Use Awaited<T> with 'await'

Author: rbuckton

src/compiler/checker.ts

@@ -1445,11 +1445,13 @@ interface Promise<T> {
  */
 type Awaited<T> =
     T extends null | undefined ? T : // special case for `null | undefined` when not in `--noImplicitAny` mode
-    T extends { then(onfulfilled: infer F): any } ? // thenable, extracts the first argument to `then()`
-        F extends ((value: infer V) => any) ? // if the argument to `then` is callable, extracts the argument
-            Awaited<V> : // recursively unwrap the value
-            never : // the argument to `then` was not callable.
-    T; // non-thenable
+        T extends object ? // `await` only unwraps object types with a callable then. Non-object types are not unwrapped.
+            T extends { then(onfulfilled: infer F): any } ? // thenable, extracts the first argument to `then()`
+                F extends ((value: infer V) => any) ? // if the argument to `then` is callable, extracts the argument
+                    Awaited<V> : // recursively unwrap the value
+                    never : // the argument to `then` was not callable.
+            T : // argument was not an object
+        T; // non-thenable
 
 interface ArrayLike<T> {
     readonly length: number;

src/lib/es5.d.ts

@@ -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_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

@@ -1,14 +1,14 @@
-tests/cases/compiler/doNotElaborateAssignabilityToTypeParameters.ts(3,3): error TS2322: Type 'T | Yadda' is not assignable to type 'T'.
-  'T' could be instantiated with an arbitrary type which could be unrelated to 'T | Yadda'.
+tests/cases/compiler/doNotElaborateAssignabilityToTypeParameters.ts(3,3): error TS2322: Type 'Yadda | Awaited<T>' is not assignable to type 'T'.
+  'T' could be instantiated with an arbitrary type which could be unrelated to 'Yadda | Awaited<T>'.
 
 
 ==== tests/cases/compiler/doNotElaborateAssignabilityToTypeParameters.ts (1 errors) ====
     async function foo<T>(x: T): Promise<T> {
       let yaddable = await getXOrYadda(x);
       return yaddable;
       ~~~~~~~~~~~~~~~~
-!!! error TS2322: Type 'T | Yadda' is not assignable to type 'T'.
-!!! error TS2322:   'T' could be instantiated with an arbitrary type which could be unrelated to 'T | Yadda'.
+!!! error TS2322: Type 'Yadda | Awaited<T>' is not assignable to type 'T'.
+!!! error TS2322:   'T' could be instantiated with an arbitrary type which could be unrelated to 'Yadda | Awaited<T>'.
     }
 
     interface Yadda {

tests/baselines/reference/doNotElaborateAssignabilityToTypeParameters.errors.txt

@@ -4,14 +4,14 @@ async function foo<T>(x: T): Promise<T> {
 >x : T
 
   let yaddable = await getXOrYadda(x);
->yaddable : T | Yadda
->await getXOrYadda(x) : T | Yadda
+>yaddable : Yadda | Awaited<T>
+>await getXOrYadda(x) : Yadda | Awaited<T>
 >getXOrYadda(x) : T | Yadda
 >getXOrYadda : <T>(x: T) => T | Yadda
 >x : T
 
   return yaddable;
->yaddable : T | Yadda
+>yaddable : Yadda | Awaited<T>
 }
 
 interface Yadda {

tests/baselines/reference/doNotElaborateAssignabilityToTypeParameters.types

@@ -4,7 +4,7 @@ async function f<T>(source: Iterable<T> | AsyncIterable<T>) {
 >source : Iterable<T> | AsyncIterable<T>
 
     for await (const x of source) {
->x : T
+>x : Awaited<T>
 >source : Iterable<T> | AsyncIterable<T>
     }
 }