rustc fails to see a bound that is present, but also requires that the bound not be present

In certain cases, rustc will emit an error message on what appears to be valid code, claiming that

• a bound that is present is not present, and must be present
• but the bound is present and must not be present

Here is a MWE (or playground):

trait Foo<T> { type Quux; }
trait Bar<B> {
    type Baz;
    fn arthur() where B: Foo<Self::Baz>, B::Quux: Send;
}
impl<B> Bar<B> for () {
    type Baz = ();
    fn arthur() where B: Foo<()>, B::Quux: Send { }
}

This produces the pair of errors:

rustc 1.16.0 (30cf806ef 2017-03-10)
error[E0277]: the trait bound `B: Foo<()>` is not satisfied
 --> <anon>:8:5
  |
8 |     fn arthur() where B: Foo<()>, B::Quux: Send { }
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Foo<()>` is not implemented for `B`
  |
  = help: consider adding a `where B: Foo<()>` bound

error[E0276]: impl has stricter requirements than trait
 --> <anon>:8:5
  |
4 |     fn arthur() where B: Foo<Self::Baz>, B::Quux: Send;
  |     --------------------------------------------------- definition of `arthur` from trait
...
8 |     fn arthur() where B: Foo<()>, B::Quux: Send { }
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ impl has extra requirement `B: Foo<()>`

error: aborting due to 2 previous errors

1. It's interesting to note that the bound B::Quux: Send is an integral part of this MWE: removing it bypasses the error.

   James Kay at 2017-04-06 18:20:32

2. afaict this is caused by our current approach to param env normalization, see https://github.com/rust-lang/rust/blob/c225c45bce099e4ff6c61c73a425326d1625ed2f/compiler/rustc_trait_selection/src/traits/mod.rs#L328-L345

   fixed in the new solver because we defer projection equality there, so the unnormalized B: Foo<Self::Baz> candidate now applies to the B: Foo<()> goal.

   lcnr at 2023-11-13 11:43:56