fix: Always ensure num_args is initialized

As we move people off takes_value/multiple_values, this will provide
something to check.

This was previously blocked on other issues related to flag handling
(clap-rs#4023)

src/builder/arg.rs

@@ -3984,6 +3984,14 @@ impl<'help> Arg<'help> {
         if val_names_len > 1 {
             self.settings.set(ArgSettings::MultipleValues);
             self.num_vals.get_or_insert(val_names_len.into());
+        } else {
+            if self.is_multiple_values_set() {
+                self.num_vals.get_or_insert((1..).into());
+            } else if self.is_takes_value_set() {
+                self.num_vals.get_or_insert(1.into());
+            } else {
+                self.num_vals.get_or_insert(0.into());
+            }
         }
     }
 

src/builder/debug_asserts.rs

@@ -692,12 +692,15 @@ fn assert_arg(arg: &Arg) {
 
     if let Some(num_vals) = arg.get_num_args() {
         // This can be the cause of later asserts, so put this first
-        let num_val_names = arg.get_value_names().unwrap_or(&[]).len();
-        if num_vals.max_values() < num_val_names {
-            panic!(
-                "Argument {}: Too many value names ({}) compared to number_of_values ({})",
-                arg.name, num_val_names, num_vals
-            );
+        if num_vals != 0.into() {
+            // HACK: Don't check for flags to make the derive easier
+            let num_val_names = arg.get_value_names().unwrap_or(&[]).len();
+            if num_vals.max_values() < num_val_names {
+                panic!(
+                    "Argument {}: Too many value names ({}) compared to number_of_values ({})",
+                    arg.name, num_val_names, num_vals
+                );
+            }
         }
 
         assert_eq!(