Control flow type analysis doesn't flow to a callback closure

[zakjan]

TypeScript Version: 2.0.0 (installed today with npm install typescript@beta) with strictNullChecks enabled

Code

function plusOne(x: number): number {
    return x + 1;
}

function test(x?: number): Promise<number> {
    if (!x) {
        return Promise.resolve(0);
    }

    return Promise.resolve().then(() => plusOne(x));
}

Expected behavior:
no type error

Actual behavior:
error TS2345: Argument of type 'number | undefined' is not assignable to parameter of type 'number'.
Type 'undefined' is not assignable to type 'number'.

[zakjan]

However if the closure is used directly, it is ok.

function test2(x?: number): number {
    if (!x) {
        return 0;
    }

    return (() => plusOne(x))();
}

This emits no error.

[kitsonk]

Essentially a dupe of #9263, #7719, #8541, #8552, #9491 (and many more...). The root reason is discussed here.

[zakjan]

Ah, ok, thanks a lot.