We have a problem with Boost.Rational and in Boost.Utility in C++20 standards mode. I've fixed Boost.Rational in develop. Relevant GitHub issues: 1. https://github.com/boostorg/rational/issues/43 2. https://github.com/boostorg/utility/issues/65 My fix for Boost.Rational that I will merge to master for the 1.75 release: https://github.com/boostorg/rational/pull/45 The root cause is the same. C++20 adds new language rules that implicitly generate some operators, and the current implementations result in an infinite loop. (In both cases an equality operator ends up calling itself recursively). GCC does emit a warning diagnostic in C++20 mode, but as usual these tend to be unnoticed at build time. ../../boost/rational.hpp:779:16: warning: in C++20 this comparison calls the current function recursively with reversed arguments 779 | return a == b; | ~~^~~~ So this issue was found by three independent users of Rational (one who reported a Github issue, the other who complained on Reddit, and a third that tried running the Rational unit tests with GCC10 and found two failures). in Boost.Utility still needs a fix for C++20 mode, but it has an active maintainer unlike Rational. The GitHub issue for it was closed (I've reopened it) because it was assumed to be a defect in the standard and one that vendors would fix. We did have some discussion on the C++ standard committee reflector about this, however - whatever the future change to the standard is - GCC and Clang with -std=c++20 that implement these rules have already been released and users are using it, so we should fix this in Boost 1.75. Glen