Proposed resolution:

This wording is relative to N4986.

1. Modify [time.format] as indicated:

   template<classDuration, class charT>
     struct formatter<chrono::local-time-format-t<Duration>, charT>;
   

   -17- Let `f` be a locale-time-format-t<Duration> object passed to `formatter::format`.

   -18- Remarks: If the chrono-specs is omitted, the result is equivalent to using `%F %T %Z` as the chrono-specs. If `%Z` is used, it is replaced with `*f.abbrev` if `f.abbrev` is not a null pointer value. If `%Z` is used and `f.abbrev` is a null pointer value, an exception of type `format_error` is thrown. If `%z` (or a modified variant of `%z`) is used, it is formatted with the value of `*f.offset_sec` if `f.offset_sec` is not a null pointer value. If `%z` (or a modified variant of `%z`) is used and `f.offset_sec` is a null pointer value, then an exception of type `format_error` is thrown.

     template<class Duration, class TimeZonePtr, class charT>
     struct formatter<chrono::zoned_time<Duration, TimeZonePtr>, charT>
         : formatter<chrono::local-time-format-t<common_type_t<Duration, seconds>>, charT> {
       template<class FormatContext>
         typename FormatContext::iterator
         format(const chrono::zoned_time<Duration, TimeZonePtr>& tp, FormatContext& ctx) const;
     };
   

   template<class FormatContext>
     typename FormatContext::iterator
       format(const chrono::zoned_time<Duration, TimeZonePtr>& tp, FormatContext& ctx) const;
   

   -19- Effects: Equivalent to:

   sys_info info = tp.get_info();
   return formatter<chrono::local-time-format-t<common_type_t<Duration, seconds>>, charT>::
            format({tp.get_local_time(), &info.abbrev, &info.offset}, ctx);
   

[ Wrocław 2024-11-23; Status changed: Voting → WP. ]

[ 2024-08-02; Reflector poll ]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

The std::formatter<std::chrono::zoned_time<Duration, TimeZonePtr>> specialization calls `tp.get_local_time()` for the object it passes to its base class' `format` function. But `get_local_time()` does not return a local_time<Duration>, it returns local_time<common_type_t<Duration, seconds>>. The base class' `format` function is only defined for local_time<Duration>. That means this is ill-formed, even though the static assert passes:

using namespace std::chrono;
  static_assert( std::formattable<zoned_time<minutes>, char> );
  zoned_time<minutes> zt;
  (void) std::format("{}", zt); // error: cannot convert local_time<seconds> to local_time<minutes>

Additionally, it's not specified what output you should get for:

`std::format("{}", local_time_format(zt.get_local_time()));`

The proposed resolution below has been implemented in libstdc++.