TypeScript Version: 2.2.0-dev.20170126

Code

declare function takeA(val: 'A'): void;
export function bounceAndTakeIfA<AB extends 'A' | 'B'>(value: AB): AB {
    if (value === 'A') {
        takeA(value);
        return value;
    }
    else {
        return value;
    }
}

Expected behavior:
Compiles without error.

Actual behavior:

Argument of type 'AB' is not assignable to parameter of type '"A"'.

It works correctly if I just use value: 'A' | 'B' as the argument to bounceAndTakeIfA, but since I want the return value to type-match the input, I need to use the generic (overloading could do it, but overloading is brittle and error-prone, since there is no error-checking that the overload signatures are actually correct; my team has banned them for that reason). But (I'm guessing) since AB extends 'A' | 'B', narrowing doesn't happen. In reality, I am just using extends to mean AB ⊂ 'A' | 'B', but extends can mean more than that, which (I suspect) is why TS is refusing to narrow on it.

Some alternative to extends that more specifically means ⊂ (subsets maybe?) would probably be the best solution here?