for...in loop not narrowing key

[nsgundy]

TypeScript Version: 2.2.1

Code

type AB = 'A' | 'B';

type ABMAP = {
    [P in AB]: string
}

const abMap: ABMAP = {
    'A': 'a',
    'B': 'b'
}

function log(p: AB) {
    //
}

for (const p in abMap) {
    log(p); // <<-- Argument of type 'string' is not assignable to parameter of type 'AB'.
}

Expected behavior:
p is of type AB.

Actual behavior:
p is of type string

[Jessidhia]

Any object can match your ABMAP type as long as it has at least A and B as keys. There is no exact object type declaration 😢

[nsgundy]

That's a bummer. Any other way for the log function to keep its parameter typed as AB?

[mhegazy]

Same explanation as in #13989

[nsgundy]

Thanks all, closing 👍