Defer generic awaited type

Author: jablko

src/compiler/checker.ts

@@ -842,6 +842,7 @@ namespace ts {
         let deferredGlobalESSymbolConstructorSymbol: Symbol | undefined;
         let deferredGlobalESSymbolType: ObjectType;
         let deferredGlobalTypedPropertyDescriptorType: GenericType;
+        let deferredGlobalAwaitedSymbol: Symbol | undefined;
         let deferredGlobalPromiseType: GenericType;
         let deferredGlobalPromiseLikeType: GenericType;
         let deferredGlobalPromiseConstructorSymbol: Symbol | undefined;
@@ -896,7 +897,6 @@ namespace ts {
         const potentialThisCollisions: Node[] = [];
         const potentialNewTargetCollisions: Node[] = [];
         const potentialWeakMapCollisions: Node[] = [];
-        const awaitedTypeStack: number[] = [];
 
         const diagnostics = createDiagnosticCollection();
         const suggestionDiagnostics = createDiagnosticCollection();
@@ -11455,6 +11455,10 @@ namespace ts {
             return deferredGlobalESSymbolType || (deferredGlobalESSymbolType = getGlobalType("Symbol" as __String, /*arity*/ 0, reportErrors)) || emptyObjectType;
         }
 
+        function getGlobalAwaitedSymbol(reportErrors: boolean) {
+            return deferredGlobalAwaitedSymbol || (deferredGlobalAwaitedSymbol = getGlobalTypeSymbol("Awaited" as __String, reportErrors));
+        }
+
         function getGlobalPromiseType(reportErrors: boolean) {
             return deferredGlobalPromiseType || (deferredGlobalPromiseType = getGlobalType("Promise" as __String, /*arity*/ 1, reportErrors)) || emptyGenericType;
         }
@@ -26270,8 +26274,6 @@ namespace ts {
             // creates a `Promise<T>` type where `T` is the promisedType argument
             const globalPromiseType = getGlobalPromiseType(/*reportErrors*/ true);
             if (globalPromiseType !== emptyGenericType) {
-                // if the promised type is itself a promise, get the underlying type; otherwise, fallback to the promised type
-                promisedType = getAwaitedType(promisedType) || unknownType;
                 return createTypeReference(globalPromiseType, [promisedType]);
             }
 
@@ -26282,8 +26284,6 @@ namespace ts {
             // creates a `PromiseLike<T>` type where `T` is the promisedType argument
             const globalPromiseLikeType = getGlobalPromiseLikeType(/*reportErrors*/ true);
             if (globalPromiseLikeType !== emptyGenericType) {
-                // if the promised type is itself a promise, get the underlying type; otherwise, fallback to the promised type
-                promisedType = getAwaitedType(promisedType) || unknownType;
                 return createTypeReference(globalPromiseLikeType, [promisedType]);
             }
 
@@ -29743,98 +29743,26 @@ namespace ts {
                 return typeAsAwaitable.awaitedTypeOfType = type;
             }
 
-            if (type.flags & TypeFlags.Union) {
-                let types: Type[] | undefined;
-                for (const constituentType of (<UnionType>type).types) {
-                    types = append<Type>(types, getAwaitedType(constituentType, errorNode, diagnosticMessage, arg0));
-                }
-
-                if (!types) {
-                    return undefined;
-                }
-
-                return typeAsAwaitable.awaitedTypeOfType = getUnionType(types);
+            const symbol = getGlobalAwaitedSymbol(/*reportErrors*/ false);
+            if (!symbol) {
+                return typeAsAwaitable.awaitedTypeOfType = type;
             }
 
