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narrowing doesn't work #11433
Description
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);
}
}
Activity
zpdDG4gta8XKpMCd commentedon Oct 7, 2016
despite #10417 (comment)
ahejlsberg commentedon Oct 13, 2016
I don't think this is a bug. We don't know for sure that Z's
kindproperty isn't'A'or'B'(it could basically be any string value), so we can't safely narrow toAorBin the switch cases.zpdDG4gta8XKpMCd commentedon Oct 13, 2016
i see your point, negating type constraints (#7993) could have guaranteed that
Z.kindis neitherAnorB: