bug: Include groups in get_arg_conflicts_with

So that it doesn't panic if trying to get the conflicts for an Arg that
conflicts with a group.

Fixes: #3900

src/builder/command.rs

@@ -3556,15 +3556,22 @@ impl<'help> App<'help> {
         if arg.is_global_set() {
             self.get_global_arg_conflicts_with(arg)
         } else {
-            arg.blacklist
-                .iter()
-                .map(|id| {
-                    self.args.args().find(|arg| arg.id == *id).expect(
-                        "Command::get_arg_conflicts_with: \
-                    The passed arg conflicts with an arg unknown to the cmd",
-                    )
-                })
-                .collect()
+            let mut result = Vec::new();
+            for id in arg.blacklist.iter() {
+                if let Some(arg) = self.find(id) {
+                    result.push(arg);
+                } else if let Some(group) = self.find_group(id) {
+                    result.extend(group.args.iter().map(|id| {
+                        self.find(id).expect(
+                            "Command::get_arg_conflicts_with: \
+                                The passed arg conflicts with a group containing an unknown arg",
+                        )
+                    }));
+                } else {
+                    panic!("Command::get_arg_conflicts_with: The passed arg conflicts with an arg unknown to the cmd");
+                }
+            }
+            result
         }
     }
 

tests/builder/conflicts.rs

@@ -154,6 +154,8 @@ fn arg_conflicts_with_group() {
     if let Err(err) = result {
         panic!("{}", err);
     }
+
+    cmd.build();
 }
 
 #[test]
@@ -296,6 +298,24 @@ fn required_group_conflicts_with_arg() {
     }
 }
 
+#[test]
+fn get_arg_conflicts_with_group() {
+    let flag = arg!(--flag).conflicts_with("gr");
+    let mut cmd = Command::new("group_conflict")
+        .arg(&flag)
+        .group(ArgGroup::new("gr").arg("some").arg("other"))
+        .arg(arg!(--some))
+        .arg(arg!(--other));
+
+    cmd.build();
+
+    let result = cmd.get_arg_conflicts_with(&flag);
+
+    assert_eq!(result.len(), 2);
+    assert_eq!(result[0].get_id(), "some");
+    assert_eq!(result[1].get_id(), "other");
+}
+
 #[test]
 fn conflict_output() {
     utils::assert_output(