-            const promisedType = getPromisedTypeOfPromise(type);
-            if (promisedType) {
-                if (type.id === promisedType.id || awaitedTypeStack.indexOf(promisedType.id) >= 0) {
-                    // Verify that we don't have a bad actor in the form of a promise whose
-                    // promised type is the same as the promise type, or a mutually recursive
-                    // promise. If so, we return undefined as we cannot guess the shape. If this
-                    // were the actual case in the JavaScript, this Promise would never resolve.
-                    //
-                    // An example of a bad actor with a singly-recursive promise type might
-                    // be:
-                    //
-                    //  interface BadPromise {
-                    //      then(
-                    //          onfulfilled: (value: BadPromise) => any,
-                    //          onrejected: (error: any) => any): BadPromise;
-                    //  }
-                    // The above interface will pass the PromiseLike check, and return a
-                    // promised type of `BadPromise`. Since this is a self reference, we
-                    // don't want to keep recursing ad infinitum.
-                    //
-                    // An example of a bad actor in the form of a mutually-recursive
-                    // promise type might be:
-                    //
-                    //  interface BadPromiseA {
-                    //      then(
-                    //          onfulfilled: (value: BadPromiseB) => any,
-                    //          onrejected: (error: any) => any): BadPromiseB;
-                    //  }
-                    //
-                    //  interface BadPromiseB {
-                    //      then(
-                    //          onfulfilled: (value: BadPromiseA) => any,
-                    //          onrejected: (error: any) => any): BadPromiseA;
-                    //  }
-                    //
-                    if (errorNode) {
-                        error(errorNode, Diagnostics.Type_is_referenced_directly_or_indirectly_in_the_fulfillment_callback_of_its_own_then_method);
-                    }
-                    return undefined;
-                }
-
-                // Keep track of the type we're about to unwrap to avoid bad recursive promise types.
-                // See the comments above for more information.
-                awaitedTypeStack.push(type.id);
-                const awaitedType = getAwaitedType(promisedType, errorNode, diagnosticMessage, arg0);
-                awaitedTypeStack.pop();
-
-                if (!awaitedType) {
-                    return undefined;
-                }
+            if (type.aliasSymbol === symbol) {
+                return typeAsAwaitable.awaitedTypeOfType = type;
+            }
 
-                return typeAsAwaitable.awaitedTypeOfType = awaitedType;
+            const result = getTypeAliasInstantiation(symbol, [type]);
+            if (result !== unknownType || type === unknownType || getPromisedTypeOfPromise(type) === unknownType) {
+                return typeAsAwaitable.awaitedTypeOfType = result;
             }
 
-            // The type was not a promise, so it could not be unwrapped any further.
-            // As long as the type does not have a callable "then" property, it is
-            // safe to return the type; otherwise, an error will be reported in
-            // the call to getNonThenableType and we will return undefined.
-            //
-            // An example of a non-promise "thenable" might be:
-            //
-            //  await { then(): void {} }
-            //
-            // The "thenable" does not match the minimal definition for a promise. When
-            // a Promise/A+-compatible or ES6 promise tries to adopt this value, the promise
-            // will never settle. We treat this as an error to help flag an early indicator
-            // of a runtime problem. If the user wants to return this value from an async
-            // function, they would need to wrap it in some other value. If they want it to
-            // be treated as a promise, they can cast to <any>.
-            const thenFunction = getTypeOfPropertyOfType(type, "then" as __String);
-            if (thenFunction && getSignaturesOfType(thenFunction, SignatureKind.Call).length > 0) {
-                if (errorNode) {
-                    if (!diagnosticMessage) return Debug.fail();
-                    error(errorNode, diagnosticMessage, arg0);
-                }
-                return undefined;
+            if (errorNode) {
+                if (!diagnosticMessage) return Debug.fail();
+                error(errorNode, diagnosticMessage, arg0);
             }
 
-            return typeAsAwaitable.awaitedTypeOfType = type;
+            return undefined;
         }
 
