improve tests as suggested by review comments

src/test/ui/traits/trait-object-with-self-in-projection-output-bad.rs

@@ -1,3 +1,7 @@
+// Regression test for #56288. Checks that if a supertrait defines an associated type
+// projection that references `Self`, then that associated type must still be explicitly
+// specified in the `dyn Trait` variant, since we don't know what `Self` is anymore.
+
 trait Base {
     type Output;
 }
@@ -16,7 +20,31 @@ impl Helper for u32
     type Target = i32;
 }
 
+trait ConstI32 {
+    type Out;
+}
+
+impl<T: ?Sized> ConstI32 for T {
+    type Out = i32;
+}
+
+// Test that you still need to manually give a projection type if the Output type
+// is normalizable.
+trait NormalizableHelper:
+    Base<Output=<Self as ConstI32>::Out>
+{
+    type Target;
+}
+
+impl NormalizableHelper for u32
+{
+    type Target = i32;
+}
+
 fn main() {
     let _x: Box<dyn Helper<Target=i32>> = Box::new(2u32);
     //~^ ERROR the value of the associated type `Output` (from the trait `Base`) must be specified
+
+    let _y: Box<dyn NormalizableHelper<Target=i32>> = Box::new(2u32);
+    //~^ ERROR the value of the associated type `Output` (from the trait `Base`) must be specified
 }