Fix #20453: Implement safe widening of singleton types

[noti0na1]

Fix #20453

[sjrd]

Attention: Match type reduction is governed by a spec, and any "improvement" is a spec change that can alter erasure of some things, breaking binaries and tasty. Such a change is a no-go by default. Any change needs to be highly motivated and debated.

[noti0na1]

Attention: Match type reduction is governed by a spec, and any "improvement" is a spec change that can alter erasure of some things, breaking binaries and tasty. Such a change is a no-go by default. Any change needs to be highly motivated and debated.

I see. The issue was assigned to me a while ago, so I thought it's just a simple improvement ¯⁠\⁠_⁠(⁠ツ⁠)⁠_⁠/⁠¯

[soronpo]

I think there is enough demand for a separate inline match types feature that act like the macro type match without disjointness restriction. Most user cases and expectations require that behavior, IMO.