error[E0562]: `impl Trait` is not allowed in paths
  --> $DIR/issue-57979-impl-trait-in-path.rs:8:48
   |
LL | pub fn demo(_: impl Quux<(), Assoc=<() as Quux<impl Bar>>::Assoc>) { }
   |                                                ^^^^^^^^
   |
   = note: `impl Trait` is only allowed in arguments and return types of functions and methods

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0562`.