Better typings for Promise.then(), like #31117

Author: jablko

src/lib/es5.d.ts

@@ -1375,7 +1375,7 @@ interface PromiseLike<T> {
      * @param onrejected The callback to execute when the Promise is rejected.
      * @returns A Promise for the completion of which ever callback is executed.
      */
-    then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): PromiseLike<TResult1 | TResult2>;
+    then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1) | undefined | null, onrejected?: ((reason: any) => TResult2) | undefined | null): PromiseLike<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>;
 }
 
 /**
@@ -1388,14 +1388,14 @@ interface Promise<T> {
      * @param onrejected The callback to execute when the Promise is rejected.
      * @returns A Promise for the completion of which ever callback is executed.
      */
-    then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): Promise<TResult1 | TResult2>;
+    then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1) | undefined | null, onrejected?: ((reason: any) => TResult2) | undefined | null): Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>;
 
     /**
      * Attaches a callback for only the rejection of the Promise.
      * @param onrejected The callback to execute when the Promise is rejected.
      * @returns A Promise for the completion of the callback.
      */
-    catch<TResult = never>(onrejected?: ((reason: any) => TResult | PromiseLike<TResult>) | undefined | null): Promise<T | TResult>;
+    catch<TResult = never>(onrejected?: ((reason: any) => TResult) | undefined | null): Promise<T | (TResult extends PromiseLike<infer UResult> ? UResult : TResult)>;
 }
 
 interface ArrayLike<T> {

tests/baselines/reference/asyncFunctionDeclaration15_es5.errors.txt

@@ -7,7 +7,7 @@ tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration1
 tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(10,23): error TS1055: Type 'typeof Thenable' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
   Construct signature return types 'Thenable' and 'PromiseLike<T>' are incompatible.
     The types returned by 'then(...)' are incompatible between these types.
-      Type 'void' is not assignable to type 'PromiseLike<TResult1 | TResult2>'.
+      Type 'void' is not assignable to type 'PromiseLike<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>'.
 tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(17,16): error TS1058: The return type of an async function must either be a valid promise or must not contain a callable 'then' member.
 tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(23,25): error TS1320: Type of 'await' operand must either be a valid promise or must not contain a callable 'then' member.
 
@@ -39,7 +39,7 @@ tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration1
 !!! error TS1055: Type 'typeof Thenable' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
 !!! error TS1055:   Construct signature return types 'Thenable' and 'PromiseLike<T>' are incompatible.
 !!! error TS1055:     The types returned by 'then(...)' are incompatible between these types.
-!!! error TS1055:       Type 'void' is not assignable to type 'PromiseLike<TResult1 | TResult2>'.
+!!! error TS1055:       Type 'void' is not assignable to type 'PromiseLike<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>'.
     async function fn7() { return; } // valid: Promise<void>
     async function fn8() { return 1; } // valid: Promise<number>
     async function fn9() { return null; } // valid: Promise<any>

