Add test for a still ambiguous scenario

Author: spastorino

src/test/ui/associated-type-bounds/ambiguous-associated-type2.rs

@@ -0,0 +1,10 @@
+trait Foo {
+    type Item;
+}
+trait Bar<T> {
+    type Item;
+}
+trait Baz: Foo + Bar<Self::Item> {}
+//~^ ERROR cycle detected when computing the supertraits of `Baz` [E0391]
+
+fn main() {}

src/test/ui/associated-type-bounds/ambiguous-associated-type2.stderr

@@ -0,0 +1,16 @@
+error[E0391]: cycle detected when computing the supertraits of `Baz`
+  --> $DIR/ambiguous-associated-type2.rs:7:1
+   |
+LL | trait Baz: Foo + Bar<Self::Item> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: ...which again requires computing the supertraits of `Baz`, completing the cycle
+note: cycle used when computing the supertraits of `Baz`
+  --> $DIR/ambiguous-associated-type2.rs:7:1
+   |
+LL | trait Baz: Foo + Bar<Self::Item> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.