Compilation fails with GCC 5.5 on Ubuntu 16.04

[maximilianriemensberger]

GCC 5.5 (e.g. Ubuntu 16.04) complains about this line calling into a non-constexpr function

date/include/date/date.h

Line 3920 in dca8ddc

h -= hours{12};

external/date/include/date/date.h: In function 'constexpr std::chrono::hours date::make12(std::chrono::hours)':
external/date/include/date/date.h:3920:15: error: call to non-constexpr function 'std::chrono::duration<_Rep, _Period>& std::chrono::duration<_Rep, _Period>::operator-=(const std::chrono::duration<_Rep, _Period>&) [with _Rep = long int; _Period = std::ratio<3600l>]'
             h -= hours{12};
               ^
external/date/include/date/date.h: In function 'constexpr std::chrono::hours date::make24(std::chrono::hours, bool)':
external/date/include/date/date.h:3934:15: error: call to non-constexpr function 'std::chrono::duration<_Rep, _Period>& std::chrono::duration<_Rep, _Period>::operator+=(const std::chrono::duration<_Rep, _Period>&) [with _Rep = long int; _Period = std::ratio<3600l>]'
             h += hours{12};
               ^

[maximilianriemensberger]

Replacing the two failing lines with h = h - hours{12} and h = h + hours{12} fixes it for me.

[HowardHinnant]

gcc-5.5 did not fully implement the C++14 constexpr rules in <chrono>.

[maximilianriemensberger]

An alternative to the proposed solution would be to conditionally macro away the constexpr from those functions which are already conditionally constexpr. Would you be open to any of those fixes?

[HowardHinnant]

Yes. I haven't decided on the best way forward yet. Your PR is one good way. Dialing back on when CONSTCD14 gets set to constexpr for gcc (based on gcc version) is another.