Add a test showing that a similar example compiles

src/test/ui/impl-trait/nested-return-type3.rs

@@ -0,0 +1,20 @@
+// check-pass
+
+trait Duh {}
+
+impl Duh for i32 {}
+
+trait Trait {
+    type Assoc: Duh;
+}
+
+impl<F: Duh> Trait for F {
+    type Assoc = F;
+}
+
+fn foo() -> impl Trait<Assoc = impl Send> {
+    42
+}
+
+fn main() {
+}