Index: Lib/argparse.py
===================================================================
--- Lib/argparse.py
+++ Lib/argparse.py
@@ -1847,31 +1847,31 @@
                         action_tuples.append((action, args, option_string))
                         break
 
                     # error if a double-dash option did not use the
                     # explicit argument
                     else:
                         msg = _('ignored explicit argument %r')
                         raise ArgumentError(action, msg % explicit_arg)
 
                 # if there is no explicit argument, try to match the
                 # optional's string arguments with the following strings
                 # if successful, exit the loop
                 else:
                     start = start_index + 1
                     selected_patterns = arg_strings_pattern[start:]
-                    arg_count = match_argument(action, selected_patterns)
+                    arg_count = match_argument(action, selected_patterns, arg_strings, start_index)
                     stop = start + arg_count
                     args = arg_strings[start:stop]
                     action_tuples.append((action, args, option_string))
                     break
 
             # add the Optional to the list and return the index at which
             # the Optional's string args stopped
             assert action_tuples
             for action, args, option_string in action_tuples:
                 take_action(action, args, option_string)
             return stop
 
         # the list of Positionals left to be parsed; this is modified
         # by consume_positionals()
         positionals = self._get_positional_actions()
@@ -1998,44 +1998,49 @@
                                 arg_strings.append(arg)
                         arg_strings = self._read_args_from_files(arg_strings)
                         new_arg_strings.extend(arg_strings)
                     finally:
                         args_file.close()
                 except IOError:
                     err = _sys.exc_info()[1]
                     self.error(str(err))
 
         # return the modified argument list
         return new_arg_strings
 
     def convert_arg_line_to_args(self, arg_line):
         return [arg_line]
 
-    def _match_argument(self, action, arg_strings_pattern):
+    def _match_argument(self, action, arg_strings_pattern, arg_strings, start_index):
         # match the pattern for this action to the arg strings
         nargs_pattern = self._get_nargs_pattern(action)
         match = _re.match(nargs_pattern, arg_strings_pattern)
 
         # raise an exception if we weren't able to find a match
         if match is None:
             nargs_errors = {
                 None: _('expected one argument'),
                 OPTIONAL: _('expected at most one argument'),
                 ONE_OR_MORE: _('expected at least one argument'),
             }
             default = _('expected %s argument(s)') % action.nargs
             msg = nargs_errors.get(action.nargs, default)
+
+            if arg_strings[start_index + 1].startswith("-"):
+                msg += " (if you intended {argument} to be the argument of {key}, pass {key}={argument} instead)".format(
+                    argument=arg_strings[start_index+1], key=arg_strings[start_index])
+            
             raise ArgumentError(action, msg)
 
         # return the number of arguments matched
         return len(match.group(1))
 
     def _match_arguments_partial(self, actions, arg_strings_pattern):
         # progressively shorten the actions list by slicing off the
         # final actions until we find a match
         result = []
         for i in range(len(actions), 0, -1):
             actions_slice = actions[:i]
             pattern = ''.join([self._get_nargs_pattern(action)
                                for action in actions_slice])
             match = _re.match(pattern, arg_strings_pattern)
             if match is not None: