[Bug c++/88173] `std::numeric_limits::quiet_NaN()` is not constexpr

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88173

--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot 
com> ---
An ordered comparison (< <= > >=) with qNaN raises the "invalid" 
exception.  An equality comparison (== !=) does not, and nor do 
__builtin_isless etc.

I don't know how C++ constexpr rules relate to floating-point exceptions.