Spurious "bounds [...] not enforced in type aliases" when using TAIT

[Patryk27]

Code

#![feature(type_alias_impl_trait)]

use std::fmt::Debug;

type Foo<T: Debug> = (impl Debug, usize);

fn foo<U: Debug>() -> Foo<U> {
    (Vec::<U>::new(), 1234)
}

Current output

warning: bounds on generic parameters are not enforced in type aliases
 --> src/main.rs:5:13
  |
5 | type Foo<T: Debug> = (impl Debug, usize);
  |             ^^^^^
  |
  = note: `#[warn(type_alias_bounds)]` on by default
help: the bound will not be checked when the type alias is used, and should be removed
  |
5 - type Foo<T: Debug> = (impl Debug, usize);
5 + type Foo<T> = (impl Debug, usize);
  |

Desired output

No warning at all, I think, since the bound is actually checked; applying the suggestion yields code that does not compile.

Rationale and extra context

No response

Other cases

No response

Anything else?

No response

[mu001999]

Currently, this lint passes type X = impl Trait. It seems like there shouldn't be a warning if the type has any opaque types.

[mu001999]

@rustbot claim

[oli-obk]

In #108663 we figured out that we could add ty::AliasKind::Alias which could then represent type aliases properly. But instead of converting all type aliases, which has its own problems, we just do it for type aliases with opaque types, which already have all the restrictions needed to just solve this issue.