mGCA: Confusing error on ill-formed const projection (e.g., "expected `usize`, found a different `usize`")

In Const Param Defaults that Reference Params

#![feature(min_generic_const_args)]

use core::direct_const_arg as lift;

struct Ty<T, const N: usize = { lift!(<T as Trait>::N) }>(T);

trait Trait { type const N: usize; }

In the code above, type parameter T doesn't implement trait Trait rendering the const projection <T as Trait>::N ill-formed … except that we intentionally don't check generic parameter defaults for well-formedness if they reference generic parameters (via)[^1]. Therefore, we won't detect & report the trait bound `T: Trait` is not satisfied. That's expected but interestingly we emit a very weird+broken error afterward:

error: the constant `<T as Trait>::N` is not of type `usize`
 --> src/lib.rs:7:14
  |
7 | struct Ty<T, const N: usize = { lift!(<T as Trait>::N) }>(T);
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `usize`, found a different `usize`

Elsewhere

This error gets emitted in (all?) other (relevant) places, too. Consider:

#![feature(min_generic_const_args)]

trait Trait { type const N: usize; }

type const _: usize = <() as Trait>::N;

error[E0277]: the trait bound `(): Trait` is not satisfied
 --> src/lib.rs:5:1
  |
5 | type const _: usize = <() as Trait>::N;
  | ^^^^^^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `()`
  |
help: this trait has no implementations, consider adding one
 --> src/lib.rs:3:1
  |
3 | trait Trait { type const N: usize; }
  | ^^^^^^^^^^^

error: the constant `<() as Trait>::N` is not of type `usize`
 --> src/lib.rs:5:1
  |
5 | type const _: usize = <() as Trait>::N;
  | ^^^^^^^^^^^^^^^^^^^ expected `usize`, found a different `usize`

However, since we do perform well-formedness checking in locations like this, we get a more reasonable error diagnostic alongside the problematic one. So it's only half-bad.

[^1]: If you ask me, I don't like that decision.

Reproduce the examples in src/lib.rs, then start with compiler/rustc_hir_analysis/src/check/wfcheck.rs around the linked generic-parameter-default check. Trace the const projection type-checking and diagnostic path in the reported locations. Done means the ill-formed projection no longer produces the misleading “expected usize, found a different usize” error while the relevant trait-bound diagnostic remains.