fix(parser): Only add ArgGroup to ArgMatches for command-line

This will fix `clap_derive`s behavior for optional-flattened groups as
it will properly detect when the group is present (#3566).

While I consider this a bug and not part of compatibility guarentees, I
still want to keep in mind user impact which could still prevent this.
Defaults will make the group always-present which has little value and
if anything is relying on this, it is probably an application bug.

src/parser/parser.rs

@@ -1490,13 +1490,15 @@ impl<'cmd> Parser<'cmd> {
             self.remove_overrides(arg, matcher);
         }
         matcher.start_custom_arg(arg, source);
-        for group in self.cmd.groups_for_arg(arg.get_id()) {
-            matcher.start_custom_group(group.clone(), source);
-            matcher.add_val_to(
-                &group,
-                AnyValue::new(arg.get_id().clone()),
-                OsString::from(arg.get_id().as_str()),
-            );
+        if source.is_explicit() {
+            for group in self.cmd.groups_for_arg(arg.get_id()) {
+                matcher.start_custom_group(group.clone(), source);
+                matcher.add_val_to(
+                    &group,
+                    AnyValue::new(arg.get_id().clone()),
+                    OsString::from(arg.get_id().as_str()),
+                );
+            }
         }
     }
 }

tests/builder/groups.rs

@@ -73,9 +73,10 @@ fn group_single_value() {
 #[test]
 fn group_empty() {
     let res = Command::new("group")
+        .arg(arg!(-f --flag "some flag"))
         .arg(arg!(-c --color [color] "some option"))
         .arg(arg!(-n --hostname <name> "another option"))
-        .group(ArgGroup::new("grp").args(["hostname", "color"]))
+        .group(ArgGroup::new("grp").args(["hostname", "color", "flag"]))
         .try_get_matches_from(vec![""]);
     assert!(res.is_ok(), "{}", res.unwrap_err());
 
@@ -87,12 +88,13 @@ fn group_empty() {
 #[test]
 fn group_required_flags_empty() {
     let result = Command::new("group")
+        .arg(arg!(-f --flag "some flag"))
         .arg(arg!(-c --color "some option"))
         .arg(arg!(-n --hostname <name> "another option"))
         .group(
             ArgGroup::new("grp")
                 .required(true)
-                .args(["hostname", "color"]),
+                .args(["hostname", "color", "flag"]),
         )
         .try_get_matches_from(vec![""]);
     assert!(result.is_err());
@@ -103,9 +105,10 @@ fn group_required_flags_empty() {
 #[test]
 fn group_multi_value_single_arg() {
     let res = Command::new("group")
+        .arg(arg!(-f --flag "some flag"))
         .arg(arg!(-c --color <color> "some option").num_args(1..))
         .arg(arg!(-n --hostname <name> "another option"))
-        .group(ArgGroup::new("grp").args(["hostname", "color"]))
+        .group(ArgGroup::new("grp").args(["hostname", "color", "flag"]))
         .try_get_matches_from(vec!["", "-c", "blue", "red", "green"]);
     assert!(res.is_ok(), "{:?}", res.unwrap_err().kind());