Bounds on associated types prevent a trait from being dyn-compatible

[RustyYato]

playground

trait Foo {
    type Bar: Other<Self::Bar>;
}

impl dyn Foo<Bar = ()> {}

trait Other<T> {}

impl<T> Other<T> for T {}

Doesn't compile with the error,

error[E0038]: the trait `Foo` cannot be made into an object
 --> src/lib.rs:5:6
  |
5 | impl dyn Foo<Bar = ()> {}
  |      ^^^^^^^^^^^^^^^^^ the trait `Foo` cannot be made into an object
  |
  = note: the trait cannot use `Self` as a type parameter in the supertraits or where-clauses

But this doesn't make sense, because Self (being the type erased thing in dyn Foo) is never referenced in any bound. Self::Bar is referenced, but this should be fine in a dyn-compatible¹ trait because all associated types have to be specified to use the dyn-compatible trait (i.e. I can't do dyn Foo in this case). This can be seen in Iterator which is dyn-compatible.

I think this should work and allow these sorts of bounds.

Note that moving the bound to a function is perfectly fine as seen here, but not on associated types or the trait itself.

I think this may be related to #27675

Footnotes

1. Formerly known as "object safe". ↩