trait upcasting doesn't handle duplicate projection bounds

trait Super<T> {
    type Assoc;
}
trait Trait<T, U>: Super<T, Assoc = T> + Super<U, Assoc = U> {}

fn compiles(x: Box<dyn Trait<u32, i32>>) -> Box<dyn Super<u32, Assoc = u32>> {
    x
}

fn does_not(x: Box<dyn Trait<u32, u32>>) -> Box<dyn Super<u32, Assoc = u32>> {
    x // ERROR mismatched types
}

both of these should work or the second one should at least fail with an ambiguity error. It's really weird that we just hard reject this

Start by compiling the provided Rust example and compare the successful and failing trait-upcast cases. Trace the compiler's trait upcasting and duplicate projection-bound handling; done means the second case either upcasts successfully or reports an explicit ambiguity error instead of a type mismatch.