VMS Help CC, Messages, NONPORTDEFINED  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Message        "defined" is treated as an identifier here, not an
                operator.

 Description    For compatibility with older C compilers, in certain
                modes the compiler will treat #ifdef defined(foo) as
                #ifdef defined, and #ifndef defined(foo) as #ifndef
                defined.  This might not have been what you intended.

 User Action    Do not mix #ifdef/#ifndef with the defined operator.