         /**

src/harness/fourslashInterfaceImpl.ts

@@ -961,6 +961,7 @@ namespace FourSlashInterface {
             typeEntry("PropertyDecorator"),
             typeEntry("MethodDecorator"),
             typeEntry("ParameterDecorator"),
+            typeEntry("Awaited"),
             typeEntry("PromiseConstructorLike"),
             interfaceEntry("PromiseLike"),
             interfaceEntry("Promise"),

src/lib/es2015.iterable.d.ts

@@ -200,18 +200,18 @@ interface PromiseConstructor {
     /**
      * Creates a Promise that is resolved with an array of results when all of the provided Promises
      * resolve, or rejected when any Promise is rejected.
-     * @param values An array of Promises.
+     * @param values An iterable of Promises.
      * @returns A new Promise.
      */
-    all<TAll>(values: Iterable<TAll | PromiseLike<TAll>>): Promise<TAll[]>;
+    all<TAll>(values: Iterable<TAll>): Promise<Awaited<TAll>[]>;
 
     /**
      * Creates a Promise that is resolved or rejected when any of the provided Promises are resolved
      * or rejected.
      * @param values An iterable of Promises.
      * @returns A new Promise.
      */
-    race<T>(values: Iterable<T>): Promise<T extends PromiseLike<infer U> ? U : T>;
+    race<T>(values: Iterable<T>): Promise<Awaited<T>>;
 }
 
 declare namespace Reflect {

src/lib/es2015.promise.d.ts

@@ -18,87 +18,15 @@ interface PromiseConstructor {
      * @param values An array of Promises.
      * @returns A new Promise.
      */
-    all<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>, T7 | PromiseLike<T7>, T8 | PromiseLike<T8>, T9 | PromiseLike<T9>, T10 | PromiseLike<T10>]): Promise<[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10]>;
-
-    /**
-     * Creates a Promise that is resolved with an array of results when all of the provided Promises
-     * resolve, or rejected when any Promise is rejected.
-     * @param values An array of Promises.
-     * @returns A new Promise.
-     */
-    all<T1, T2, T3, T4, T5, T6, T7, T8, T9>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>, T7 | PromiseLike<T7>, T8 | PromiseLike<T8>, T9 | PromiseLike<T9>]): Promise<[T1, T2, T3, T4, T5, T6, T7, T8, T9]>;
-
-    /**
-     * Creates a Promise that is resolved with an array of results when all of the provided Promises
-     * resolve, or rejected when any Promise is rejected.
-     * @param values An array of Promises.
-     * @returns A new Promise.
-     */
-    all<T1, T2, T3, T4, T5, T6, T7, T8>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>, T7 | PromiseLike<T7>, T8 | PromiseLike<T8>]): Promise<[T1, T2, T3, T4, T5, T6, T7, T8]>;
-
-    /**
-     * Creates a Promise that is resolved with an array of results when all of the provided Promises
-     * resolve, or rejected when any Promise is rejected.
-     * @param values An array of Promises.
-     * @returns A new Promise.
-     */
-    all<T1, T2, T3, T4, T5, T6, T7>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>, T7 | PromiseLike<T7>]): Promise<[T1, T2, T3, T4, T5, T6, T7]>;
-
-    /**
-     * Creates a Promise that is resolved with an array of results when all of the provided Promises
-     * resolve, or rejected when any Promise is rejected.
-     * @param values An array of Promises.
-     * @returns A new Promise.
-     */
-    all<T1, T2, T3, T4, T5, T6>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>]): Promise<[T1, T2, T3, T4, T5, T6]>;
-
-    /**
-     * Creates a Promise that is resolved with an array of results when all of the provided Promises
-     * resolve, or rejected when any Promise is rejected.
-     * @param values An array of Promises.
-     * @returns A new Promise.
-     */
-    all<T1, T2, T3, T4, T5>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>]): Promise<[T1, T2, T3, T4, T5]>;
-
-    /**
-     * Creates a Promise that is resolved with an array of results when all of the provided Promises
-     * resolve, or rejected when any Promise is rejected.
-     * @param values An array of Promises.
-     * @returns A new Promise.
-     */
-    all<T1, T2, T3, T4>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>]): Promise<[T1, T2, T3, T4]>;
-
-    /**
-     * Creates a Promise that is resolved with an array of results when all of the provided Promises
-     * resolve, or rejected when any Promise is rejected.
-     * @param values An array of Promises.
-     * @returns A new Promise.
-     */
-    all<T1, T2, T3>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>]): Promise<[T1, T2, T3]>;
-
-    /**
-     * Creates a Promise that is resolved with an array of results when all of the provided Promises
-     * resolve, or rejected when any Promise is rejected.
-     * @param values An array of Promises.
-     * @returns A new Promise.
-     */
-    all<T1, T2>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>]): Promise<[T1, T2]>;
-
-    /**
-     * Creates a Promise that is resolved with an array of results when all of the provided Promises
-     * resolve, or rejected when any Promise is rejected.
-     * @param values An array of Promises.
-     * @returns A new Promise.
-     */
-    all<T>(values: readonly (T | PromiseLike<T>)[]): Promise<T[]>;
+    all<T extends readonly any[]>(values: T): Promise<{ -readonly [P in keyof T]: Awaited<T[P]> }>;
 
