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awaited operator is not introduced to async generators #37528

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@falsandtru

Description

@falsandtru

@rbuckton Can you?

TypeScript Version: 3.7.x-dev.201xxxxx

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Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.

Expected behavior:

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<awaited T, awaited TReturn>>;
    return(value: TReturn | PromiseLike<TReturn>): Promise<IteratorResult<awaited T, awaited TReturn>>;
    throw(e: any): Promise<IteratorResult<awaited T, awaited TReturn>>;
    [Symbol.asyncIterator](): AsyncGenerator<awaited T, awaited TReturn, TNext>;
}
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<awaited T, awaited TReturn>>;
    return?(value?: TReturn | PromiseLike<TReturn>): Promise<IteratorResult<awaited T, awaited TReturn>>;
    throw?(e?: any): Promise<IteratorResult<awaited T, awaited TReturn>>;
}

Actual behavior:

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>>;
    [Symbol.asyncIterator](): AsyncGenerator<T, TReturn, TNext>;
}
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>>;
}

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