Fix an assertion failure in bind_generator_hidden_types_above.

Fixes rust-lang#110941.

Author: nnethercote

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -3016,6 +3016,7 @@ fn bind_generator_hidden_types_above<'tcx>(
                         counter += 1;
                         tcx.mk_re_late_bound(current_depth, br)
                     }
+                    ty::RePlaceholder(_) => r,
                     r => bug!("unexpected region: {r:?}"),
                 })
             }

tests/ui/regions/issue-110941.rs

@@ -0,0 +1,37 @@
+// run-pass
+// compile-flags: --edition=2018 -Zdrop-tracking-mir=yes
+
+use std::future::{Future, Ready};
+async fn read() {}
+async fn connect<A: ToSocketAddr>(addr: A) {
+    let _ = addr.to_socket_addr().await;
+}
+pub trait ToSocketAddr {
+    type Future: Future;
+    fn to_socket_addr(&self) -> Self::Future;
+}
+impl ToSocketAddr for &() {
+    type Future = Ready<()>;
+    fn to_socket_addr(&self) -> Self::Future {
+        unimplemented!()
+    }
+}
+struct Server;
+impl Server {
+    fn and_then<F>(self, _fun: F) -> AndThen<F> {
+        unimplemented!()
+    }
+}
+struct AndThen<F> {
+    _marker: std::marker::PhantomData<F>,
+}
+pub async fn run<F>(_: F) {
+    let _ = connect(&()).await;
+}
+fn main() {
+    let _ = async {
+        let server = Server;
+        let verification_route = server.and_then(read);
+        run(verification_route).await;
+    };
+}