     /**
      * Creates a Promise that is resolved or rejected when any of the provided Promises are resolved
      * or rejected.
      * @param values An array of Promises.
      * @returns A new Promise.
      */
-    race<T>(values: readonly T[]): Promise<T extends PromiseLike<infer U> ? U : T>;
+    race<T extends readonly any[]>(values: T): Promise<Awaited<T[number]>>;
 
     /**
      * Creates a new rejected promise for the provided reason.
@@ -112,10 +40,10 @@ interface PromiseConstructor {
      * @param value A promise.
      * @returns A promise whose internal state matches the provided promise.
      */
-    resolve<T>(value: T | PromiseLike<T>): Promise<T>;
+    resolve<T>(value: T): Promise<Awaited<T>>;
 
     /**
-     * Creates a new resolved promise .
+     * Creates a new resolved promise.
      * @returns A resolved promise.
      */
     resolve(): Promise<void>;

src/lib/es5.d.ts

@@ -1378,6 +1378,11 @@ declare type PropertyDecorator = (target: Object, propertyKey: string | symbol)
 declare type MethodDecorator = <T>(target: Object, propertyKey: string | symbol, descriptor: TypedPropertyDescriptor<T>) => TypedPropertyDescriptor<T> | void;
 declare type ParameterDecorator = (target: Object, propertyKey: string | symbol, parameterIndex: number) => void;
 
+// The undefined case is for strictNullChecks false, in which case
+// undefined extends PromiseLike<infer U> is true, which would otherwise
+// make Awaited<undefined> -> unknown.
+type Awaited<T> = T extends undefined ? T : T extends PromiseLike<infer U> ? U : T extends { then: Function } ? unknown : T;
+
 declare type PromiseConstructorLike = new <T>(executor: (resolve: (value?: T | PromiseLike<T>) => void, reject: (reason?: any) => void) => void) => PromiseLike<T>;
 
 interface PromiseLike<T> {

tests/baselines/reference/asyncArrowFunction11_es5.types

@@ -11,9 +11,9 @@ class A {
         await Promise.resolve();
 >await Promise.resolve() : void
 >Promise.resolve() : Promise<void>
->Promise.resolve : { <T>(value: T | PromiseLike<T>): Promise<T>; (): Promise<void>; }
+>Promise.resolve : { <T>(value: T): Promise<Awaited<T>>; (): Promise<void>; }
 >Promise : PromiseConstructor
->resolve : { <T>(value: T | PromiseLike<T>): Promise<T>; (): Promise<void>; }
+>resolve : { <T>(value: T): Promise<Awaited<T>>; (): Promise<void>; }
 
