[Bug libstdc++/113500] Using std::format with float or double based std::chrono::time_point causes error: no match for 'operator<<'

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113500

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Hirthammer from comment #5)
> This whole thing with std::format and std::chrono::time_point is currently a
> total minefield.

That seems like an exaggeration.

> 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.

It compiles fine with GCC for me.

> 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.

That's because operator<< is not defined in general for time_point, only for
specific clocks. It's defined for utc_time and gps_time unconditionally. It's
only defined for sys_time for non-floating-point representations and periods
less than a day:
https://en.cppreference.com/w/cpp/chrono/system_clock/operator_ltlt

Because std::format for chrono types is specified to use operator<< in some
cases, that constraint for sys_time also affects std::format.