TAIT fails where RPIT works

[oli-obk]

I tried this code:

trait Duh {}

impl Duh for i32 {}

trait Trait {
    type Assoc: Duh;
}

impl<R: Duh, F: FnMut() -> R> Trait for F {
    type Assoc = R;
}

type Sendable = impl Send;

type Foo = impl Trait<Assoc = Sendable>;

fn foo() -> Foo {
    || 42
}

I expected to see this happen: it compiles, just like

trait Duh {}

impl Duh for i32 {}

trait Trait {
    type Assoc: Duh;
}

impl<R: Duh, F: FnMut() -> R> Trait for F {
    type Assoc = R;
}

type Sendable = impl Send;

fn foo() -> impl Trait<Assoc = Sendable> {
    || 42
}

Instead, this happened:

error[[E0277]](https://doc.rust-lang.org/nightly/error-index.html#E0277): the trait bound `Sendable: Duh` is not satisfied
  --> src/lib.rs:19:5
   |
19 |     || 42
   |     ^^^^^ the trait `Duh` is not implemented for `Sendable`
   |
   = help: the trait `Duh` is implemented for `i32`
note: required for `[closure@src/lib.rs:19:5: 19:7]` to implement `Trait`
  --> src/lib.rs:10:31
   |
10 | impl<R: Duh, F: FnMut() -> R> Trait for F {
   |                               ^^^^^     ^

[ShoyuVanilla]

This seems to have been fixed somehow 🤔
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=f9e8edc6895aa6e5015b103594c8db5c
1.79.0-nightly(2024-03-21 0ad927c0c07b65fc0dae)

[ShoyuVanilla]

Well, I've neglected the warning because it is compiled, but

warning: opaque type `Foo` does not satisfy its associated type bounds
  --> src/main.rs:18:23
   |
9  |     type Assoc: Duh;
   |                 --- this associated type bound is unsatisfied for `Sendable`
...
18 | type Foo = impl Trait<Assoc = Sendable>;
   |                       ^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(opaque_hidden_inferred_bound)]` on by default

the warning is almost same as before (though not a hard error) and the rust_lint doc has the same code in this issue and explains why it should be linted 😅

I'll add a test that addressing these.