https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113500
--- Comment #5 from hirtham...@allterra-dno.de --- (In reply to Jonathan Wakely from comment #4) > MSVC rejects this the same way, although libc++ from LLVM 17 compiles it. > > AFAICT std::format("{}", tp) would be invalid because that formats tp by > writing to a stream, and there is no operator<< for > sys_time<duration<floating-point>>. > > Arguably, std::format("{:%S}", tp) could work, because it doesn't need to > use operator<< but the problem is that we can't detect at compile-time > whether or not we need to use operator<< for a given format string. So it > always fails to compile. > > The reason it works with libc++ is because they have a bug in their > operator<< for sys_time. > > I think libstdc++ is correct here, but I've asked the C++ committee whether > we might want to change the standard to support this case. This whole thing with std::format and std::chrono::time_point is currently a total minefield. In MSVC it is even more complicated and I already reported the bug in October 2023. See: https://developercommunity.visualstudio.com/t/Using-std::format-with-unsigned-integer-/10501153 If you change the clock to utc_clock or gps_clock the code compiles with MSVC (but not with GCC) on Compiler Explorer. I don't know much about the exact definitions of the standard. But I find it very confusing if std::format generally supports a std::chrono::time_point as an input, but depending on the chosen template arguments of the time_point it does or does not compile.