[libc++][NFC] Remove unimplemented parts of chrono synopsis

[jloser]

Summary

Several parts in the chrono synopsis for C++20 are not yet
implemented. The current recommendation is that things are added to the
synopsis when implemented -- not beforehand. As such, remove the
not-yet-implemented parts to avoid confusion.

[Mordante]

@jloser Nothing to do with your patch. Normally we only add a synopsis when we implement that part of the code. That way it's possible to track our implementation status. Clearly we added a part of the synopsis that's not yet implemented.

I'm somewhat in doubt whether we should accept this patch or remove that part of the synopsis.
@ldionne WDYT?

[• Quuxplusone]

For the record, I think we should just remove this part of the synopsis comment until it's been implemented. This was @mclow.lists in da77fe4a77485f5f4c4eb7f460554fc14e88d3b6 (July 2018); it's possible that back then the synopsis comment was more of a "to-do list" than a "done list." These days I agree with @Mordante's assessment.

[jloser]

I'm OK with removing this part of the synopsis instead. I'd vote that we remove *all* of the C++20-but-not-yet-implemented chrono parts in the synopsis then, such as time zone database, time zones, exception classes, parsing, etc (basically all of C++20 chrono stuff mentioned in the synopsis).

[Mordante]

In D111922#3068734, @jloser wrote:

I'm OK with removing this part of the synopsis instead. I'd vote that we remove *all* of the C++20-but-not-yet-implemented chrono parts in the synopsis then, such as time zone database, time zones, exception classes, parsing, etc (basically all of C++20 chrono stuff mentioned in the synopsis).

Agreed I too prefer to remove the not implemented parts.

[ldionne]

Agreed with the removal. I believe things changed and we now use the synopsis for a very clear purpose - back then I don't think we were as systematic with the way we'd use the synopsis comments.

Let's remove what's not implemented yet from the synopsis (this will also make it easier to figure out what we'd need to work on).

Accepting assuming what we all agree on will be implemented, not the current diff :-).

Thanks!

[jloser]

Remove unimplemented parts of chrono synopsis

[jloser]

@ldionne @Quuxplusone @Mordante - I've updated this patch to remove the unimplemented parts of chrono from the synopsis. Please give it a look over to make sure everything seems OK. If so, I'll land it.

[jloser]

In D111922#3072836, @jloser wrote:

@ldionne @Quuxplusone @Mordante - I've updated this patch to remove the unimplemented parts of chrono from the synopsis. Please give it a look over to make sure everything seems OK. If so, I'll land it.

@ldionne are you happy with this? I know you approved before I even made the change - so just checking in. :)

[jloser]

gentle ping @Quuxplusone @Mordante

[• Quuxplusone]

LGTM!

libcxx/include/chrono

 template <class Rep1, class Period1, class Rep2, class Period2>
   struct common_type<chrono::duration<Rep1, Period1>, chrono::duration<Rep2, Period2>>;
 
 template <class Clock, class Duration1, class Duration2>
   struct common_type<chrono::time_point<Clock, Duration1>, chrono::time_point<Clock, Duration2>>;
 
 namespace chrono {
 
-
-template<class T> struct is_clock;  // C++20
-template<class T> inline constexpr bool is_clock_v = is_clock<T>::value;   // C++20
-
-
 // duration arithmetic
 template <class Rep1, class Period1, class Rep2, class Period2>
   constexpr
   typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
   operator+(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
 template <class Rep1, class Period1, class Rep2, class Period2>
   constexpr
   typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
public:
     static time_point from_time_t(time_t __t) noexcept;
 };
 
 template <class Duration>
   using sys_time  = time_point<system_clock, Duration>; // C++20
 using sys_seconds = sys_time<seconds>;                  // C++20
 using sys_days    = sys_time<days>;                     // C++20
 
