1 files changed, 34 insertions, 0 deletions
diff --git a/src/test/ui/auto-traits/auto-trait-projection-recursion.rs b/src/test/ui/auto-traits/auto-trait-projection-recursion.rs
new file mode 100644
index 000000000..a36f26f02
--- /dev/null
+++ b/src/test/ui/auto-traits/auto-trait-projection-recursion.rs
@@ -0,0 +1,34 @@
+// Checking the `Send` bound in `main` requires:
+//
+// checking             <C<'static> as Y>::P: Send
+// which normalizes to  Box<X<C<'?1>>>: Send
+// which needs          X<C<'?1>>: Send
+// which needs          <C<'?1> as Y>::P: Send
+//
+// At this point we used to normalize the predicate to `Box<X<C<'?2>>>: Send`
+// and continue in a loop where we created new region variables to the
+// recursion limit. To avoid this we now "canonicalize" region variables to
+// lowest unified region vid. This means we instead have to prove
+// `Box<X<C<'?1>>>: Send`, which we can because auto traits are coinductive.
+
+// check-pass
+
+// Avoid a really long error message if this regresses.
+#![recursion_limit="20"]
+
+trait Y {
+    type P;
+}
+
+impl<'a> Y for C<'a> {
+    type P = Box<X<C<'a>>>;
+}
+
+struct C<'a>(&'a ());
+struct X<T: Y>(T::P);
+
+fn is_send<S: Send>() {}
+
+fn main() {
+    is_send::<X<C<'static>>>();
+}