tests/baselines/reference/declarationEmitExportAliasVisibiilityMarking.types

@@ -28,10 +28,10 @@ export let lazyCard = () => import('./Card').then(a => a.default);
 >lazyCard : () => Promise<(suit: import("tests/cases/compiler/Types").Suit, rank: import("tests/cases/compiler/Types").Rank) => { suit: import("tests/cases/compiler/Types").Suit; rank: import("tests/cases/compiler/Types").Rank; }>
 >() => import('./Card').then(a => a.default) : () => Promise<(suit: import("tests/cases/compiler/Types").Suit, rank: import("tests/cases/compiler/Types").Rank) => { suit: import("tests/cases/compiler/Types").Suit; rank: import("tests/cases/compiler/Types").Rank; }>
 >import('./Card').then(a => a.default) : Promise<(suit: import("tests/cases/compiler/Types").Suit, rank: import("tests/cases/compiler/Types").Rank) => { suit: import("tests/cases/compiler/Types").Suit; rank: import("tests/cases/compiler/Types").Rank; }>
->import('./Card').then : <TResult1 = typeof import("tests/cases/compiler/Card"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/compiler/Card")) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>import('./Card').then : <TResult1 = typeof import("tests/cases/compiler/Card"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/compiler/Card")) => TResult1, onrejected?: (reason: any) => TResult2) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >import('./Card') : Promise<typeof import("tests/cases/compiler/Card")>
 >'./Card' : "./Card"
->then : <TResult1 = typeof import("tests/cases/compiler/Card"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/compiler/Card")) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>then : <TResult1 = typeof import("tests/cases/compiler/Card"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/compiler/Card")) => TResult1, onrejected?: (reason: any) => TResult2) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >a => a.default : (a: typeof import("tests/cases/compiler/Card")) => (suit: import("tests/cases/compiler/Types").Suit, rank: import("tests/cases/compiler/Types").Rank) => { suit: import("tests/cases/compiler/Types").Suit; rank: import("tests/cases/compiler/Types").Rank; }
 >a : typeof import("tests/cases/compiler/Card")
 >a.default : (suit: import("tests/cases/compiler/Types").Suit, rank: import("tests/cases/compiler/Types").Rank) => { suit: import("tests/cases/compiler/Types").Suit; rank: import("tests/cases/compiler/Types").Rank; }

tests/baselines/reference/esModuleInteropImportCall.types

@@ -9,10 +9,10 @@ export = foo;
 === tests/cases/compiler/index.ts ===
 import("./foo").then(f => {
 >import("./foo").then(f => {    f.default;}) : Promise<void>
->import("./foo").then : <TResult1 = { default: () => void; }, TResult2 = never>(onfulfilled?: (value: { default: () => void; }) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>import("./foo").then : <TResult1 = { default: () => void; }, TResult2 = never>(onfulfilled?: (value: { default: () => void; }) => TResult1, onrejected?: (reason: any) => TResult2) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >import("./foo") : Promise<{ default: () => void; }>
 >"./foo" : "./foo"
->then : <TResult1 = { default: () => void; }, TResult2 = never>(onfulfilled?: (value: { default: () => void; }) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>then : <TResult1 = { default: () => void; }, TResult2 = never>(onfulfilled?: (value: { default: () => void; }) => TResult1, onrejected?: (reason: any) => TResult2) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >f => {    f.default;} : (f: { default: () => void; }) => void
 >f : { default: () => void; }
 

tests/baselines/reference/genericFunctionInference1.types

@@ -842,9 +842,9 @@ const promise = Promise.resolve(1);
 
 promise.then(
 >promise.then(    pipe(        x => x + 1,        x => x * 2,    ),) : Promise<number>
->promise.then : <TResult1 = number, TResult2 = never>(onfulfilled?: ((value: number) => TResult1 | PromiseLike<TResult1>) | null | undefined, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | null | undefined) => Promise<TResult1 | TResult2>
+>promise.then : <TResult1 = number, TResult2 = never>(onfulfilled?: ((value: number) => TResult1) | null | undefined, onrejected?: ((reason: any) => TResult2) | null | undefined) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >promise : Promise<number>
->then : <TResult1 = number, TResult2 = never>(onfulfilled?: ((value: number) => TResult1 | PromiseLike<TResult1>) | null | undefined, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | null | undefined) => Promise<TResult1 | TResult2>
+>then : <TResult1 = number, TResult2 = never>(onfulfilled?: ((value: number) => TResult1) | null | undefined, onrejected?: ((reason: any) => TResult2) | null | undefined) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 
     pipe(
 >pipe(        x => x + 1,        x => x * 2,    ) : (x: number) => number

tests/baselines/reference/importCallExpression1ESNext.types

@@ -15,9 +15,9 @@ var p1 = import("./0");
 
