Making union types even more useful

[smarter]

@megri

However, despite the compiler telling us that the Y branch is unchecked, it still infers the union for a “naked” match

This happens because exhaustivity checking is run at a later phase than typechecking. Trying to move some of the logic from exhaustivity checking into the typer to give more precise types to the default case branch would be an interesting thing to try but no on has attempted that experiment so far.