1 files changed, 39 insertions, 0 deletions
diff --git a/src/test/ui/auto-traits/typeck-auto-trait-no-supertraits.rs b/src/test/ui/auto-traits/typeck-auto-trait-no-supertraits.rs
new file mode 100644
index 000000000..2a76893fe
--- /dev/null
+++ b/src/test/ui/auto-traits/typeck-auto-trait-no-supertraits.rs
@@ -0,0 +1,39 @@
+// This test is for #29859, we need to ensure auto traits,
+// (also known previously as default traits), do not have
+// supertraits. Since the compiler synthesizes these
+// instances on demand, we are essentially enabling
+// users to write axioms if we view trait selection,
+// as a proof system.
+//
+// For example the below test allows us to add the rule:
+//  forall (T : Type), T : Copy
+//
+// Providing a copy instance for *any* type, which
+// is most definitely unsound. Imagine copying a
+// type that contains a mutable reference, enabling
+// mutable aliasing.
+//
+// You can imagine an even more dangerous test,
+// which currently compiles on nightly.
+//
+// fn main() {
+//    let mut i = 10;
+//    let (a, b) = copy(&mut i);
+//    println!("{:?} {:?}", a, b);
+// }
+
+#![feature(auto_traits)]
+#![feature(negative_impls)]
+
+auto trait Magic: Copy {} //~ ERROR E0568
+impl<T:Magic> Magic for T {}
+
+fn copy<T: Magic>(x: T) -> (T, T) { (x, x) }
+
+#[derive(Debug)]
+struct NoClone;
+
+fn main() {
+    let (a, b) = copy(NoClone);
+    println!("{:?} {:?}", a, b);
+}