feat(assert): Help people know about implicit ArgGroups

src/builder/debug_asserts.rs

@@ -280,20 +280,28 @@ pub(crate) fn assert_app(cmd: &Command) {
     }
 
     for group in cmd.get_groups() {
+        let derive_hint = if cfg!(feature = "derive") {
+            " (note: `Args` implicitly creates `ArgGroup`s; disable with `#[group(skip)]`"
+        } else {
+            ""
+        };
+
         // Name conflicts
         assert!(
             cmd.get_groups().filter(|x| x.id == group.id).count() < 2,
-            "Command {}: Argument group name must be unique\n\n\t'{}' is already in use",
+            "Command {}: Argument group name must be unique\n\n\t'{}' is already in use{}",
             cmd.get_name(),
             group.get_id(),
+            derive_hint
         );
 
         // Groups should not have naming conflicts with Args
         assert!(
             !cmd.get_arguments().any(|x| x.get_id() == group.get_id()),
-            "Command {}: Argument group name '{}' must not conflict with argument name",
+            "Command {}: Argument group name '{}' must not conflict with argument name{}",
             cmd.get_name(),
             group.get_id(),
+            derive_hint
         );
 
         for arg in &group.args {

tests/derive/groups.rs

@@ -90,3 +90,32 @@ fn skip_group_avoids_duplicate_ids() {
     assert_eq!(Compose::<Empty, Empty>::group_id(), None);
     assert_eq!(Opt::group_id(), Some(clap::Id::from("Opt")));
 }
+
+#[test]
+#[should_panic = "\
+Command clap: Argument group name must be unique
+
+\t'Compose' is already in use (note: `Args` implicitly creates `ArgGroup`s; disable with `#[group(skip)]`"]
+fn helpful_panic_on_duplicate_groups() {
+    #[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();
+}