Error(s):

830561152/source.cpp:7:20: error: constexpr variable 'nan' must be initialized by a constant expression
    constexpr bool nan = std::isnan(300.0);
                   ^     ~~~~~~~~~~~~~~~~~
830561152/source.cpp:7:26: note: non-constexpr function 'isnan<double>' cannot be used in a constant expression
    constexpr bool nan = std::isnan(300.0);
                         ^
/usr/include/c++/v1/math.h:505:1: note: declared here
isnan(_A1 __lcpp_x) _NOEXCEPT
^
1 error generated.