[Bug c/68907] bogus warning: right-hand operand of comma expression has no effect on an atomic_int preincrement

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68907

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
GCC 4.9 warns too.