Record coroutine kind in generics

tests/ui/coroutine/polymorphize-args.rs

@@ -0,0 +1,17 @@
+// compile-flags: -Zpolymorphize=on
+// build-pass
+
+#![feature(coroutines, coroutine_trait)]
+
+use std::ops::Coroutine;
+use std::pin::Pin;
+use std::thread;
+
+fn main() {
+    let mut foo = || yield;
+    thread::spawn(move || match Pin::new(&mut foo).resume(()) {
+        s => panic!("bad state: {:?}", s),
+    })
+    .join()
+    .unwrap();
+}