make { a: string, b: number } & { a: number } resolve to never

[devanshj]

This should compile... Or at least find a good reason why it should not :P

import { p, pa } from "@sthir/predicate"

let x = {} as { a: number; b: string } | { a: string; b: number }
if (pa(x, p(".a typeof ===", "number"))) {
  let test: string = x.b    
  //  ~~~~
  // Type 'string | number' is not assignable to type 'string'.
}

[devanshj]

Previously we'd normalize an object to never only if all values were never, but now we do that if we find any value that is never, which is theoretically correct, so I hope there wouldn't be any regressions :P

Although some people use never as "not allowed" as in { a: string, b?: never }, so if they have objects like this in the tree they'd just disappear. In order to prevent this we could only check for never produced by intersections, ie normalizing { a: string, b: number } & { a: number } to never is fine, but normalizing { a: never, b: number } to never is not ideal.

Opened #16.