add known-bug test for unsound issue 84591

tests/ui/implied-bounds/implied-bounds-on-trait-hierarchy.rs

@@ -0,0 +1,39 @@
+// check-pass
+// known-bug: #84591
+
+// Should fail. Subtrait can incorrectly extend supertrait lifetimes even when
+// supertrait has weaker implied bounds than subtrait. Strongly related to
+// issue #25860.
+
+trait Subtrait<T>: Supertrait {}
+trait Supertrait {
+    fn action(self);
+}
+
+fn subs_to_soup<T, U>(x: T)
+where
+    T: Subtrait<U>,
+{
+    soup(x)
+}
+
+fn soup<T: Supertrait>(x: T) {
+    x.action();
+}
+
+impl<'a, 'b: 'a> Supertrait for (&'b str, &mut &'a str) {
+    fn action(self) {
+        *self.1 = self.0;
+    }
+}
+
+impl<'a, 'b> Subtrait<&'a &'b str> for (&'b str, &mut &'a str) {}
+
+fn main() {
+    let mut d = "hi";
+    {
+        let x = "Hello World".to_string();
+        subs_to_soup((x.as_str(), &mut d));
+    }
+    println!("{}", d);
+}