/sys$common/syshlp/helplib.hlb CC, Messages, SIZEBIT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Message        <Context> "<expression>" is a bitfield, and so has no
                size.

 Description    A bitfield expression cannot be used as the argument
                to the sizeof operator or the __builtin_alignof
                builtin.

 User Action    Pass an expression with a valid type to the operator
                or builtin.