Add regression test for #80409

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

Author: JohnTitor

Diff for: tests/ui/inference/issue-80409.rs

@@ -0,0 +1,36 @@
+// check-pass
+
+#![allow(unreachable_code, unused)]
+
+use std::marker::PhantomData;
+
+struct FsmBuilder<TFsm> {
+    _fsm: PhantomData<TFsm>,
+}
+
+impl<TFsm> FsmBuilder<TFsm> {
+    fn state(&mut self) -> FsmStateBuilder<TFsm> {
+        todo!()
+    }
+}
+
+struct FsmStateBuilder<TFsm> {
+    _state: PhantomData<TFsm>,
+}
+
+impl<TFsm> FsmStateBuilder<TFsm> {
+    fn on_entry<TAction: Fn(&mut StateContext<'_, TFsm>)>(&self, _action: TAction) {}
+}
+
+trait Fsm {
+    type Context;
+}
+
+struct StateContext<'a, TFsm: Fsm> {
+    context: &'a mut TFsm::Context,
+}
+
+fn main() {
+    let mut builder: FsmBuilder<usize> = todo!();
+    builder.state().on_entry(|_| {});
+}