Fix usage of positional grouped arguments uses arg name rather than val_name

src/build/app/mod.rs

@@ -2575,8 +2575,10 @@ impl<'help> App<'help> {
             .filter_map(|x| self.find(x))
             .map(|x| {
                 if x.index.is_some() {
-                    x.name.to_owned()
+                    // Print val_name for positional arguments. e.g. <file_name>
+                    x.name_no_brackets().to_string()
                 } else {
+                    // Print useage string for flags arguments, e.g. <--help>
                     x.to_string()
                 }
             })

tests/groups.rs

@@ -274,6 +274,30 @@ fn group_multiple_args_error() {
     assert_eq!(err.kind, ErrorKind::ArgumentConflict);
 }
 
+#[test]
+fn group_usage_use_val_name() {
+    static GROUP_USAGE_USE_VAL_NAME: &str = "prog 
+
+USAGE:
+    prog <A>
+
+ARGS:
+    <A>    
+
+FLAGS:
+    -h, --help       Prints help information
+    -V, --version    Prints version information";
+    let app = App::new("prog")
+        .arg(Arg::new("a").value_name("A"))
+        .group(ArgGroup::new("group").arg("a").required(true));
+    assert!(utils::compare_output(
+        app,
+        "prog --help",
+        GROUP_USAGE_USE_VAL_NAME,
+        false,
+    ));
+}
+
 #[test]
 fn group_acts_like_arg() {
     let m = App::new("prog")