Add a test for default trait method with RPITITs

tests/ui/impl-trait/in-trait/default-method-constraint.rs

@@ -0,0 +1,17 @@
+// check-pass
+
+// This didn't work in the previous default RPITIT method hack attempt
+
+#![feature(return_position_impl_trait_in_trait)]
+//~^ WARN the feature `return_position_impl_trait_in_trait` is incomplete
+
+trait Foo {
+    fn bar(x: bool) -> impl Sized {
+        if x {
+            let _: u32 = Self::bar(!x);
+        }
+        Default::default()
+    }
+}
+
+fn main() {}