Add test for known issue with higher-ranked fn ptr in duplicated associated types in dyn.

This issue previously only affected trait aliases. This PR causes
the same issue to also affect supertraits.

Author: theemathas

tests/ui/associated-type-bounds/duplicate-bound-in-dyn-higher-ranked.rs

@@ -0,0 +1,37 @@
+// Alpha-equivalent associated types are currently treated as different types
+// for the purposes of prohibiting conflicting associated types in dyn.
+// This is undesirable, but we don't have a good way of fixing this.
+// See also https://github.com/rust-lang/rust/issues/146548
+
+#![feature(trait_alias)]
+
+trait Trait {
+    type Assoc;
+}
+trait Alias = Trait<Assoc = for<'a> fn(&'a ())>;
+fn via_trait_alias(_: &dyn Alias<Assoc = for<'b> fn(&'b ())>) {}
+//~^ ERROR conflicting associated type bindings for `Assoc`
+
+trait Super<T> {
+    type Assoc;
+}
+trait Sub: Super<i32, Assoc = for<'a> fn(&'a ())> + Super<i64, Assoc = for<'b> fn(&'b ())> {}
+fn via_supertrait(_: &dyn Sub) {}
+//~^ ERROR conflicting associated type bindings for `Assoc`
+
+struct Thing;
+impl Trait for Thing {
+    type Assoc = fn(&());
+}
+impl Super<i32> for Thing {
+    type Assoc = fn(&());
+}
+impl Super<i64> for Thing {
+    type Assoc = fn(&());
+}
+impl Sub for Thing {}
+
+fn main() {
+    via_trait_alias(&Thing);
+    via_supertrait(&Thing);
+}

tests/ui/associated-type-bounds/duplicate-bound-in-dyn-higher-ranked.stderr

@@ -0,0 +1,21 @@
+error: conflicting associated type bindings for `Assoc`
+  --> $DIR/duplicate-bound-in-dyn-higher-ranked.rs:12:24
+   |
+LL | trait Alias = Trait<Assoc = for<'a> fn(&'a ())>;
+   |                     -------------------------- `Assoc` (in `Trait`) is specified to be `for<'a> fn(&'a ())` here
+LL | fn via_trait_alias(_: &dyn Alias<Assoc = for<'b> fn(&'b ())>) {}
+   |                        ^^^^^^^^^^--------------------------^
+   |                                  |
+   |                                  `Assoc` (in `Trait`) is specified to be `for<'b> fn(&'b ())` here
+
+error: conflicting associated type bindings for `Assoc`
+  --> $DIR/duplicate-bound-in-dyn-higher-ranked.rs:19:23
+   |
+LL | fn via_supertrait(_: &dyn Sub) {}
+   |                       ^^^^^^^
+   |
+   = note: `Assoc` (in `Super<i64>`) is specified to be `for<'b> fn(&'b ())`
+   = note: `Assoc` (in `Super<i32>`) is also specified to be `for<'a> fn(&'a ())`
+
+error: aborting due to 2 previous errors
+