[Web API type definition issue] Extending `ReadableStream` throws "Base constructors must all have the same return type" error

[yume-chan]

Summary

Extending ReadableStream throws "Base constructors must all have the same return type" error

Expected vs. Actual Behavior

Extending ReadableStream should be allowed (as mentioned in whatwg/streams#160 and whatwg/streams#965), but changes in #1362 cause a TS error when trying to do that

class A extends ReadableStream {
//              ^ Base constructors must all have the same return type. ts(2510)
}

Playground Link

https://www.typescriptlang.org/play/?#code/MYGwhgzhAECC0FMAeAXBA7AJjASgsmYARiAgMooBO+AttAN4BQjAvkA

Browser Support

• This API is supported in at least two major browser engines (not two Chromium-based browsers).

Have Tried The Latest Releases

• This issue applies to the latest release of TypeScript.
• This issue applies to the latest release of @types/web.

Additional Context

No response

[HolgerJeromin]

cc @MattiasBuelens

[MattiasBuelens]

This seems to be caused by the overloads on declare var ReadableStream here:

TypeScript-DOM-lib-generator/baselines/webworker.generated.d.ts

Lines 9239 to 9244 in 4a002c9

	declare var ReadableStream: {
	prototype: ReadableStream;
	new(underlyingSource: UnderlyingByteSource, strategy?: { highWaterMark?: number }): ReadableStream<Uint8Array<ArrayBuffer>>;
	new<R = any>(underlyingSource: UnderlyingDefaultSource<R>, strategy?: QueuingStrategy<R>): ReadableStream<R>;
	new<R = any>(underlyingSource?: UnderlyingSource<R>, strategy?: QueuingStrategy<R>): ReadableStream<R>;
	};

One way to "fix" this would be to explicitly pass a generic type parameter R to ReadableStream (see playground):

class A<R = any> extends ReadableStream<R> {
}

Or, if you also need to override the constructor (see playground):

class A<R = any> extends ReadableStream<R> {
    constructor(underlyingSource: UnderlyingSource<R>, strategy: QueuingStrategy<R>) {
        super(underlyingSource, strategy);
    }
}

@saschanaz I don't know if we could express this better in the DOM types? The intention is:

• if R is Uint8Array, then allow passing an UnderlyingByteSource (or an UnderlyingDefaultSource<Uint8Array>)
• if underlyingSource is an UnderlyingByteSource, then assert (or infer) that R is Uint8Array

Is there a way to do that while still keeping the return type of all the constructors the same, i.e.:

declare var ReadableStream: {
  new<R = any>(/* ... */): ReadableStream<R>;
}

[yume-chan]

One way to "fix" this would be to explicitly pass a generic type parameter R to ReadableStream

You are right! And all of these works:

class A<T> extends ReadableStream<T> {
    constructor() {
        super({})
    }
}

class B extends ReadableStream<Uint8Array> {
    constructor() {
        super({})
    }
}

// Or any concrete generic parameter
class C extends ReadableStream<string> {
    constructor() {
        super({})
    }
}

So the original issue is not very important because everyone should use one of them.