Check for opaque types behind projections

[oli-obk]

Fixes #99840

I still dislike how fragile this is, but integrating into the regular orphan check doesn't really make sense either, it's not about orphan checking at all.

r? @lcnr

[lcnr]

that approach doesn't work i think, consider

#![feature(type_alias_impl_trait)]
trait Foo {
    type Assoc;
}

impl Foo for i32 {
    type Assoc = u32;
}
type ImplTrait = impl Sized;
fn constrain() -> ImplTrait {
    1u64
}
impl Foo for i64 {
    type Assoc = ImplTrait;
}

trait Bar<T> {}

impl<T: Foo> Bar<<T as Foo>::Assoc> for T {}

i don't think we can avoid opaque types during coherence. I think we should allow them but treat them as opaque so that they block impls for any other type which i added to #99840 (comment) as an edit 😁

[lcnr]

for #76202 we have to not return a SimplifiedType for opaque types in impls so that they're added to blanket_impls.

not sure about #86411 but it seems fixable and #84660 is caused by opaque types not being considered "opaque enough" i think

[lqd]

if this approach is not going to work or land soon, should we revert #99383 in the meantime ?

[oli-obk]

if this approach is not going to work or land soon, should we revert #99383 in the meantime ?

No, this is all behind feature gates, so we're not going to affect stable code.