Original code by zephyrkul.
This new parsing mode allows for backtracking in case of failure
when a typing.Union[..., NoneType] or a typing.Optional[...] is used.
This means that if a type would fail to parse, the view is undo'd to
a previous state, passing the default parameter to the callback, and
then continuing on the next parameter as if nothing had happened.
00a445310b