Add weird return types TAIT test

src/test/ui/type-alias-impl-trait/weird-return-types.rs

@@ -0,0 +1,16 @@
+// edition:2018
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+#![allow(dead_code)]
+
+use std::future::Future;
+use std::fmt::Debug;
+
+type Foo = impl Debug;
+
+fn f() -> impl Future<Output = Foo> {
+    async move { 22_u32 }
+}
+
+fn main() {}