Add tests for untested capabilities

tests/ui/associated-type-bounds/higher-ranked.rs

@@ -0,0 +1,17 @@
+// check-pass
+
+#![feature(associated_type_bounds)]
+
+trait A<'a> {
+    type Assoc: ?Sized;
+}
+
+impl<'a> A<'a> for () {
+    type Assoc = &'a ();
+}
+
+fn hello() -> impl for<'a> A<'a, Assoc: Sized> {
+    ()
+}
+
+fn main() {}

tests/ui/associated-type-bounds/nested-bounds-dont-eliminate-alias-bounds.rs

@@ -0,0 +1,37 @@
+// check-pass
+
+#![feature(associated_type_bounds)]
+
+trait Trait1 {
+    type Assoc1: Bar;
+
+    fn assoc(self) -> Self::Assoc1;
+}
+
+impl Trait1 for () {
+    type Assoc1 = ();
+    fn assoc(self) {}
+}
+
+trait Foo {}
+impl Foo for () {}
+trait Bar {}
+impl Bar for () {}
+
+fn hello() -> impl Trait1<Assoc1: Foo> {
+    ()
+}
+
+fn world() {
+    // Tests that `Assoc1: Foo` bound in the RPIT doesn't disqualify
+    // the `Assoc1: Bar` bound in the item, as a nested RPIT desugaring
+    // would do.
+
+    fn is_foo(_: impl Foo) {}
+    is_foo(hello().assoc());
+
+    fn is_bar(_: impl Bar) {}
+    is_bar(hello().assoc());
+}
+
+fn main() {}