Associated Type bound in GAT Parameter incorrectly errors as unsatisfied

[T0mstone]

I tried this code:

trait TypeFn<T> {
    type Output;
}
trait Boo {
    type Arg;
    type CallCopy<F: TypeFn<Self::Arg, Output: Copy>>;
}
impl Boo for () {
    type Arg = ();
    type CallCopy<F: TypeFn<Self::Arg, Output: Copy>> = F;
}

I expected to see this happen: The code compiles.

Instead, this happened: The code errored with

error[E0277]: the trait bound `F: TypeFn<()>` is not satisfied
  --> error.rs:10:5
   |
10 |     type CallCopy<F: TypeFn<Self::Arg, Output: Copy>> = F;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `TypeFn<()>` is not implemented for `F`

error[E0276]: impl has stricter requirements than trait
  --> error.rs:10:22
   |
6  |     type CallCopy<F: TypeFn<Self::Arg, Output: Copy>>;
   |     ------------------------------------------------- definition of `CallCopy` from trait
...
10 |     type CallCopy<F: TypeFn<Self::Arg, Output: Copy>> = F;
   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ impl has extra requirement `F: TypeFn<()>`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0276, E0277.
For more information about an error, try `rustc --explain E0276`.

Meta

rustc --version --verbose:

rustc 1.88.0 (6b00bc388 2025-06-23)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: x86_64-unknown-linux-gnu
release: 1.88.0
LLVM version: 20.1.5

The error persists on nightly:

rustc 1.90.0-nightly (e46629662 2025-07-17)
binary: rustc
commit-hash: e4662966273ed58b51f9ff8d682accc202aa1210
commit-date: 2025-07-17
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.8

[T0mstone]

Notably, removing Output: Copy makes the error disappear.

[compiler-errors]

This is one of the many "param env normalization" bugs that are fixed by the new trait solver.