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/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/asyncArrowFunctionCapturesThis_es2017.types

@@ -6,9 +6,9 @@ class C {
 >method : () => void
 
       var fn = async () => await this;      
->fn : () => Promise<this>
->async () => await this : () => Promise<this>
->await this : this
+>fn : () => Promise<Awaited<this>>
+>async () => await this : () => Promise<Awaited<this>>
+>await this : Awaited<this>
 >this : this
    }
 }

tests/baselines/reference/asyncArrowFunctionCapturesThis_es5.types

@@ -6,9 +6,9 @@ class C {
 >method : () => void
 
       var fn = async () => await this;
->fn : () => Promise<this>
->async () => await this : () => Promise<this>
->await this : this
+>fn : () => Promise<Awaited<this>>
+>async () => await this : () => Promise<Awaited<this>>
+>await this : Awaited<this>
 >this : this
    }
 }

tests/baselines/reference/asyncArrowFunctionCapturesThis_es6.types

@@ -6,9 +6,9 @@ class C {
 >method : () => void
 
       var fn = async () => await this;      
->fn : () => Promise<this>
->async () => await this : () => Promise<this>
->await this : this
+>fn : () => Promise<Awaited<this>>
+>async () => await this : () => Promise<Awaited<this>>
+>await this : Awaited<this>
 >this : this
    }
 }

tests/baselines/reference/asyncFunctionReturnType.js

@@ -63,7 +63,7 @@ async function fGenericIndexedTypeForExplicitPromiseOfAnyProp<TObj extends Obj>(
     return Promise.resolve<TObj["anyProp"]>(obj.anyProp);
 }
 
-async function fGenericIndexedTypeForKProp<TObj extends Obj, K extends keyof TObj>(obj: TObj, key: K): Promise<TObj[K]> {
+async function fGenericIndexedTypeForKProp<TObj extends Obj, K extends keyof TObj>(obj: TObj, key: K): Promise<Awaited<TObj[K]>> {
     return obj[key];
 }
 
@@ -73,7 +73,8 @@ async function fGenericIndexedTypeForPromiseOfKProp<TObj extends Obj, K extends
 
 async function fGenericIndexedTypeForExplicitPromiseOfKProp<TObj extends Obj, K extends keyof TObj>(obj: TObj, key: K): Promise<TObj[K]> {
     return Promise.resolve<TObj[K]>(obj[key]);
-}
+}
+
 
 //// [asyncFunctionReturnType.js]
 var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {

tests/baselines/reference/asyncFunctionReturnType.symbols

@@ -221,7 +221,7 @@ async function fGenericIndexedTypeForExplicitPromiseOfAnyProp<TObj extends Obj>(
 >anyProp : Symbol(Obj.anyProp, Decl(asyncFunctionReturnType.ts, 12, 23))
 }
 
-async function fGenericIndexedTypeForKProp<TObj extends Obj, K extends keyof TObj>(obj: TObj, key: K): Promise<TObj[K]> {
+async function fGenericIndexedTypeForKProp<TObj extends Obj, K extends keyof TObj>(obj: TObj, key: K): Promise<Awaited<TObj[K]>> {
 >fGenericIndexedTypeForKProp : Symbol(fGenericIndexedTypeForKProp, Decl(asyncFunctionReturnType.ts, 62, 1))
 >TObj : Symbol(TObj, Decl(asyncFunctionReturnType.ts, 64, 43))
 >Obj : Symbol(Obj, Decl(asyncFunctionReturnType.ts, 8, 1))
@@ -232,6 +232,7 @@ async function fGenericIndexedTypeForKProp<TObj extends Obj, K extends keyof TOb
 >key : Symbol(key, Decl(asyncFunctionReturnType.ts, 64, 93))
 >K : Symbol(K, Decl(asyncFunctionReturnType.ts, 64, 60))
 >Promise : Symbol(Promise, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --))
+>Awaited : Symbol(Awaited, Decl(lib.es5.d.ts, --, --))
 >TObj : Symbol(TObj, Decl(asyncFunctionReturnType.ts, 64, 43))
 >K : Symbol(K, Decl(asyncFunctionReturnType.ts, 64, 60))
 
@@ -285,3 +286,4 @@ async function fGenericIndexedTypeForExplicitPromiseOfKProp<TObj extends Obj, K
 >obj : Symbol(obj, Decl(asyncFunctionReturnType.ts, 72, 100))
 >key : Symbol(key, Decl(asyncFunctionReturnType.ts, 72, 110))
 }
+

tests/baselines/reference/asyncFunctionReturnType.types

@@ -180,8 +180,8 @@ async function fGenericIndexedTypeForExplicitPromiseOfAnyProp<TObj extends Obj>(
 >anyProp : any
 }
 
-async function fGenericIndexedTypeForKProp<TObj extends Obj, K extends keyof TObj>(obj: TObj, key: K): Promise<TObj[K]> {
->fGenericIndexedTypeForKProp : <TObj extends Obj, K extends keyof TObj>(obj: TObj, key: K) => Promise<TObj[K]>
+async function fGenericIndexedTypeForKProp<TObj extends Obj, K extends keyof TObj>(obj: TObj, key: K): Promise<Awaited<TObj[K]>> {
+>fGenericIndexedTypeForKProp : <TObj extends Obj, K extends keyof TObj>(obj: TObj, key: K) => Promise<Awaited<TObj[K]>>
 >obj : TObj
 >key : K
 
@@ -220,3 +220,4 @@ async function fGenericIndexedTypeForExplicitPromiseOfKProp<TObj extends Obj, K
 >obj : TObj
 >key : K
 }
+

tests/baselines/reference/compareTypeParameterConstrainedByLiteralToLiteral.errors.txt

@@ -9,5 +9,6 @@ tests/cases/compiler/compareTypeParameterConstrainedByLiteralToLiteral.ts(5,5):
         t === "x";  // Should be error
         ~~~~~~~~~
 !!! error TS2367: This condition will always return 'false' since the types 'T' and '"x"' have no overlap.
+!!! related TS2773 tests/cases/compiler/compareTypeParameterConstrainedByLiteralToLiteral.ts:5:5: Did you forget to use 'await'?
     }