 p1.then(zero => {
 >p1.then(zero => {    return zero.foo();}) : Promise<string>
->p1.then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>p1.then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1, onrejected?: (reason: any) => TResult2) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >p1 : Promise<typeof import("tests/cases/conformance/dynamicImport/0")>
->then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1, onrejected?: (reason: any) => TResult2) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >zero => {    return zero.foo();} : (zero: typeof import("tests/cases/conformance/dynamicImport/0")) => string
 >zero : typeof import("tests/cases/conformance/dynamicImport/0")
 

tests/baselines/reference/importCallExpression2ESNext.types

@@ -14,9 +14,9 @@ function foo(x: Promise<any>) {
 
     x.then(value => {
 >x.then(value => {        let b = new value.B();        b.print();    }) : Promise<void>
->x.then : <TResult1 = any, TResult2 = never>(onfulfilled?: (value: any) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>x.then : <TResult1 = any, TResult2 = never>(onfulfilled?: (value: any) => TResult1, onrejected?: (reason: any) => TResult2) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >x : Promise<any>
->then : <TResult1 = any, TResult2 = never>(onfulfilled?: (value: any) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>then : <TResult1 = any, TResult2 = never>(onfulfilled?: (value: any) => TResult1, onrejected?: (reason: any) => TResult2) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >value => {        let b = new value.B();        b.print();    } : (value: any) => void
 >value : any
 

tests/baselines/reference/importCallExpression4ESNext.types

@@ -38,11 +38,11 @@ class C {
 
         this.myModule.then(Zero => {
 >this.myModule.then(Zero => {            console.log(Zero.foo());        }, async err => {            console.log(err);            let one = await import("./1");            console.log(one.backup());        }) : Promise<void>
->this.myModule.then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>this.myModule.then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1, onrejected?: (reason: any) => TResult2) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >this.myModule : Promise<typeof import("tests/cases/conformance/dynamicImport/0")>
 >this : this
 >myModule : Promise<typeof import("tests/cases/conformance/dynamicImport/0")>
->then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1, onrejected?: (reason: any) => TResult2) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >Zero => {            console.log(Zero.foo());        } : (Zero: typeof import("tests/cases/conformance/dynamicImport/0")) => void
 >Zero : typeof import("tests/cases/conformance/dynamicImport/0")
 

tests/baselines/reference/importCallExpressionES5AMD.types

@@ -15,9 +15,9 @@ var p1 = import("./0");
 
 p1.then(zero => {
 >p1.then(zero => {    return zero.foo();}) : Promise<string>
->p1.then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>p1.then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1, onrejected?: (reason: any) => TResult2) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >p1 : Promise<typeof import("tests/cases/conformance/dynamicImport/0")>
->then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1, onrejected?: (reason: any) => TResult2) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >zero => {    return zero.foo();} : (zero: typeof import("tests/cases/conformance/dynamicImport/0")) => string
 >zero : typeof import("tests/cases/conformance/dynamicImport/0")
 

tests/baselines/reference/importCallExpressionES5CJS.types

@@ -15,9 +15,9 @@ var p1 = import("./0");
 
 p1.then(zero => {
 >p1.then(zero => {    return zero.foo();}) : Promise<string>
->p1.then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>p1.then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1, onrejected?: (reason: any) => TResult2) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >p1 : Promise<typeof import("tests/cases/conformance/dynamicImport/0")>
->then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>then : <TResult1 = typeof import("tests/cases/conformance/dynamicImport/0"), TResult2 = never>(onfulfilled?: (value: typeof import("tests/cases/conformance/dynamicImport/0")) => TResult1, onrejected?: (reason: any) => TResult2) => Promise<(TResult1 extends PromiseLike<infer UResult1> ? UResult1 : TResult1) | (TResult2 extends PromiseLike<infer UResult2> ? UResult2 : TResult2)>
 >zero => {    return zero.foo();} : (zero: typeof import("tests/cases/conformance/dynamicImport/0")) => string
 >zero : typeof import("tests/cases/conformance/dynamicImport/0")