For what it is worth, BSD/OS's "compat setreuid" code fakes the
whole thing out:

        /*
         * If requested ruid == euid, this is effectively a call to setuid.
         */

and:

        /*
         * Partially emulate old setreuid usage of exchanging ruid and euid.
         * We assume that the intent of setting ruid is to be able to get
         * back setuid privilege. So we make sure that we will be able to
         * do so, but do not actually set the ruid.
         */

Other setreuid() syscalls are rejected; this last one is turned into
a seteuid() as needed.

Chris