Normalize bounds fully when checking defaulted types

[tmandry]

When checking that the default type for <T as X>::Y is valid in this example:

trait X { type Y: PartialEq<<Self as X>::Y> }
impl X for T { default type Y = S; }

We will have to prove the bound S: PartialEq<<T as X>::Y>. In this case
we want <T as X>::Y to normalize to S. This is valid because we are
checking the default value specifically here. Add <T as X>::Y = S to the
ParamEnv for normalization of the bound we are checking only.

Fixes #73818.

---

I noticed that adding this to the env for bounds checking didn't break any tests. Not sure if this is because we can't rely on it to prove anything, or because of missing test coverage.

r? @matthewjasper, @nikomatsakis

[tmandry]

Friendly ping, can I get a review?

[matthewjasper]

Can you try replacing the value of concrete_ty_predicate (line 1277) with predicate.subst(tcx, rebased_substs). Most of the code there is manually normalizing things that should now be normalizable using the additional predicate.

[tmandry]

That worked!

Note that I now add the predicate unconditionally (removing the branch on is_final()) to avoid regressing an error message for a GAT outlives bound. Details are in the commit history.

[matthewjasper]

@bors r+

[bors]

📌 Commit e35d286 has been approved by matthewjasper

[bors]

⌛ Testing commit e35d286 with merge bb85981...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: matthewjasper
Pushing bb85981 to master...