Premature literal type widening?

[ethanresnick]

TypeScript Version: 2.4.1

Code

type AorBValuedObj = { 'test': 'a' | 'b' };

const makeTestObj = () => {
    const x = (<boolean>true) ? 'a' : 'b';
    return { 'test': x };
};

const x2: AorBValuedObj = makeTestObj();

Expected behavior:
Assignment to x2 succeeds.

Actual behavior:
Assignment fails because the result type of makeTestObj is {'test': string} rather than {'test': 'a' | 'b' }.

I vaguely recall that the rules for widening depend on whether a value is mutable, so my guess is that the widening is happening on the property's type because properties are mutable. Still, would it make sense not to widen here? Or maybe the current behavior is good, since it forces me declare explicitly that makeTestObj returns an AorBValuedObj?

[ahejlsberg]

This is working as intended. In makeTestObj you need some indication that you don't want the test property widened (e.g. a return type annotation or a type assertion on the returned value). Absent that, the compiler widens the type because a property is a mutable location.

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.