From 21b9f5c3bb55a8be44fe277891dd1c77fd6ca782 Mon Sep 17 00:00:00 2001
From: whtahy <whtahy@users.noreply.github.com>
Date: Wed, 26 Apr 2023 20:48:38 -0400
Subject: [PATCH] add known-bug test for unsound issue 74629

---
 .../coherence-overlap-negative-impls.rs       | 41 +++++++++++++++++++
 1 file changed, 41 insertions(+)
 create mode 100644 tests/ui/coherence/coherence-overlap-negative-impls.rs

diff --git a/tests/ui/coherence/coherence-overlap-negative-impls.rs b/tests/ui/coherence/coherence-overlap-negative-impls.rs
new file mode 100644
index 00000000000..cd1df53a528
--- /dev/null
+++ b/tests/ui/coherence/coherence-overlap-negative-impls.rs
@@ -0,0 +1,41 @@
+// check-pass
+// known-bug: #74629
+
+// Should fail. The `0` and `1` impls overlap, violating coherence. Eg, with
+// `T = Test, F = ()`, all bounds are true, making both impls applicable.
+// `Test: Fold<Nil>`, `Test: Fold<()>` are true because of `2`.
+// `Is<Test>: NotNil` is true because of `auto trait` and lack of negative impl.
+
+#![feature(negative_impls)]
+#![feature(auto_traits)]
+
+struct Nil;
+struct Cons<H>(H);
+struct Test;
+
+trait Fold<F> {}
+
+impl<T, F> Fold<F> for Cons<T> // 0
+where
+    T: Fold<Nil>,
+{}
+
+impl<T, F> Fold<F> for Cons<T> // 1
+where
+    T: Fold<F>,
+    private::Is<T>: private::NotNil,
+{}
+
+impl<F> Fold<F> for Test {} // 2
+
+mod private {
+    use crate::Nil;
+
+    pub struct Is<T>(T);
+    pub auto trait NotNil {}
+
+    #[allow(suspicious_auto_trait_impls)]
+    impl !NotNil for Is<Nil> {}
+}
+
+fn main() {}