Lint `type_alias_bounds` fires unnecessarily for trait bounds that define short-hand projections on the RHS about rust HOT 4 OPEN

Thanks for improving my poor issue title 😄 @fmease 🙏

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NB By the way, lazy_type_alias implements the desired semantics. See its tracking issue: #112792.

CC #21903.

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Right, that makes sense (I just had to do some advertising :P).

Yeah, the lint isn't super smart atm. This isn't the only case where bounds on type parameters of (eager) type aliases actually have a user-visible effect. Outlives-bounds for example may affect trait object lifetime defaults (we have an A-lint issue for that somewhere).

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Good to know there is progress on the "make type aliases work as expected" front 🙂.

But to avoid confusion, I do want to clarify that this is not what this issue is about, this one is a diagnostics issue, w.r.t. how type aliases currently work. That is, when using <T as Trait>::Assoc in the rhs of a type alias, this already, currently, enforces that T : ?Sized + Trait be true (and when Trait : Sized, we end up with T : Trait).
Given that, we should be allowed to add such bounds without having Rust warn us about our bounds being ignored, because given the rhs, they cannot be.

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