1 files changed, 35 insertions, 0 deletions
diff --git a/tests/ui/traits/trait-upcasting/deref-upcast-behavioral-change.rs b/tests/ui/traits/trait-upcasting/deref-upcast-behavioral-change.rs
new file mode 100644
index 000000000..e4784fa41
--- /dev/null
+++ b/tests/ui/traits/trait-upcasting/deref-upcast-behavioral-change.rs
@@ -0,0 +1,35 @@
+#![deny(deref_into_dyn_supertrait)]
+use std::ops::Deref;
+
+trait Bar<T> {}
+impl<T, U> Bar<U> for T {}
+
+trait Foo: Bar<i32> {
+    fn as_dyn_bar_u32<'a>(&self) -> &(dyn Bar<u32> + 'a);
+}
+
+impl Foo for () {
+    fn as_dyn_bar_u32<'a>(&self) -> &(dyn Bar<u32> + 'a) {
+        self
+    }
+}
+
+impl<'a> Deref for dyn Foo + 'a {
+    //~^ ERROR this `Deref` implementation is covered by an implicit supertrait coercion
+    //~| WARN this will change its meaning in a future release!
+    type Target = dyn Bar<u32> + 'a;
+
+    fn deref(&self) -> &Self::Target {
+        self.as_dyn_bar_u32()
+    }
+}
+
+fn take_dyn<T>(x: &dyn Bar<T>) -> T {
+    todo!()
+}
+
+fn main() {
+    let x: &dyn Foo = &();
+    let y = take_dyn(x);
+    let z: u32 = y;
+}