Fix #8423: Remove undefined while getting the type of the first argument of then signature

[mhegazy]

Fixes #8423

[vladima]

this function is used in two places

• when we get type of first parameter in the call signature of then
• when we get type of first parameter in the onFulfilled callback.

Do we want to strip undefined in both places or just in the first one?

[mhegazy]

well. i went back and forth on this, and then did not see a reason why not...
so we want to get T

then<U>(success?: (value: T) => U,...): IPromise<U>

the first takes care of the optionality of succes?, the second would remove the undefined if value was optional. the question is what is the promised type for this:

then<U>(success?: (value?: T) => U,...): IPromise<U>

is it T or T|undefined ?

[vladima]

what about cases like this:

declare function resolve1<T>(value: T): Promise<T>;
declare function resolve2<T>(value: T): Windows.Foundation.IPromise<T>;

async function f(x?: number) {
    let x1 = await resolve1(x);
    let x2 = await resolve2(x);
}

Currently type of x1 is number | undefined and x2 fails with error addressed by this PR.
With the fix type of x2 will be just number which will be inconsistent with type for x1

[mhegazy]

good point. fixed. thanks

[vladima]

👍