fix(parser): Always put in arg "" for external subcommands (unstable)

This allows distinguishing external subcommands from built-in
subcommands which can especially be confusing when escaping subcommands.

Fixes clap-rs#3263

src/parse/arg_matcher.rs

@@ -85,6 +85,7 @@ impl ArgMatcher {
         }
     }
 
+    #[cfg(not(feature = "unstable-v4"))]
     pub(crate) fn get_mut(&mut self, arg: &Id) -> Option<&mut MatchedArg> {
         self.0.args.get_mut(arg)
     }
@@ -142,6 +143,19 @@ impl ArgMatcher {
         ma.inc_occurrences();
     }
 
+    #[cfg(feature = "unstable-v4")]
+    pub(crate) fn inc_occurrence_of_external(&mut self, allow_invalid_utf8: bool) {
+        let id = &Id::empty_hash();
+        debug!(
+            "ArgMatcher::inc_occurrence_of_external: id={:?}, allow_invalid_utf8={}",
+            id, allow_invalid_utf8
+        );
+        let ma = self.entry(id).or_insert(MatchedArg::new());
+        ma.update_ty(ValueSource::CommandLine);
+        ma.invalid_utf8_allowed(allow_invalid_utf8);
+        ma.inc_occurrences();
+    }
+
     pub(crate) fn add_val_to(&mut self, arg: &Id, val: OsString, ty: ValueSource, append: bool) {
         if append {
             self.append_val_to(arg, val, ty);

src/parse/parser.rs

@@ -428,26 +428,34 @@ impl<'help, 'cmd> Parser<'help, 'cmd> {
 
                 // Collect the external subcommand args
                 let mut sc_m = ArgMatcher::new(self.cmd);
+                let allow_invalid_utf8 = self
+                    .cmd
+                    .is_allow_invalid_utf8_for_external_subcommands_set();
+                #[cfg(feature = "unstable-v4")]
+                {
+                    sc_m.inc_occurrence_of_external(allow_invalid_utf8);
+                }
 
                 for v in raw_args.remaining(&mut args_cursor) {
-                    let allow_invalid_utf8 = self
-                        .cmd
-                        .is_allow_invalid_utf8_for_external_subcommands_set();
                     if !allow_invalid_utf8 && v.to_str().is_none() {
                         return Err(ClapError::invalid_utf8(
                             self.cmd,
                             Usage::new(self.cmd).create_usage_with_title(&[]),
                         ));
                     }
+                    let external_id = &Id::empty_hash();
                     sc_m.add_val_to(
-                        &Id::empty_hash(),
+                        external_id,
                         v.to_os_string(),
                         ValueSource::CommandLine,
                         false,
                     );
-                    sc_m.get_mut(&Id::empty_hash())
-                        .expect("just inserted")
-                        .invalid_utf8_allowed(allow_invalid_utf8);
+                    #[cfg(not(feature = "unstable-v4"))]
+                    {
+                        sc_m.get_mut(external_id)
+                            .expect("just inserted")
+                            .invalid_utf8_allowed(allow_invalid_utf8);
+                    }
                 }
 
                 matcher.subcommand(SubCommand {

tests/builder/app_settings.rs

@@ -851,6 +851,46 @@ fn issue_1093_allow_ext_sc() {
     utils::assert_output(cmd, "clap-test --help", ALLOW_EXT_SC, false);
 }
 
+#[test]
+#[cfg(not(feature = "unstable-v4"))]
+fn allow_ext_sc_empty_args() {
+    let res = Command::new("clap-test")
+        .version("v1.4.8")
+        .allow_external_subcommands(true)
+        .allow_invalid_utf8_for_external_subcommands(true)
+        .try_get_matches_from(vec!["clap-test", "external-cmd"]);
+
+    assert!(res.is_ok(), "{}", res.unwrap_err());
+
+    match res.unwrap().subcommand() {
+        Some((name, args)) => {
+            assert_eq!(name, "external-cmd");
+            assert_eq!(args.values_of_lossy(""), None);
+        }
+        _ => unreachable!(),
+    }
+}
+
+#[test]
+#[cfg(feature = "unstable-v4")]
+fn allow_ext_sc_empty_args() {
+    let res = Command::new("clap-test")
+        .version("v1.4.8")
+        .allow_external_subcommands(true)
+        .allow_invalid_utf8_for_external_subcommands(true)
+        .try_get_matches_from(vec!["clap-test", "external-cmd"]);
+
+    assert!(res.is_ok(), "{}", res.unwrap_err());
+
+    match res.unwrap().subcommand() {
+        Some((name, args)) => {
+            assert_eq!(name, "external-cmd");
+            assert_eq!(args.values_of_lossy(""), Some(vec![]));
+        }
+        _ => unreachable!(),
+    }
+}
+
 #[test]
 fn allow_ext_sc_when_sc_required() {
     let res = Command::new("clap-test")