[Bug c++/81429] New: maybe_unused attribute triggers syntax error when used on first argument to a constructor

["jeff.benshetler at stackpath dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81429

            Bug ID: 81429
           Summary: maybe_unused attribute triggers syntax error when used
                    on first argument to a constructor
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jeff.benshetler at stackpath dot com
  Target Milestone: ---

Created attachment 41746
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41746&action=edit
Preprocessed file that triggers the bug.

maybe_unused attribute triggers syntax error when used on first argument to a
constructor

Systm Type: Linux bnx1-lab1-dfw0.stackpath.systems 3.10.0-514.10.2.el7.x86_64
#1 SMP Fri Mar 3 00:04:05 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

Options given when GCC was configured/built: 
  tar xzf gcc-7.1.0.tar.gz
  cd gcc-7.1.0
  ./contrib/download_prerequisites
  cd ..
  mkdir objdir-7.1.0
  cd objdir-7.1.0
  ../gcc-7.1.0/configure --prefix=/opt/gcc-7.1.0 --enable-languages=c,c++
--disable-multilib
  make -j60
  sudo make install

Complete command line that triggers the bug: g++ -Wall -Wextra -save-temps -c
maybe_unused.cpp

Compiler Output
  maybe_unused.cpp:3:9: error: expected unqualified-id before ‘[’ token
     Foo([[maybe_unused]] int x) // fails
         ^
  maybe_unused.cpp:3:9: error: expected ‘)’ before ‘[’ token
This bug occurs with gcc-6.3.0, gcc-6.4.0, gcc-7.1.0. clang-3.9 is fine with
the code.