number>()?.() : void | undefined
 >o5<number>() : (() => void) | undefined
 >o5 : <T>() => (() => void) | undefined
 

tests/baselines/reference/callChainInference.types

@@ -29,7 +29,7 @@ if (value) {
 }
 
 value?.foo("a");
->value?.foo("a") : void
+>value?.foo("a") : void | undefined
 >value?.foo : (<T>(this: T, arg: keyof T) => void) | undefined
 >value : Y | undefined
 >foo : (<T>(this: T, arg: keyof T) => void) | undefined

tests/baselines/reference/controlFlowOptionalChain.types

@@ -595,7 +595,7 @@ function f01(x: unknown) {
 >true : true
 
         maybeIsString?.(x);
->maybeIsString?.(x) : void
+>maybeIsString?.(x) : void | undefined
 >maybeIsString : ((value: unknown) => asserts value is string) | undefined
 >x : unknown
 

tests/baselines/reference/controlFlowSuperPropertyAccess.types

@@ -13,7 +13,7 @@ class C extends B {
 >body : () => void
 
         super.m && super.m();
->super.m && super.m() : void
+>super.m && super.m() : void | undefined
 >super.m : (() => void) | undefined
 >super : B
 >m : (() => void) | undefined

tests/baselines/reference/discriminantPropertyCheck.types

@@ -343,7 +343,7 @@ const u: U = {} as any;
 >{} : {}
 
 u.a && u.b && f(u.a, u.b);
->u.a && u.b && f(u.a, u.b) : void | ""
+>u.a && u.b && f(u.a, u.b) : void | "" | undefined
 >u.a && u.b : string | undefined
 >u.a : string | undefined
 >u : U
@@ -361,7 +361,7 @@ u.a && u.b && f(u.a, u.b);
 >b : string
 
 u.b && u.a && f(u.a, u.b);
->u.b && u.a && f(u.a, u.b) : void | ""
+>u.b && u.a && f(u.a, u.b) : void | "" | undefined
 >u.b && u.a : string | undefined
 >u.b : string | undefined
 >u : U

tests/baselines/reference/promiseTypeStrictNull.types

@@ -888,8 +888,8 @@ const p75 = p.then(() => undefined, () => null);
 >null : null
 
 const p76 = p.then(() => undefined, () => {});
->p76 : Promise<void>
->p.then(() => undefined, () => {}) : Promise<void>
+>p76 : Promise<void | undefined>
+>p.then(() => undefined, () => {}) : Promise<void | undefined>
 >p.then : <TResult1 = boolean, TResult2 = never>(onfulfilled?: ((value: boolean) => TResult1 | PromiseLike<TResult1>) | null | undefined, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | null | undefined) => Promise<TResult1 | TResult2>
 >p : Promise<boolean>
 >then : <TResult1 = boolean, TResult2 = never>(onfulfilled?: ((value: boolean) => TResult1 | PromiseLike<TResult1>) | null | undefined, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | null | undefined) => Promise<TResult1 | TResult2>
@@ -1092,8 +1092,8 @@ const p93 = p.then(() => {}, () => x);
 >x : any
 
 const p94 = p.then(() => {}, () => undefined);
->p94 : Promise<void>
->p.then(() => {}, () => undefined) : Promise<void>
+>p94 : Promise<void | undefined>
+>p.then(() => {}, () => undefined) : Promise<void | undefined>
 >p.then : <TResult1 = boolean, TResult2 = never>(onfulfilled?: ((value: boolean) => TResult1 | PromiseLike<TResult1>) | null | undefined, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | null | undefined) => Promise<TResult1 | TResult2>
 >p : Promise<boolean>
 >then : <TResult1 = boolean, TResult2 = never>(onfulfilled?: ((value: boolean) => TResult1 | PromiseLike<TResult1>) | null | undefined, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | null | undefined) => Promise<TResult1 | TResult2>

tests/baselines/reference/superMethodCall.types

@@ -11,20 +11,20 @@ class Derived extends Base {
 >Base : Base
 
     method() {
->method : () => void
+>method : () => void | undefined
 
         return super.method?.();
->super.method?.() : void
+>super.method?.() : void | undefined
 >super.method : (() => void) | undefined
 >super : Base
 >method : (() => void) | undefined
     }
 
     async asyncMethod() {
->asyncMethod : () => Promise<void>
+>asyncMethod : () => Promise<void | undefined>
 
         return super.method?.();
->super.method?.() : void
+>super.method?.() : void | undefined
 >super.method : (() => void) | undefined
 >super : Base
 >method : (() => void) | undefined

tests/baselines/reference/thisMethodCall.types

@@ -9,7 +9,7 @@ class C {
 >other : () => void
 
         this.method?.();
->this.method?.() : void
+>this.method?.() : void | undefined
 >this.method : (() => void) | undefined
 >this : this
 >method : (() => void) | undefined

tests/baselines/reference/truthinessCallExpressionCoercion2.types

@@ -60,7 +60,7 @@ function test(required1: () => boolean, required2: () => boolean, b: boolean, op
 
     // ok
     optional && console.log('optional');
->optional && console.log('optional') : void
+>optional && console.log('optional') : void | undefined
 >optional : (() => boolean) | undefined
 >console.log('optional') : void
 >console.log : (...data: any[]) => void
@@ -70,7 +70,7 @@ function test(required1: () => boolean, required2: () => boolean, b: boolean, op
 
     // ok
     1 && optional && console.log('optional');
->1 && optional && console.log('optional') : void
+>1 && optional && console.log('optional') : void | undefined
 >1 && optional : (() => boolean) | undefined
 >1 : 1
 >optional : (() => boolean) | undefined
@@ -441,7 +441,7 @@ class Foo {
 
