Updated formatting of error message, showing all variations of argument name

include/argparse/argparse.hpp

@@ -816,18 +816,18 @@ public:
     }
   }
 
-  std::string get_names_csv() const {
+  std::string get_names_csv(char separator = ',') const {
     return std::accumulate(
         m_names.begin(), m_names.end(), std::string{""},
-        [](const std::string &result, const std::string &name) {
+        [&](const std::string &result, const std::string &name) {
-          return result.empty() ? name : result + ',' + name;
+          return result.empty() ? name : result + separator + name;
         });
   }
 
   std::string get_usage_full() const {
     std::stringstream usage;
 
-    usage << get_names_csv();
+    usage << get_names_csv('/');
     const std::string metavar = !m_metavar.empty() ? m_metavar : "VAR";
     if (m_num_args_range.get_max() > 0) {
       usage << " " << metavar;

include/argparse/main.cpp

@@ -2,7 +2,7 @@
 
 int main(int argc, char* argv[]) {
 	argparse::ArgumentParser program;
-	program.add_argument("-a").required();
+	program.add_argument("-a", "--number-of-apples");
-	program.add_argument("-b", "--bro").required();
+	program.add_argument("-b", "--bro");
 	program.parse_args(argc, argv);
 }

test/test_error_reporting.cpp

@@ -35,6 +35,20 @@ TEST_CASE("Missing optional argument name" * test_suite("error_reporting")) {
   }
 }
 
+TEST_CASE("Missing optional argument name (multiple names)" *
+          test_suite("error_reporting")) {
+  argparse::ArgumentParser parser("test");
+  parser.add_argument("-a", "--number-of-apples");
+  parser.add_argument("-b");
+
+  SUBCASE("Bad case 2") {
+    REQUIRE_THROWS_WITH_AS(parser.parse_args({"test", "1", "2"}),
+                           "Zero positional arguments expected, did you mean "
+                           "-a/--number-of-apples VAR",
+                           std::runtime_error);
+  }
+}
+
 TEST_CASE("Missing optional argument name with other positional arguments" *
           test_suite("error_reporting")) {
   argparse::ArgumentParser parser("test");