Switch on typeof for keyof type loses information

[jack-williams]

TypeScript Version: master 83fe1ea

Search Terms: switch typeof keyof intersection

Code

function keyofNarrowing<S extends { [K in keyof S]: string }>(k: keyof S) {
    switch (typeof k) {
        case 'number': k; return; // k has type number, should be keyof S & number
        case 'symbol': k; return; // k has type number, should be keyof S & symbol
        case 'string': k; return; // k has type number, should be keyof S & string
    }
}

Expected behavior:
The value k should narrow to an intersection type to retain keyof type.

Actual behavior:
The value k narrows to a primitive.

Playground Link: N/A (Playground not on this version).

[ahejlsberg]

This one is fixed by #27157.

[jack-williams]

Great, I'll remove the fix I put in for #27182 (but will keep the tests). This will mean #27182 is blocked on #27157 but that seems ok I think.