fix: Broken long arg inference on conflicts

clap_builder/src/parser/parser.rs

@@ -729,7 +729,7 @@ impl<'cmd> Parser<'cmd> {
             debug!("Parser::parse_long_arg: Found valid arg or flag '{arg}'");
             Some((long_arg, arg))
         } else if self.cmd.is_infer_long_args_set() {
-            self.cmd.get_arguments().find_map(|a| {
+            let mut iter = self.cmd.get_arguments().filter_map(|a| {
                 if let Some(long) = a.get_long() {
                     if long.starts_with(long_arg) {
                         return Some((long, a));
@@ -738,7 +738,9 @@ impl<'cmd> Parser<'cmd> {
                 a.aliases
                     .iter()
                     .find_map(|(alias, _)| alias.starts_with(long_arg).then(|| (alias.as_str(), a)))
-            })
+            });
+
+            iter.next().filter(|_| iter.next().is_none())
         } else {
             None
         };

tests/builder/opts.rs

@@ -659,7 +659,7 @@ fn issue_2279() {
 }
 
 #[test]
-fn infer_long_arg() {
+fn infer_long_arg_pass() {
     let cmd = Command::new("test")
         .infer_long_args(true)
         .arg(
@@ -716,3 +716,57 @@ fn infer_long_arg() {
     let matches = cmd.clone().try_get_matches_from(["test", "--a"]).unwrap();
     assert!(*matches.get_one::<bool>("arg").expect("defaulted by clap"));
 }
+
+#[test]
+fn infer_long_arg_pass_conflicts_exact_match() {
+    let cmd = Command::new("test")
+        .infer_long_args(true)
+        .arg(Arg::new("arg").long("arg").action(ArgAction::SetTrue))
+        .arg(Arg::new("arg2").long("arg2").action(ArgAction::SetTrue));
+
+    let matches = cmd.clone().try_get_matches_from(["test", "--arg"]).unwrap();
+    assert!(*matches.get_one::<bool>("arg").expect("defaulted by clap"));
+
+    let matches = cmd
+        .clone()
+        .try_get_matches_from(["test", "--arg2"])
+        .unwrap();
+    assert!(*matches.get_one::<bool>("arg2").expect("defaulted by clap"));
+}
+
+#[test]
+fn infer_long_arg_pass_conflicting_aliases() {
+    let cmd = Command::new("test").infer_long_args(true).arg(
+        Arg::new("abc-123")
+            .long("abc-123")
+            .aliases(["a", "abc-xyz"])
+            .action(ArgAction::SetTrue),
+    );
+
+    let matches = cmd.clone().try_get_matches_from(["test", "--ab"]).unwrap();
+    assert!(*matches
+        .get_one::<bool>("abc-123")
+        .expect("defaulted by clap"));
+}
+
+#[test]
+fn infer_long_arg_fail_conflicts() {
+    let cmd = Command::new("test")
+        .infer_long_args(true)
+        .arg(
+            Arg::new("abc-123")
+                .long("abc-123")
+                .action(ArgAction::SetTrue),
+        )
+        .arg(
+            Arg::new("abc-xyz")
+                .long("abc-xyz")
+                .action(ArgAction::SetTrue),
+        );
+
+    let error = cmd
+        .clone()
+        .try_get_matches_from(["test", "--abc"])
+        .unwrap_err();
+    assert_eq!(error.kind(), ErrorKind::UnknownArgument);
+}