Better typings for AsyncIterator/Generator, like #31117

Author: jablko

src/lib/es2018.asyncgenerator.d.ts

@@ -2,9 +2,9 @@
 
 interface AsyncGenerator<T = unknown, TReturn = any, TNext = unknown> extends AsyncIterator<T, TReturn, TNext> {
     // NOTE: 'next' is defined using a tuple to ensure we report the correct assignability errors in all places.
-    next(...args: [] | [TNext]): Promise<IteratorResult<T, TReturn>>;
-    return(value: TReturn | PromiseLike<TReturn>): Promise<IteratorResult<T, TReturn>>;
-    throw(e: any): Promise<IteratorResult<T, TReturn>>;
+    next(...args: [] | [TNext]): Promise<IteratorResult<T, TReturn extends PromiseLike<infer UReturn> ? UReturn : TReturn>>;
+    return(value: TReturn): Promise<IteratorResult<T, TReturn extends PromiseLike<infer UReturn> ? UReturn : TReturn>>;
+    throw(e: any): Promise<IteratorResult<T, TReturn extends PromiseLike<infer UReturn> ? UReturn : TReturn>>;
     [Symbol.asyncIterator](): AsyncGenerator<T, TReturn, TNext>;
 }
 

src/lib/es2018.asynciterable.d.ts

@@ -11,9 +11,9 @@ interface SymbolConstructor {
 
 interface AsyncIterator<T, TReturn = any, TNext = undefined> {
     // NOTE: 'next' is defined using a tuple to ensure we report the correct assignability errors in all places.
-    next(...args: [] | [TNext]): Promise<IteratorResult<T, TReturn>>;
-    return?(value?: TReturn | PromiseLike<TReturn>): Promise<IteratorResult<T, TReturn>>;
-    throw?(e?: any): Promise<IteratorResult<T, TReturn>>;
+    next(...args: [] | [TNext]): Promise<IteratorResult<T, TReturn extends PromiseLike<infer UReturn> ? UReturn : TReturn>>;
+    return?(value?: TReturn): Promise<IteratorResult<T, TReturn extends PromiseLike<infer UReturn> ? UReturn : TReturn>>;
+    throw?(e?: any): Promise<IteratorResult<T, TReturn extends PromiseLike<infer UReturn> ? UReturn : TReturn>>;
 }
 
 interface AsyncIterable<T> {
@@ -22,4 +22,4 @@ interface AsyncIterable<T> {
 
 interface AsyncIterableIterator<T> extends AsyncIterator<T> {
     [Symbol.asyncIterator](): AsyncIterableIterator<T>;
-}
+}