1 files changed, 33 insertions, 0 deletions
diff --git a/tests/ui/auto-traits/issue-83857-ub.rs b/tests/ui/auto-traits/issue-83857-ub.rs
new file mode 100644
index 000000000..626e60c37
--- /dev/null
+++ b/tests/ui/auto-traits/issue-83857-ub.rs
@@ -0,0 +1,33 @@
+#![allow(suspicious_auto_trait_impls)]
+// Tests that we don't incorrectly allow overlap between a builtin auto trait
+// impl and a user written one. See #83857 for more details
+
+struct Always<T, U>(T, U);
+unsafe impl<T, U> Send for Always<T, U> {}
+struct Foo<T, U>(Always<T, U>);
+
+trait False {}
+unsafe impl<U: False> Send for Foo<u32, U> {}
+
+trait WithAssoc {
+    type Output;
+}
+impl<T: Send> WithAssoc for T {
+    type Output = Self;
+}
+impl WithAssoc for Foo<u32, ()> {
+    type Output = Box<i32>;
+}
+
+fn generic<T, U>(v: Foo<T, U>, f: fn(<Foo<T, U> as WithAssoc>::Output) -> i32) {
+    //~^ ERROR `Foo<T, U>` cannot be sent between threads safely
+    f(foo(v));
+}
+
+fn foo<T: Send>(x: T) -> <T as WithAssoc>::Output {
+    x
+}
+
+fn main() {
+    generic(Foo(Always(0, ())), |b| *b);
+}