1 files changed, 35 insertions, 0 deletions
diff --git a/tests/ui/c-variadic/not-dyn-compatible.rs b/tests/ui/c-variadic/not-dyn-compatible.rs
new file mode 100644
index 00000000000..b40a13e5847
--- /dev/null
+++ b/tests/ui/c-variadic/not-dyn-compatible.rs
@@ -0,0 +1,35 @@
+// Traits where a method is c-variadic are not dyn compatible.
+//
+// Creating a function pointer from a method on an `&dyn T` value creates a ReifyShim.
+// This shim cannot reliably forward C-variadic arguments. Thus the trait as a whole
+// is dyn-incompatible to prevent invalid shims from being created.
+#![feature(c_variadic)]
+
+#[repr(transparent)]
+struct Struct(u64);
+
+trait Trait {
+    fn get(&self) -> u64;
+
+    unsafe extern "C" fn dyn_method_ref(&self, mut ap: ...) -> u64 {
+        self.get() + unsafe { ap.arg::<u64>() }
+    }
+}
+
+impl Trait for Struct {
+    fn get(&self) -> u64 {
+        self.0
+    }
+}
+
+fn main() {
+    unsafe {
+        let dyn_object: &dyn Trait = &Struct(64);
+        //~^ ERROR the trait `Trait` is not dyn compatible
+        assert_eq!(dyn_object.dyn_method_ref(100), 164);
+        assert_eq!(
+            (Trait::dyn_method_ref as unsafe extern "C" fn(_, ...) -> u64)(dyn_object, 100),
+            164
+        );
+    }
+}