Type inference on ternary-selected promises fails

[calebegg]

Example:

let x = {then(cb: (x: string)=>any) { cb('x'); }};
let y = {then(cb: (x: Object)=>any) { cb({}); }};

(Math.random() < .5 ? x : y).then((z) => z); // z is string
(Math.random() < .5 ? y : x).then((z) => z); // z is Object

z should probably be string | Object in both cases.

Might be related, but if I have x: number in the second function above, the last lines fail with "Cannot invoke an expression whose type lacks a call signature", which is weird.

[RyanCavanaugh]

In general you should never declare things as Object, it rarely does what you want (for example, string is assignable to Object). See #1809.

The primary issue is that Object is a supertype of string, and function parameters are bivariant (see the entry in the FAQ), so the type of x is assignable to y and the type of y is assignable to x, so we're basically just picking the first one here when resolving the type of expr ? x: y.

The related issue is because call signatures don't appear on union types unless the signatures are identical. See #5640