Also expand weak alias tys inside consts inside expand_weak_alias_tys

[fmease]

Ever since #121344 has been merged, I couldn't let go of the fear that I might've slipped a tiny bug into rustc (:P).

Checking the type flags of the Const is strictly more correct than only checking the ones of the Const's Ty. I don't think it's possible to trigger an ICE rn (i.e., one of the two bug!("unexpected weak alias type") I added in branches where expand_weak_alias_tys should've expanded all weak alias tys) because presently const exprs aren't allowed to capture late-bound vars. To be future-proof however, we should iron this out.

A possible reproducer would be the following if I'm not mistaken (currently fails to compile due to the aforementioned restriction):

#![feature(lazy_type_alias, adt_const_params, generic_const_exprs)]

type F = for<'a> fn(A<{ S::<Weak<'a>>(loop {}) }>) -> &'a ();

type A<const N: S<Weak<'static>>> = ();

#[derive(PartialEq, Eq, std::marker::ConstParamTy)]
struct S<T>(T);

type Weak<'a> = &'a ();

Whether a late-bound region should actually be considered constrained by a const expr is a separate question — one which we don't need to answer until / unless we actually allow them in such contexts (probable answer: only inside the return exprs of a block but not inside the stmts).

r? oli-obk (he's not available rn but that's fine) or types or compiler

[compiler-errors]

r? compiler-errors @bors r+ rollup

[bors]

📌 Commit 35a5be2 has been approved by compiler-errors

It is now in the queue for this repository.