Add regression test for #104678

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

Author: JohnTitor

src/test/ui/async-await/in-trait/issue-104678.rs

@@ -0,0 +1,31 @@
+// edition:2021
+// check-pass
+
+#![feature(async_fn_in_trait)]
+#![allow(incomplete_features)]
+
+use std::future::Future;
+pub trait Pool {
+    type Conn;
+
+    async fn async_callback<'a, F: FnOnce(&'a Self::Conn) -> Fut, Fut: Future<Output = ()>>(
+        &'a self,
+        callback: F,
+    ) -> ();
+}
+
+pub struct PoolImpl;
+pub struct ConnImpl;
+
+impl Pool for PoolImpl {
+    type Conn = ConnImpl;
+
+    async fn async_callback<'a, F: FnOnce(&'a Self::Conn) -> Fut, Fut: Future<Output = ()>>(
+        &'a self,
+        _callback: F,
+    ) -> () {
+        todo!()
+    }
+}
+
+fn main() {}