Regression: TypeScript no longer awaits custom Promises

[RReverser]

TypeScript Version:

1.9.0-dev.20160502

Code

declare namespace Windows.Foundation {
    interface IPromise<TResult> {
        then<U>(success?: (value: TResult) => IPromise<U>, error?: (error: any) => IPromise<U>, progress?: (progress: any) => void): IPromise<U>;
        then<U>(success?: (value: TResult) => IPromise<U>, error?: (error: any) => U, progress?: (progress: any) => void): IPromise<U>;
        then<U>(success?: (value: TResult) => U, error?: (error: any) => IPromise<U>, progress?: (progress: any) => void): IPromise<U>;
        then<U>(success?: (value: TResult) => U, error?: (error: any) => U, progress?: (progress: any) => void): IPromise<U>;
        done<U>(success?: (value: TResult) => any, error?: (error: any) => any, progress?: (progress: any) => void): void;

        cancel(): void;
    }
}

async function sample(promise: Windows.Foundation.IPromise<number>) {
    var number = await promise;
}

Expected behavior:
No error, as this is an await-compatible Promise definition; more concretely, those are definitions for UWP APIs and they worked well with a stable TypeScript.

Actual behavior:
Compiler emits an error Operand for 'await' does not have a valid callable 'then' member..

[mhegazy]

Please see #6068 for more details.

[RReverser]

@mhegazy Doesn't look like a duplicate - that issue seems to be describing setting global Promise to another value, while this one is about awaiting a custom implementation that exists under a custom name, while does fulfill thenable interface.

[mhegazy]

sorry about that. indeed the wrong reference.

[mhegazy]

but i do not get an error for this locally. looks like there is something else going on.. can you share more code?

[RReverser]

Well, I limited to this minimal example and I'm certainly seeing this error. Perhaps something in the config, let me try without one.

[RReverser]

Okay, this seems to be happening only when strictNullChecks is enabled. Although I don't really see the connection yet.

[RReverser]

@mhegazy Please let me know if any other details are needed for this bug.

[vladima]

I can repro this.

[Arnavion]

While you're at it, could you also make that error list exactly how the then is not the expected signature in the general case? For example could you print out the types for both and how they disagree (just like the error for mismatching types in general does)?

[RReverser]

@mhegazy Thank you! Fast as always, it's a pleasure to be beta-tester :)

[mhegazy]

While you're at it, could you also make that error list exactly how the then is not the expected signature in the general case? For example could you print out the types for both and how they disagree (just like the error for mismatching types in general does)?

For this specific check, it actually passed. First we try to get the promised type from the then signatures if that failed, we want to make sure the type does not have a then property on it, or it might be used, and you can get an unexpected behavior at runtime. so we compare it to something that has a then in this case it passes.

the issue was a bit earlier than this specif check, during getting the promised type; now that undefined is added to optional arguments by default, we try to get the signature of a union type of first arg in all overloads and it fails as undefined does not have any signatures.

[mhegazy]

Thanks @RReverser! we do appreciate you feedback, keep the reports coming.