Add broken test for AFIT with RPITIT

src/test/ui/impl-trait/in-trait/default-body-with-rpit.rs

@@ -0,0 +1,21 @@
+// known-bug: #102688
+// edition:2021
+
+#![feature(async_fn_in_trait, return_position_impl_trait_in_trait)]
+#![allow(incomplete_features)]
+
+use std::fmt::Debug;
+
+trait Foo {
+    async fn baz(&self) -> impl Debug {
+        ""
+    }
+}
+
+struct Bar;
+
+impl Foo for Bar {}
+
+fn main() {
+    let _ = Bar.baz();
+}

src/test/ui/impl-trait/in-trait/default-body-with-rpit.stderr

@@ -0,0 +1,12 @@
+error[E0720]: cannot resolve opaque type
+  --> $DIR/default-body-with-rpit.rs:10:28
+   |
+LL |     async fn baz(&self) -> impl Debug {
+   |                            ^^^^^^^^^^ cannot resolve opaque type
+   |
+   = note: these returned values have a concrete "never" type
+   = help: this error will resolve once the item's body returns a concrete type
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0720`.