-class utc_clock;                                        // C++20
-
-template <class Duration>
-  using utc_time  = time_point<utc_clock, Duration>;    // C++20
-using utc_seconds = utc_time<seconds>;                  // C++20
-
-class tai_clock;                                        // C++20
-
-template <class Duration>
-  using tai_time  = time_point<tai_clock, Duration>;    // C++20
-using tai_seconds = tai_time<seconds>;                  // C++20
-
 class file_clock;                                       // C++20
 
 template<class Duration>
   using file_time = time_point<file_clock, Duration>;   // C++20
 
 class steady_clock
 {
 public:

 
 // 25.7.8, local time           // C++20
 struct local_t {};
 template<class Duration>
   using local_time  = time_point<local_t, Duration>;
 using local_seconds = local_time<seconds>;
 using local_days    = local_time<days>;
 
-// 25.7.9, time_point conversions template<class DestClock, class SourceClock>    // C++20
-struct clock_time_conversion;
-
-template<class DestClock, class SourceClock, class Duration>
-  auto clock_cast(const time_point<SourceClock, Duration>& t);
-
 // 25.8.2, class last_spec    // C++20
 struct last_spec;
 
 // 25.8.3, class day          // C++20
 
 class day;
 constexpr bool operator==(const day& x, const day& y) noexcept;
 constexpr bool operator!=(const day& x, const day& y) noexcept;
public:
     constexpr chrono::minutes minutes() const noexcept;
     constexpr chrono::seconds seconds() const noexcept;
     constexpr precision subseconds() const noexcept;
 
     constexpr explicit operator  precision()   const noexcept;
     constexpr          precision to_duration() const noexcept;
 };
 
-template <class charT, class traits, class Duration>
-  basic_ostream<charT, traits>&
-    operator<<(basic_ostream<charT, traits>& os, hh_mm_ss<Duration> const& hms);
-
 // 26.10, 12/24 hour functions
 constexpr bool is_am(hours const& h) noexcept;
 constexpr bool is_pm(hours const& h) noexcept;
 constexpr hours make12(const hours& h) noexcept;
 constexpr hours make24(const hours& h, bool is_pm) noexcept;
 
-
-// 25.10.2, time zone database     // C++20
-struct tzdb;
-class tzdb_list;
-
-// 25.10.2.3, time zone database access    // C++20
-const tzdb& get_tzdb();
-tzdb_list& get_tzdb_list();
-const time_zone* locate_zone(string_view tz_name);
-const time_zone* current_zone();
-
-// 25.10.2.4, remote time zone database support    // C++20
-const tzdb& reload_tzdb();
-string remote_version();
-
-// 25.10.3, exception classes    // C++20
-class nonexistent_local_time;
-class ambiguous_local_time;
-
-// 25.10.4, information classes    // C++20
-struct sys_info;
-struct local_info;
-
 // 25.10.5, class time_zone    // C++20
 enum class choose {earliest, latest};
 class time_zone;
 bool operator==(const time_zone& x, const time_zone& y) noexcept;
 bool operator!=(const time_zone& x, const time_zone& y) noexcept;
 bool operator<(const time_zone& x, const time_zone& y) noexcept;
 bool operator>(const time_zone& x, const time_zone& y) noexcept;
 bool operator<=(const time_zone& x, const time_zone& y) noexcept;
 bool operator>=(const time_zone& x, const time_zone& y) noexcept;
 
