[Bug libstdc++/78996] New: <variant> uses macro as name

[webrown.cpp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78996

            Bug ID: 78996
           Summary: <variant> uses macro as name
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: webrown.cpp at gmail dot com
  Target Milestone: ---

Upon upgrading to g++-mp-7 (MacPorts gcc7 7-20170101_0) 7.0.0 20170101
(experimental) and compiling with significant options -std=c++1z -fconcepts, I
find a previously-unseen conflict.

At line 607, header <variant> employs the identifier __unused as the name of a
template parameter pack.

Unfortunately, before #including <variant>, my test program has already
#included <sys/cdefs.h>, which on my system contains the line:

  #define __unused      __attribute__((unused))

This seems the source of the conflict that now produces diagnostics from the
unchanged test program that last month compiled cleanly.

Preprocessed source is attached.  Thank you.