If the given dtype is a DatetimeTZDtype, extract the implied
    tzinfo object from it and check that it does not conflict with the given
    tz.

    Parameters
    ----------
    dtype : dtype, str
    tz : None, tzinfo

    Returns
    -------
    tz : consensus tzinfo

    Raises
    ------
    ValueError : on tzinfo mismatch

def validate_tz_from_dtype(dtype, tz):
    """
    If the given dtype is a DatetimeTZDtype, extract the implied
    tzinfo object from it and check that it does not conflict with the given
    tz.

    Parameters
    ----------
    dtype : dtype, str
    tz : None, tzinfo

    Returns
    -------
    tz : consensus tzinfo

    Raises
    ------
    ValueError : on tzinfo mismatch
    """
    if dtype is not None:
        if isinstance(dtype, str):
            try:
                dtype = DatetimeTZDtype.construct_from_string(dtype)
            except TypeError:
                # Things like `datetime64[ns]`, which is OK for the
                # constructors, but also nonsense, which should be validated
                # but not by us. We *do* allow non-existent tz errors to
                # go through
                pass
        dtz = getattr(dtype, 'tz', None)
        if dtz is not None:
            if tz is not None and not timezones.tz_compare(tz, dtz):
                raise ValueError("cannot supply both a tz and a dtype"
                                 " with a tz")
            tz = dtz

        if tz is not None and is_datetime64_dtype(dtype):
            # We also need to check for the case where the user passed a
            #  tz-naive dtype (i.e. datetime64[ns])
            if tz is not None and not timezones.tz_compare(tz, dtz):
                raise ValueError("cannot supply both a tz and a "
                                 "timezone-naive dtype (i.e. datetime64[ns])")

    return tz