1 files changed, 43 insertions, 0 deletions
diff --git a/src/test/ui/associated-types/normalize-cycle-in-eval.rs b/src/test/ui/associated-types/normalize-cycle-in-eval.rs
new file mode 100644
index 000000000..dff4c9051
--- /dev/null
+++ b/src/test/ui/associated-types/normalize-cycle-in-eval.rs
@@ -0,0 +1,43 @@
+// regression test for #74868
+
+// check-pass
+
+trait BoxedDsl<'a> {
+    type Output;
+}
+
+impl<'a, T> BoxedDsl<'a> for T
+where
+    T: BoxedDsl<'a>,
+{
+    type Output = <T as BoxedDsl<'a>>::Output;
+}
+
+// Showing this trait is wf requires proving
+// Self: HandleUpdate
+//
+// The impl below is a candidate for this projection, as well as the `Self:
+// HandleUpdate` bound in the environment.
+// We evaluate both candidates to see if we need to consider both applicable.
+// Evaluating the impl candidate requires evaluating
+// <T as BoxedDsl<'static>>::Output == ()
+// The above impl cause normalizing the above type normalize to itself.
+//
+// This previously compiled because we would generate a new region
+// variable each time around the cycle, and evaluation would eventually return
+// `EvaluatedToErr` from the `Self: Sized` in the impl, which would in turn
+// leave the bound as the only candidate.
+//
+// #73452 changed this so that region variables are canonicalized when we
+// normalize, which means that the projection cycle is detected before
+// evaluation returns EvaluatedToErr. The cycle resulted in an error being
+// emitted immediately, causing this to fail to compile.
+//
+// To fix this, normalization doesn't directly emit errors when it finds a
+// cycle, instead letting the caller handle it. This restores the original
+// behavior.
+trait HandleUpdate {}
+
+impl<T> HandleUpdate for T where T: BoxedDsl<'static, Output = ()> {}
+
+fn main() {}