1 files changed, 35 insertions, 0 deletions

diff --git a/src/test/ui/issues/issue-31299.rs b/src/test/ui/issues/issue-31299.rs
new file mode 100644
index 000000000..78c3252d3
--- /dev/null
+++ b/src/test/ui/issues/issue-31299.rs
@@ -0,0 +1,35 @@
+// run-pass
+// Regression test for #31299. This was generating an overflow error
+// because of eager normalization:
+//
+// proving `M: Sized` requires
+// - proving `PtrBack<Vec<M>>: Sized` requires
+//   - normalizing `Vec<<Vec<M> as Front>::Back>>: Sized` requires
+//     - proving `Vec<M>: Front` requires
+//       - `M: Sized` <-- cycle!
+//
+// If we skip the normalization step, though, everything goes fine.
+//
+// This could be fixed by implementing lazy normalization everywhere.
+//
+// However, we want this to work before then. For that, when checking
+// whether a type is Sized we only check that the tails are Sized. As
+// PtrBack does not have a tail, we don't need to normalize anything
+// and this compiles
+
+trait Front {
+    type Back;
+}
+
+impl<T> Front for Vec<T> {
+    type Back = Vec<T>;
+}
+
+struct PtrBack<T: Front>(#[allow(unused_tuple_struct_fields)] Vec<T::Back>);
+
+struct M(#[allow(unused_tuple_struct_fields)] PtrBack<Vec<M>>);
+
+#[allow(unused_must_use)]
+fn main() {
+    std::mem::size_of::<M>();
+}