Wrong union type matching

[dfilatov]

TypeScript Version: 2.3.2

Code

type Action = { type: 'activate' } | { type: 'disactivate' };

function dispatchAction(action: Action): void {

}

const active = true;

dispatchAction({ type : (active? 'disactivate' : 'activate') });

Expected behavior:
Everything is correct. And if I change last line to:

dispatchAction(active? { type : 'disactivate' } : { type : 'activate' });

then no errors will be produced.

Actual behavior:
Error:
Argument of type '{ type: "activate" | "disactivate"; }' is not assignable to parameter of type 'Action'.
Type '{ type: "activate" | "disactivate"; }' is not assignable to type '{ type: "disactivate"; }'.
Types of property 'type' are incompatible.
Type '"activate" | "disactivate"' is not assignable to type '"disactivate"'.
Type '"activate"' is not assignable to type '"disactivate"'.

[ahejlsberg]

This is working as intended. I would suggest writing the declaration of type Action as follows to clearly indicate that the type property has two possible values regardless of the rest of the properties:

type Action = { type: 'activate' | 'disactivate' };

In general, the unification you propose is only possible for a small subset of types. See my comment in #12052.

[mhegazy]

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