fix(multicall): Disallow args on multicall binary

Set expectations for how this can be used and to make sure the right
errors are given.

src/build/command.rs

@@ -4027,7 +4027,7 @@ impl<'help> App<'help> {
 
             #[cfg(feature = "unstable-multicall")]
             {
-                if self.is_set(AppSettings::Multicall) {
+                if self.is_multicall_set() {
                     self.settings.insert(AppSettings::SubcommandRequired.into());
                     self.settings.insert(AppSettings::DisableHelpFlag.into());
                     self.settings.insert(AppSettings::DisableVersionFlag.into());

src/build/debug_asserts.rs

@@ -56,6 +56,16 @@ pub(crate) fn assert_app(cmd: &Command) {
     for arg in cmd.get_arguments() {
         assert_arg(arg);
 
+        #[cfg(feature = "unstable-multicall")]
+        {
+            assert!(
+                !cmd.is_multicall_set(),
+                "Command {}: Arguments like {} cannot be set on a multicall command",
+                cmd.get_name(),
+                arg.name
+            );
+        }
+
         if let Some(s) = arg.short.as_ref() {
             short_flags.push(Flag::Arg(format!("-{}", s), &*arg.name));
         }

tests/builder/subcommands.rs

@@ -602,3 +602,17 @@ For more information try help
         .unwrap_err();
     assert_eq!(err.kind(), ErrorKind::DisplayVersion);
 }
+
+#[cfg(feature = "unstable-multicall")]
+#[test]
+#[should_panic = "Command repl: Arguments like oh-no cannot be set on a multicall command"]
+fn cant_have_args_with_multicall() {
+    let mut cmd = Command::new("repl")
+        .version("1.0.0")
+        .propagate_version(true)
+        .multicall(true)
+        .subcommand(Command::new("foo"))
+        .subcommand(Command::new("bar"))
+        .arg(Arg::new("oh-no"));
+    cmd.build();
+}