-// 25.10.6, class template zoned_traits    // C++20
-template<class T> struct zoned_traits;
-
-// 25.10.7, class template zoned_time    // C++20
-template<class Duration, class TimeZonePtr = const time_zone*> class zoned_time;
-using zoned_seconds = zoned_time<seconds>;
-
-template<class Duration1, class Duration2, class TimeZonePtr>
-  bool operator==(const zoned_time<Duration1, TimeZonePtr>& x,
-                  const zoned_time<Duration2, TimeZonePtr>& y);
-template<class Duration1, class Duration2, class TimeZonePtr>
-  bool operator!=(const zoned_time<Duration1, TimeZonePtr>& x,
-                  const zoned_time<Duration2, TimeZonePtr>& y);
-
-// 25.10.8, leap second support    // C++20
-class leap;
-
-bool operator==(const leap& x, const leap& y);
-bool operator!=(const leap& x, const leap& y);
-bool operator< (const leap& x, const leap& y);
-bool operator> (const leap& x, const leap& y);
-bool operator<=(const leap& x, const leap& y);
-bool operator>=(const leap& x, const leap& y);
-template<class Duration>
-  bool operator==(const leap& x, const sys_time<Duration>& y);
-template<class Duration>
-  bool operator==(const sys_time<Duration>& x, const leap& y);
-template<class Duration>
-  bool operator!=(const leap& x, const sys_time<Duration>& y);
-template<class Duration>
-  bool operator!=(const sys_time<Duration>& x, const leap& y);
-template<class Duration>
-  bool operator< (const leap& x, const sys_time<Duration>& y);
-template<class Duration>
-  bool operator< (const sys_time<Duration>& x, const leap& y);
-template<class Duration>
-  bool operator> (const leap& x, const sys_time<Duration>& y);
-template<class Duration>
-  bool operator> (const sys_time<Duration>& x, const leap& y);
-template<class Duration>
-  bool operator<=(const leap& x, const sys_time<Duration>& y);
-template<class Duration>
-  bool operator<=(const sys_time<Duration>& x, const leap& y);
-template<class Duration>
-  bool operator>=(const leap& x, const sys_time<Duration>& y);
-template<class Duration>
-  bool operator>=(const sys_time<Duration>& x, const leap& y);
-
-// 25.10.9, class link    // C++20
-class link;
-bool operator==(const link& x, const link& y);
-bool operator!=(const link& x, const link& y);
-bool operator< (const link& x, const link& y);
-bool operator> (const link& x, const link& y);
-bool operator<=(const link& x, const link& y);
-bool operator>=(const link& x, const link& y);
-
-// 25.11, formatting    // C++20
-template<class charT, class Streamable>
-  basic_string<charT>
-    format(const charT* fmt, const Streamable& s);
-
-template<class charT, class Streamable>
-  basic_string<charT>
-    format(const locale& loc, const charT* fmt, const Streamable& s);
-
-template<class charT, class traits, class Alloc, class Streamable>
-  basic_string<charT, traits, Alloc>
-    format(const basic_string<charT, traits, Alloc>& fmt, const Streamable& s);
-
-template<class charT, class traits, class Alloc, class Streamable>
-  basic_string<charT, traits, Alloc>
-    format(const locale& loc, const basic_string<charT, traits, Alloc>& fmt,
-           const Streamable& s);
-
-// 25.12, parsing    // C++20
-template<class charT, class traits, class Alloc, class Parsable>
-unspecified
-    parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp);
-
-template<class charT, class traits, class Alloc, class Parsable>
-unspecified
-    parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp,
-          basic_string<charT, traits, Alloc>& abbrev);
-
-template<class charT, class traits, class Alloc, class Parsable>
-unspecified
-    parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp,
-          minutes& offset);
-
-template<class charT, class traits, class Alloc, class Parsable>
-unspecified
-    parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp,
-          basic_string<charT, traits, Alloc>& abbrev, minutes& offset);
-
 // calendrical constants
 inline constexpr last_spec                              last{};       // C++20
 inline constexpr chrono::weekday                        Sunday{0};    // C++20
 inline constexpr chrono::weekday                        Monday{1};    // C++20
 inline constexpr chrono::weekday                        Tuesday{2};   // C++20
 inline constexpr chrono::weekday                        Wednesday{3}; // C++20
 inline constexpr chrono::weekday                        Thursday{4};  // C++20
 inline constexpr chrono::weekday                        Friday{5};    // C++20