error[E0271]: type mismatch resolving `<() as Bar>::Foo == ()`
  --> $DIR/issue-63355.rs:34:20
   |
LL | pub type FooImpl = impl Foo;
   |                    -------- the found opaque type
LL | pub type BarImpl = impl Bar<Foo = FooImpl>;
   |                    ^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found opaque type
   |
   = note: expected unit type `()`
            found opaque type `impl Foo`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0271`.