From 465e373542b529055cd1302849f79db13a617a98 Mon Sep 17 00:00:00 2001
From: lcnr <rust@lcnr.de>
Date: Tue, 23 Sep 2025 07:55:43 +0200
Subject: [PATCH] add regression test

---
 tests/ui/methods/overflow-if-subtyping.rs | 30 +++++++++++++++++++++++
 1 file changed, 30 insertions(+)
 create mode 100644 tests/ui/methods/overflow-if-subtyping.rs

diff --git a/tests/ui/methods/overflow-if-subtyping.rs b/tests/ui/methods/overflow-if-subtyping.rs
new file mode 100644
index 0000000000000..a97f29f1f6dfc
--- /dev/null
+++ b/tests/ui/methods/overflow-if-subtyping.rs
@@ -0,0 +1,30 @@
+//@ check-pass
+
+// Regression test for #128887.
+#![allow(unconditional_recursion)]
+trait Mappable<T> {
+    type Output;
+}
+
+trait Bound<T> {}
+// Deleting this impl made it compile on beta
+impl<T> Bound<T> for T {}
+
+trait Generic<M> {}
+
+// Deleting the `: Mappable<T>` already made it error on stable.
+struct IndexWithIter<I, M: Mappable<T>, T>(I, M, T);
+
+impl<I, M, T> IndexWithIter<I, M, T>
+where
+    <M as Mappable<T>>::Output: Bound<T>,
+    // Flipping these where bounds causes this to succeed, even when removing
+    // the where-clause on the struct definition.
+    M: Mappable<T>,
+    I: Generic<M>,
+{
+    fn new(x: I) {
+        IndexWithIter::<_, _, _>::new(x);
+    }
+}
+fn main() {}