         // ok
         1 && this.optional && console.log('optional');
->1 && this.optional && console.log('optional') : void
+>1 && this.optional && console.log('optional') : void | undefined
 >1 && this.optional : (() => boolean) | undefined
 >1 : 1
 >this.optional : (() => boolean) | undefined

tests/baselines/reference/typeVariableTypeGuards.types

@@ -16,7 +16,7 @@ class A<P extends Partial<Foo>> {
 >doSomething : () => void
 
         this.props.foo && this.props.foo()
->this.props.foo && this.props.foo() : void
+>this.props.foo && this.props.foo() : void | undefined
 >this.props.foo : P["foo"] | undefined
 >this.props : Readonly<P>
 >this : this

tests/baselines/reference/voidReturnIndexUnionInference.types

@@ -51,7 +51,7 @@ function bad<P extends Props>(props: Readonly<P>) {
     // ERROR HERE!!!
     // Type R in signature of safeInvoke incorrectly inferred as {} instead of void!
     safeInvoke(props.onBar, "blah");
->safeInvoke(props.onBar, "blah") : void
+>safeInvoke(props.onBar, "blah") : void | undefined
 >safeInvoke : <A1, R>(func: ((arg1: A1) => R) | null | undefined, arg1: A1) => R | undefined
 >props.onBar : P["onBar"] | undefined
 >props : Readonly<P>

tests/baselines/reference/voidUndefinedReduction.js

@@ -0,0 +1,23 @@
+//// [voidUndefinedReduction.ts]
+// Repro from #42786
+
+function isDefined<T>(value: T | undefined | null | void): value is T {
+  return value !== undefined && value !== null;
+}
+
+declare const foo: string | undefined;
+
+if (isDefined(foo)) {
+  console.log(foo.toUpperCase()); 
+}
+
+
+//// [voidUndefinedReduction.js]
+"use strict";
+// Repro from #42786
+function isDefined(value) {
+    return value !== undefined && value !== null;
+}
+if (isDefined(foo)) {
+    console.log(foo.toUpperCase());
+}

tests/baselines/reference/voidUndefinedReduction.symbols

@@ -0,0 +1,33 @@
+=== tests/cases/compiler/voidUndefinedReduction.ts ===
+// Repro from #42786
+
+function isDefined<T>(value: T | undefined | null | void): value is T {
+>isDefined : Symbol(isDefined, Decl(voidUndefinedReduction.ts, 0, 0))
+>T : Symbol(T, Decl(voidUndefinedReduction.ts, 2, 19))
+>value : Symbol(value, Decl(voidUndefinedReduction.ts, 2, 22))
+>T : Symbol(T, Decl(voidUndefinedReduction.ts, 2, 19))
+>value : Symbol(value, Decl(voidUndefinedReduction.ts, 2, 22))
+>T : Symbol(T, Decl(voidUndefinedReduction.ts, 2, 19))
+
+  return value !== undefined && value !== null;
+>value : Symbol(value, Decl(voidUndefinedReduction.ts, 2, 22))
+>undefined : Symbol(undefined)
+>value : Symbol(value, Decl(voidUndefinedReduction.ts, 2, 22))
+}
+
+declare const foo: string | undefined;
+>foo : Symbol(foo, Decl(voidUndefinedReduction.ts, 6, 13))
+
+if (isDefined(foo)) {
+>isDefined : Symbol(isDefined, Decl(voidUndefinedReduction.ts, 0, 0))
+>foo : Symbol(foo, Decl(voidUndefinedReduction.ts, 6, 13))
+
+  console.log(foo.toUpperCase()); 
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>foo.toUpperCase : Symbol(String.toUpperCase, Decl(lib.es5.d.ts, --, --))
+>foo : Symbol(foo, Decl(voidUndefinedReduction.ts, 6, 13))
+>toUpperCase : Symbol(String.toUpperCase, Decl(lib.es5.d.ts, --, --))
+}
+

tests/baselines/reference/voidUndefinedReduction.types

@@ -0,0 +1,37 @@
+=== tests/cases/compiler/voidUndefinedReduction.ts ===
+// Repro from #42786
+
+function isDefined<T>(value: T | undefined | null | void): value is T {
+>isDefined : <T>(value: T | undefined | null | void) => value is T
+>value : void | T | null | undefined
+>null : null
+
+  return value !== undefined && value !== null;
+>value !== undefined && value !== null : boolean
+>value !== undefined : boolean
+>value : void | T | null | undefined
+>undefined : undefined
+>value !== null : boolean
+>value : T | null
+>null : null
+}
+
+declare const foo: string | undefined;
+>foo : string | undefined
+
+if (isDefined(foo)) {
+>isDefined(foo) : boolean
+>isDefined : <T>(value: void | T | null | undefined) => value is T
+>foo : string | undefined
+
+  console.log(foo.toUpperCase()); 
+>console.log(foo.toUpperCase()) : void
+>console.log : (...data: any[]) => void
+>console : Console
+>log : (...data: any[]) => void
+>foo.toUpperCase() : string
+>foo.toUpperCase : () => string
+>foo : string
+>toUpperCase : () => string
+}
+

tests/cases/compiler/voidUndefinedReduction.ts

@@ -0,0 +1,13 @@
+// @strict: true
+
+// Repro from #42786
+
+function isDefined<T>(value: T | undefined | null | void): value is T {
+  return value !== undefined && value !== null;
+}
+
+declare const foo: string | undefined;
+
+if (isDefined(foo)) {
+  console.log(foo.toUpperCase()); 
+}