From 232d685e6149227abb28c8fef05b00c4f50bbd28 Mon Sep 17 00:00:00 2001
From: whtahy <whtahy@users.noreply.github.com>
Date: Tue, 18 Apr 2023 21:30:21 -0400
Subject: [PATCH] add known-bug test for unsound issue 57893

---
 .../indirect-impl-for-trait-obj-coherence.rs  | 25 +++++++++++++++++++
 1 file changed, 25 insertions(+)
 create mode 100644 tests/ui/coherence/indirect-impl-for-trait-obj-coherence.rs

diff --git a/tests/ui/coherence/indirect-impl-for-trait-obj-coherence.rs b/tests/ui/coherence/indirect-impl-for-trait-obj-coherence.rs
new file mode 100644
index 00000000000..bb46498f90e
--- /dev/null
+++ b/tests/ui/coherence/indirect-impl-for-trait-obj-coherence.rs
@@ -0,0 +1,25 @@
+// check-pass
+// known-bug: #57893
+
+// Should fail. Because we see an impl that uses a certain associated type, we
+// type-check assuming that impl is used. However, this conflicts with the
+// "implicit impl" that we get for trait objects, violating coherence.
+
+trait Object<U> {
+    type Output;
+}
+
+impl<T: ?Sized, U> Object<U> for T {
+    type Output = U;
+}
+
+fn foo<T: ?Sized, U>(x: <T as Object<U>>::Output) -> U {
+    x
+}
+
+#[allow(dead_code)]
+fn transmute<T, U>(x: T) -> U {
+    foo::<dyn Object<U, Output = T>, U>(x)
+}
+
+fn main() {}