narrowing doesn't work

[zpdDG4gta8XKpMCd]

latest nightly build as of October 6, 2016:

interface A { kind: 'A'; a: number; }
interface B { kind: 'B'; b: number; }
type AB = A | B;
export function toResult<Z extends { kind: K; }, K extends string>(
    value: AB | Z,
    orElse: (value: Z) => number
): number {
    switch (value.kind) {
        case 'A': return value.a; // <-- problem, value isn't narrowed to A
        case 'B': return value.b;
        default: return orElse(value);
    }
}

[zpdDG4gta8XKpMCd]

despite #10417 (comment)

[ahejlsberg]

I don't think this is a bug. We don't know for sure that Z's kind property isn't 'A' or 'B' (it could basically be any string value), so we can't safely narrow to A or B in the switch cases.

[zpdDG4gta8XKpMCd]

i see your point, negating type constraints (#7993) could have guaranteed that Z.kind is neither A nor B:

export function toResult<Z extends { kind: K; }, K extends string unlike 'A', 'B'>(