test(derive): Show failure in clap-rs#4279

tests/derive/groups.rs

@@ -21,3 +21,29 @@ fn test_safely_nest_parser() {
         Opt::try_parse_from(&["test", "--foo"]).unwrap()
     );
 }
+
+#[test]
+#[should_panic = "'Compose' is already in use"]
+fn skip_group_avoids_duplicate_ids() {
+    #[derive(Parser, Debug)]
+    struct Opt {
+        #[command(flatten)]
+        first: Compose<Empty, Empty>,
+        #[command(flatten)]
+        second: Compose<Empty, Empty>,
+    }
+
+    #[derive(clap::Args, Debug)]
+    pub struct Compose<L: clap::Args, R: clap::Args> {
+        #[clap(flatten)]
+        pub left: L,
+        #[clap(flatten)]
+        pub right: R,
+    }
+
+    #[derive(clap::Args, Clone, Copy, Debug)]
+    pub struct Empty;
+
+    use clap::CommandFactory;
+    Opt::command().debug_assert();
+}