def _type_check(arg, msg):
    """Check that the argument is a type, and return it (internal helper).

    As a special case, accept None and return type(None) instead.
    Also, _TypeAlias instances (e.g. Match, Pattern) are acceptable.

    The msg argument is a human-readable error message, e.g.

        "Union[arg, ...]: arg should be a type."

    We append the repr() of the actual value (truncated to 100 chars).
    """
    if arg is None:
        return type(None)
    if isinstance(arg, str):
        arg = _ForwardRef(arg)
    if (
        isinstance(arg, _TypingBase) and type(arg).__name__ == '_ClassVar' or
        not isinstance(arg, (type, _TypingBase)) and not callable(arg)
    ):
        raise TypeError(msg + " Got %.100r." % (arg,))
    # Bare Union etc. are not valid as type arguments
    if (
        type(arg).__name__ in ('_Union', '_Optional') and
        not getattr(arg, '__origin__', None) or
        isinstance(arg, TypingMeta) and arg._gorg in (Generic, _Protocol)
    ):
        raise TypeError("Plain %s is not valid as type argument" % arg)
    return arg