Generalize `never` type handling for control flow analysis based type guard

[vilicvane]

I guess there would be some duplicates, but I cannot find it.

TypeScript Version: 2.1.14

Code

function throwError(): never {
    throw new Error();
}

let foo: string | undefined;

if (!foo) {
    throwError();
}

foo; // Expected to be `string` instead of `string | undefined`.

[akarzazi]

It would be a great way to control the flow.

For now, you may use return if your are inside a function.

function throwError(): never {
    throw new Error();
}

let foo: string | undefined;
if (!foo) {
    throwError();
    return:
}
foo; // foo is string

or use a check typeguard

function throwError(): never {
    throw new Error();
}

// Inferred return type is T
function check<T>(x: T | undefined) {
    return x || throwError();
}

let foo: string | undefined;

let foosafe = check(foo); // foosafe is string

[yortus]

Related: #8655

[dhedey]

I'd like to echo my support for this. Currently to get control flow analysis to work correctly in our codebase (which make use of a framework requiring the use of an external throwError style method), we explicitly throw ''; // See https://github.com/Microsoft/TypeScript/issues/12825 in the following line.

ADDENDUM:

I've just noted RyanCavanaugh's comment on a duplicate thread.

The recommended workaround is to write return throwError();

We'll use this workaround from now on. It appears to not interfere with the returned type. 👍

if (!foo) {
    return throwError();
}