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

 Message        <Context> the assignment expression "<expression>" is
                used as the controlling expression of an if, while or
                for statement.

 Description    A common user mistake is to accidentally use
                assignment operator "=" instead of the equality
                operator "==" in an expression that controls a
                transfer.  For example saying if (a = b) instead of
                if (a == b).  While using the assignment operator is
                valid, it is often not what was intended.  When this
                message is enabled, the compiler will detect these
                cases at compile-time.  This can often avoid long
                debugging sessions needed to find the bug in the
                user's program.

 User Action    Make sure that the assignment operator is what is
                expected.