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
 }

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

@@ -1,5 +1,5 @@
 error[E0191]: the value of the associated type `Output` (from the trait `Base`) must be specified
-  --> $DIR/trait-object-with-self-in-projection-output-bad.rs:20:17
+  --> $DIR/trait-object-with-self-in-projection-output-bad.rs:45:17
    |
 LL |     type Output;
    |     ------------ `Output` defined here
@@ -7,6 +7,15 @@ LL |     type Output;
 LL |     let _x: Box<dyn Helper<Target=i32>> = Box::new(2u32);
    |                 ^^^^^^^^^^^^^^^^^^^^^^ associated type `Output` must be specified
 
-error: aborting due to previous error
+error[E0191]: the value of the associated type `Output` (from the trait `Base`) must be specified
+  --> $DIR/trait-object-with-self-in-projection-output-bad.rs:48:17
+   |
+LL |     type Output;
+   |     ------------ `Output` defined here
+...
+LL |     let _y: Box<dyn NormalizableHelper<Target=i32>> = Box::new(2u32);
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ associated type `Output` must be specified
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0191`.

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

@@ -1,5 +1,10 @@
 // compile-pass
 
+// Regression test related to #56288. Check that a supertrait projection (of
+// `Output`) that references `Self` can be ok if it is referencing a projection (of
+// `Self::Target`, in this case). Note that we still require the user to manually
+// specify both `Target` and `Output` for now.
+
 trait Base {
     type Output;
 }

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

@@ -0,0 +1,48 @@
+// compile-pass
+
+// Regression test related to #56288. Check that a supertrait projection (of
+// `Output`) that references `Self` is ok if there is another occurence of
+// the same supertrait that specifies the projection explicitly, even if
+// the projection's associated type is not explicitly specified in the object type.
+//
+// Note that in order for this to compile, we need the `Self`-referencing projection
+// to normalize fairly directly to a concrete type, otherwise the trait resolver
+// will hate us.
+//
+// There is a test in `trait-object-with-self-in-projection-output-bad.rs` that
+// having a normalizing, but `Self`-containing projection does not *by itself*
+// allow you to avoid writing the projected type (`Output`, in this example)
+// explicitly.
+
+trait ConstI32 {
+    type Out;
+}
+
+impl<T: ?Sized> ConstI32 for T {
+    type Out = i32;
+}
+
+trait Base {
+    type Output;
+}
+
+trait NormalizingHelper: Base<Output=<Self as ConstI32>::Out> + Base<Output=i32> {
+    type Target;
+}
+
+impl Base for u32
+{
+    type Output = i32;
+}
+
+impl NormalizingHelper for u32
+{
+    type Target = i32;
+}
+
+fn main() {
+    // Make sure this works both with and without the associated type
+    // being specified.
+    let _x: Box<dyn NormalizingHelper<Target=i32>> = Box::new(2u32);
+    let _y: Box<dyn NormalizingHelper<Target=i32, Output=i32>> = Box::new(2u32);
+}