fixup! Fix replacing supertrait aliases in ReplaceProjectionWith

Author: compiler-errors

Diff for: compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs

@@ -955,7 +955,6 @@ where
             // If there's more than one projection that we can unify here, then we
             // need to stall until inference constrains things so that there's only
             // one choice.
-            // println!("ambiguity!");
             return Err(Ambiguous);
         }
 

Diff for: tests/ui/traits/next-solver/supertrait-alias-4.rs

@@ -0,0 +1,24 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+// Exercises the ambiguity that comes from replacing the associated types within the bounds
+// that are required for a `impl Trait for dyn Trait` built-in object impl to hold.
+
+trait Sup<T> {
+    type Assoc;
+}
+
+trait Foo<A, B>: Sup<A, Assoc = A> + Sup<B, Assoc = B> {
+    type Other: Bar<<Self as Sup<A>>::Assoc>;
+}
+
+trait Bar<T> {}
+impl Bar<i32> for () {}
+
+fn foo<A, B>(x: &(impl Foo<A, B> + ?Sized)) {}
+
+fn main() {
+    let x: &dyn Foo<_, _, Other = ()> = todo!();
+    foo(x);
+    let y: &dyn Foo<i32, u32, Other = ()> = x;
+}