fix(parser): Exclusive overrides required

`Arg::exclusive` is just another way of defining conflicts, so a
present-exclusive arg should override required like other conflicts.

Instead of going through the message of enumerating all other arguments
as exclusive, I shortcutted it and special case exclusive in the
required check like we do with conflicts.  The big downside is the
implicit coupling between the code paths rather than having a consistent
abstraction for covering conflicts.

Fixes clap-rs#3595

src/parse/validator.rs

@@ -479,11 +479,22 @@ impl<'help, 'cmd> Validator<'help, 'cmd> {
         debug!("Validator::validate_required: required={:?}", self.required);
         self.gather_requires(matcher);
 
+        let is_exclusive_present = matcher.arg_names().any(|name| {
+            self.cmd
+                .find(name)
+                .map(|arg| arg.is_exclusive_set())
+                .unwrap_or_default()
+        });
+        debug!(
+            "Validator::validate_required: is_exclusive_present={}",
+            is_exclusive_present
+        );
+
         for arg_or_group in self.required.iter().filter(|r| !matcher.contains(r)) {
             debug!("Validator::validate_required:iter:aog={:?}", arg_or_group);
             if let Some(arg) = self.cmd.find(arg_or_group) {
                 debug!("Validator::validate_required:iter: This is an arg");
-                if !self.is_missing_required_ok(arg, matcher, conflicts) {
+                if !is_exclusive_present && !self.is_missing_required_ok(arg, matcher, conflicts) {
                     return self.missing_required_error(matcher, vec![]);
                 }
             } else if let Some(group) = self.cmd.find_group(arg_or_group) {

tests/builder/conflicts.rs

@@ -490,3 +490,20 @@ fn group_conflicts_with_default_arg() {
     assert_eq!(m.value_of("opt"), Some("default"));
     assert!(m.is_present("flag"));
 }
+
+#[test]
+fn exclusive_with_required() {
+    let cmd = Command::new("bug")
+        .arg(Arg::new("test").long("test").exclusive(true))
+        .arg(Arg::new("input").takes_value(true).required(true));
+
+    cmd.clone()
+        .try_get_matches_from(["bug", "--test", "required"])
+        .unwrap_err();
+
+    cmd.clone()
+        .try_get_matches_from(["bug", "required"])
+        .unwrap();
+
+    cmd.clone().try_get_matches_from(["bug", "--test"]).unwrap();
+}