Proposed resolution:

This wording is relative to N5014.

1. Modify [time.format] as indicated:

   -8- For `chrono::duration`, `chrono::hh_mm_ss`, `chrono::sys_time`, `chrono::utc_time`, `chrono::tai_time`, `chrono::gps_time`, `chrono::file_time`, `chrono::local_time`, chrono::local-time-format-t, and `chrono::zoned_time` the library only provides the following specializations of `enable_nonlocking_formatter_optimization`:

   template<class Rep, class Period>
     constexpr bool enable_nonlocking_formatter_optimization<
       chrono::duration<Rep, Period>> =
         enable_nonlocking_formatter_optimizationis_arithmetic_v<Rep>;
         
   template<class Duration> 
     constexpr bool enable_nonlocking_formatter_optimization<chrono::hh_mm_ss<Duration>> 
        = enable_nonlocking_formatter_optimization<Duration>;
   
   template<class Duration> 
     constexpr bool enable_nonlocking_formatter_optimization<chrono::sys_time<Duration>> 
        = enable_nonlocking_formatter_optimization<Duration>;
   
   template<class Duration> 
     constexpr bool enable_nonlocking_formatter_optimization<chrono::utc_time<Duration>> 
        = enable_nonlocking_formatter_optimization<Duration>;
   
   template<class Duration> 
     constexpr bool enable_nonlocking_formatter_optimization<chrono::tai_time<Duration>> 
        = enable_nonlocking_formatter_optimization<Duration>;
   
   template<class Duration> 
     constexpr bool enable_nonlocking_formatter_optimization<chrono::gps_time<Duration>> 
        = enable_nonlocking_formatter_optimization<Duration>;
   
   template<class Duration> 
     constexpr bool enable_nonlocking_formatter_optimization<chrono::file_time<Duration>> 
        = enable_nonlocking_formatter_optimization<Duration>;
   
   template<class Duration> 
     constexpr bool enable_nonlocking_formatter_optimization<chrono::local_time<Duration>> 
        = enable_nonlocking_formatter_optimization<Duration>;
   
   template<class Duration> 
     constexpr bool enable_nonlocking_formatter_optimization<chrono::local-time-format-t<Duration>> 
        = enable_nonlocking_formatter_optimization<Duration>;
   

   -9- For `chrono::zoned_time` the library only provides the following specialization of `enable_nonlocking_formatter_optimization`:

   template<class Duration>
     constexpr bool enable_nonlocking_formatter_optimization<
       chrono::zoned_time<Duration, const std::chrono::time_zone*>> 
         = trueenable_nonlocking_formatter_optimization<Duration>;
   

Currently the `enable_nonlocking_formatter_optimization` is enabled for duration<Rep, Ratio> if it is enabled for `Rep`.

template<class Rep, class Period> 
  constexpr bool enable_nonlocking_formatter_optimization<chrono::duration<Rep, Period>>
    =  enable_nonlocking_formatter_optimization<Rep>;

However, this does not take into the consideration that for custom `Rep` types, the arithmetic operations on `Rep` may also lock the stream leading to deadlock (for example log on overflow). Since they are required to handle the specifiers such as `%S` we should specialize `enable_nonlocking_formatter_optimization` only for built-in types:

template<class Rep, class Period> 
  constexpr bool enable_nonlocking_formatter_optimization<chrono::duration<Rep, Period>>
    =  is_arithmetic_v<Rep>;

Furtheremore, for all types that are currently templated on `Duration` (`hh_mm_ss`, `sys_time`, `local_time`, etc.), we `enable_nonlocking_formatter_optimization` by default. This again does not take into consideration the arithmetic operations performed as duration. We should specialize `enable_nonlocking_formatter_optimization` for all of them to be enabled if `enable_nonlocking_formatter_optimization` is enabled for `duration`:

template<class Duration> 
  constexpr bool enable_nonlocking_formatter_optimization<chrono::hh_mm_ss<Duration>>
    =  enable_nonlocking_formatter_optimization<Duration>;

Note, that forwarding to `enable_nonlocking_formatter_optimization` on `Duration` instead of checking `Duration::rep` allows users to specialize `enable_nonlocking_formatter_optimization` for Durations with there custom representation types.

The proposed wording has recently been implemented in gcc's libstdc++.