         const obj = { ["a"]: () => this }; // computed property name after `await` triggers case
 >obj : { a: () => this; }

tests/baselines/reference/asyncArrowFunction5_es2017.errors.txt

@@ -16,7 +16,7 @@ tests/cases/conformance/async/es2017/asyncArrowFunction/asyncArrowFunction5_es20
 !!! error TS1005: ',' expected.
                              ~~~~~~~
 !!! error TS2403: Subsequent variable declarations must have the same type.  Variable 'Promise' must be of type 'PromiseConstructor', but here has type 'any'.
-!!! related TS6203 /.ts/lib.es2015.promise.d.ts:144:13: 'Promise' was also declared here.
+!!! related TS6203 /.ts/lib.es2015.promise.d.ts:72:13: 'Promise' was also declared here.
                                     ~
 !!! error TS1005: ',' expected.
                                            ~~

tests/baselines/reference/asyncArrowFunction5_es5.errors.txt

@@ -16,7 +16,7 @@ tests/cases/conformance/async/es5/asyncArrowFunction/asyncArrowFunction5_es5.ts(
 !!! error TS1005: ',' expected.
                              ~~~~~~~
 !!! error TS2403: Subsequent variable declarations must have the same type.  Variable 'Promise' must be of type 'PromiseConstructor', but here has type 'any'.
-!!! related TS6203 /.ts/lib.es2015.promise.d.ts:144:13: 'Promise' was also declared here.
+!!! related TS6203 /.ts/lib.es2015.promise.d.ts:72:13: 'Promise' was also declared here.
                                     ~
 !!! error TS1005: ',' expected.
                                            ~~

tests/baselines/reference/asyncArrowFunction5_es6.errors.txt

@@ -16,7 +16,7 @@ tests/cases/conformance/async/es6/asyncArrowFunction/asyncArrowFunction5_es6.ts(
 !!! error TS1005: ',' expected.
                              ~~~~~~~
 !!! error TS2403: Subsequent variable declarations must have the same type.  Variable 'Promise' must be of type 'PromiseConstructor', but here has type 'any'.
-!!! related TS6203 /.ts/lib.es2015.promise.d.ts:144:13: 'Promise' was also declared here.
+!!! related TS6203 /.ts/lib.es2015.promise.d.ts:72:13: 'Promise' was also declared here.
                                     ~
 !!! error TS1005: ',' expected.
                                            ~~

tests/baselines/reference/asyncArrowFunction9_es2017.errors.txt

@@ -16,7 +16,7 @@ tests/cases/conformance/async/es2017/asyncArrowFunction/asyncArrowFunction9_es20
 !!! error TS1005: ',' expected.
                                           ~~~~~~~
 !!! error TS2403: Subsequent variable declarations must have the same type.  Variable 'Promise' must be of type 'PromiseConstructor', but here has type 'any'.
-!!! related TS6203 /.ts/lib.es2015.promise.d.ts:144:13: 'Promise' was also declared here.
+!!! related TS6203 /.ts/lib.es2015.promise.d.ts:72:13: 'Promise' was also declared here.
                                                  ~
 !!! error TS1005: ',' expected.
                                                         ~~

tests/baselines/reference/asyncArrowFunction9_es5.errors.txt

@@ -16,7 +16,7 @@ tests/cases/conformance/async/es5/asyncArrowFunction/asyncArrowFunction9_es5.ts(
 !!! error TS1005: ',' expected.
                                           ~~~~~~~
 !!! error TS2403: Subsequent variable declarations must have the same type.  Variable 'Promise' must be of type 'PromiseConstructor', but here has type 'any'.
-!!! related TS6203 /.ts/lib.es2015.promise.d.ts:144:13: 'Promise' was also declared here.
+!!! related TS6203 /.ts/lib.es2015.promise.d.ts:72:13: 'Promise' was also declared here.
                                                  ~
 !!! error TS1005: ',' expected.
                                                         ~~