add known-bug test for unsound issue 40582

tests/ui/specialization/issue-40582.rs

@@ -0,0 +1,35 @@
+// check-pass
+// known-bug: #40582
+
+// Should fail. Should not be possible to implement `make_static`.
+
+#![feature(specialization)]
+#![allow(incomplete_features)]
+
+trait FromRef<'a, T: ?Sized> {
+    fn from_ref(r: &'a T) -> Self;
+}
+
+impl<'a, T: ?Sized> FromRef<'a, T> for &'a T {
+    fn from_ref(r: &'a T) -> Self {
+        r
+    }
+}
+
+impl<'a, T: ?Sized, R> FromRef<'a, T> for R {
+    default fn from_ref(_: &'a T) -> Self {
+        unimplemented!()
+    }
+}
+
+fn make_static<T: ?Sized>(data: &T) -> &'static T {
+    fn helper<T: ?Sized, R>(data: &T) -> R {
+        R::from_ref(data)
+    }
+    helper(data)
+}
+
+fn main() {
+    let s = "specialization".to_owned();
+    println!("{:?}", make_static(s.as_str()));
+}