Unintuitive error message with 'unreachable' code

TypeScript Version: 2.0.02

Code

const enum Keys {
    Tab = 10,
    Shift = 13
}

function bug() {
    let key: Keys;

    if (key === Keys.Tab) {
        return;
    }

    if (key === Keys.Tab || key === Keys.Shift) {
        console.log('Bug');
    }
}

On line 13 you get an error message that === can’t be applied as an operator. Took me quite some time to find out that the same comparison was above (the real code was a little bit more complicated) and was leaving the function.

[RyanCavanaugh]

The problem is that there’s no never type in play here.

If we wanted to get really fancy, when this kind of check fails, we could recompute the relation using the original declared types instead of the narrowed types. If that relation succeeded, then the error message could say something like

Type `X` is not assignable type `Y`
  Earlier code in this block changed the type of the expressions
  in a way that makes this operation invalid. Do you have a logic error?

[dbaeumer]

+1 for @RyanCavanaugh proposal. Although I would leave the ‘Do you have a logic error’ out.