def _install_mode_validator(mode: T.List[T.Union[str, bool, int]]) -> T.Optional[str]:
    """Validate the `install_mode` keyword argument.

    This is a rather odd thing, it's a scalar, or an array of 3 values in the form:
    [(str | False), (str | int | False) = False, (str | int | False) = False]
    where the second and third components are not required and default to False.
    """
    if not mode:
        return None
    if True in mode:
        return 'components can only be permission strings, numbers, or False'
    if len(mode) > 3:
        return 'may have at most 3 elements'

    perms = mode[0]
    if not isinstance(perms, (str, bool)):
        return 'first component must be a permissions string or False'

    if isinstance(perms, str):
        if not len(perms) == 9:
            return ('permissions string must be exactly 9 characters in the form rwxr-xr-x,'
                    f' got {len(perms)}')
        for i in [0, 3, 6]:
            if perms[i] not in {'-', 'r'}:
                return f'permissions character {i+1} must be "-" or "r", not {perms[i]}'
        for i in [1, 4, 7]:
            if perms[i] not in {'-', 'w'}:
                return f'permissions character {i+1} must be "-" or "w", not {perms[i]}'
        for i in [2, 5]:
            if perms[i] not in {'-', 'x', 's', 'S'}:
                return f'permissions character {i+1} must be "-", "s", "S", or "x", not {perms[i]}'
        if perms[8] not in {'-', 'x', 't', 'T'}:
            return f'permission character 9 must be "-", "t", "T", or "x", not {perms[8]}'

        if len(mode) >= 2 and not isinstance(mode[1], (int, str, bool)):
            return 'second component can only be a string, number, or False'
        if len(mode) >= 3 and not isinstance(mode[2], (int, str, bool)):
            return 'third component can only be a string, number, or False'

    return None
Validate the `install_mode` keyword argument.

    This is a rather odd thing, it's a scalar, or an array of 3 values in the form:
    [(str | False), (str | int | False) = False, (str | int | False) = False]
    where the second and third components are not required and default to False.
    
mesonbuild/interpreter/type_checking.py