1 files changed, 32 insertions, 0 deletions
diff --git a/tests/ui/traits/new-solver/cycles/coinduction/fixpoint-exponential-growth.rs b/tests/ui/traits/new-solver/cycles/coinduction/fixpoint-exponential-growth.rs
new file mode 100644
index 000000000..fcafdcf63
--- /dev/null
+++ b/tests/ui/traits/new-solver/cycles/coinduction/fixpoint-exponential-growth.rs
@@ -0,0 +1,32 @@
+// compile-flags: -Ztrait-solver=next
+
+// Proving `W<?0>: Trait` instantiates `?0` with `(W<?1>, W<?2>)` and then
+// proves `W<?1>: Trait` and `W<?2>: Trait`, resulting in a coinductive cycle.
+//
+// Proving coinductive cycles runs until we reach a fixpoint. This fixpoint is
+// never reached here and each step doubles the amount of nested obligations.
+//
+// This previously caused a hang in the trait solver, see
+// https://github.com/rust-lang/trait-system-refactor-initiative/issues/13.
+
+#![feature(rustc_attrs)]
+
+#[rustc_coinductive]
+trait Trait {}
+
+struct W<T>(T);
+
+impl<T, U> Trait for W<(W<T>, W<U>)>
+where
+    W<T>: Trait,
+    W<U>: Trait,
+{
+}
+
+fn impls<T: Trait>() {}
+
+fn main() {
+    impls::<W<_>>();
+    //~^ ERROR type annotations needed
+    //~| ERROR overflow evaluating the requirement
+}