std::isnan is there since C++11 [1]. Let's replace home-grown rtl::math::isNan (declared/defined in include/rtl/math.hxx) with this standard function. [1] https://en.cppreference.com/w/cpp/numeric/math/isnan
It is fine to remove uses of that rtl::math function across the LO code base. But the function is part of the stable URE interface, so cannot easily itself be removed. (We can think about an [API CHANGE] to drop this and similar inline functions from rtl/math.hxx once all of them are unused across the LO code base.)
Yukio Siraichi committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/core/commit/3bf3face224a7e12ba95888821a0ac21525af22c tdf#130975 replace `rtl::math::isNan` with `std::isnan`. It will be available in 7.0.0. The patch should be included in the daily builds available at https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: https://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.