[commands] reset view when Optional argument encounters parsing error

discord/ext/commands/core.py

@@ -564,7 +564,14 @@ class Command(_BaseCommand, Generic[CogT, P, T]):
         if consume_rest_is_special:
             argument = view.read_rest().strip()
         else:
-            argument = view.get_quoted_word()
+            try:
+                argument = view.get_quoted_word()
+            except ArgumentParsingError as exc:
+                if self._is_typing_optional(param.annotation):
+                    view.index = previous
+                    return None
+                else:
+                    raise exc
         view.previous = previous
 
         # type-checker fails to narrow argument