Add test for issue 28994

Author: clubby789

tests/ui/traits/fn-pointer/hrtb-assoc-fn-traits-28994.rs

@@ -0,0 +1,22 @@
+//@ check-pass
+//! Tests that a HRTB + FnOnce bound involving an associated type don't prevent
+//! a function pointer from implementing `Fn` traits.
+//! Test for <https://github.com/rust-lang/rust/issues/28994>
+
+trait LifetimeToType<'a> {
+    type Out;
+}
+
+impl<'a> LifetimeToType<'a> for () {
+    type Out = &'a ();
+}
+
+fn id<'a>(val: &'a ()) -> <() as LifetimeToType<'a>>::Out {
+    val
+}
+
+fn assert_fn<F: for<'a> FnOnce(&'a ()) -> <() as LifetimeToType<'a>>::Out>(_func: F) { }
+
+fn main() {
+    assert_fn(id);
+}