discuss projection equality

[BoxyUwU]

Summary

Talk about rust-lang/rust#96912 (and generally how projection equality should look like)

Rationale

for<'a> <_ as Trait<'a>>::Assoc: Other bounds resulting in an error is a problem i keep seeing encountered

• came across it in bevy_ecs
• known generic_associated_types issue
• blocker for supporting some kind of F: Fn(T) -> _ for an unconstrained output type as the only reason to want to use -> _ would involve the fn being higher ranked which basically guarantees encountering this bug playground

Also there's a weird hacky change to selection in this PR to fix a typenum regression which definitely seems good to talk about before merged :thinking:

[BoxyUwU]

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