add mut test

Author: b-naber

src/test/ui/resolve/issue-82825-mut.rs

@@ -0,0 +1,20 @@
+// check-pass
+
+trait Trait {
+    fn static_call(&mut self) where Self: Sized;
+    fn maybe_dynamic_call(&self) {
+        unimplemented!("unsupported maybe_dynamic_call");
+    }
+}
+
+impl<T: ?Sized + Trait> Trait for &mut T {
+    fn static_call(&mut self) where Self: Sized {
+        (**self).maybe_dynamic_call();
+    }
+}
+
+fn foo(mut x: &mut dyn Trait) {
+    x.static_call();
+}
+
+fn main() {}