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?

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 ?

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

good point. fixed. thanks