[libc++] Clarify clock behavior before epoch (LWG 3318)

[benedekaibas]

This patch resolves LWG issue 3318
by clarifying in the <chrono> header whether each clock supports time
before its epoch (i.e., negative time points).

The clocks with signed rep (e.g., system_clock, utc_clock) now
explicitly state they support time before their epoch. Clocks with
unspecified signedness clarify that behavior before epoch is unspecified.

This is a documentation-only change. No runtime behavior is modified.

Fixes #100430

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[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Benedek Kaibas (benedekaibas)

Changes

This patch resolves LWG issue 3318
by clarifying in the <chrono> header whether each clock supports time
before its epoch (i.e., negative time points).

The clocks with signed rep (e.g., system_clock, utc_clock) now
explicitly state they support time before their epoch. Clocks with
unspecified signedness clarify that behavior before epoch is unspecified.

This is a documentation-only change. No runtime behavior is modified.

Fixes #100430

---

Full diff: https://github.com/llvm/llvm-project/pull/144697.diff

1 Files Affected:

• (modified) libcxx/include/chrono (+14-2)

diff --git a/libcxx/include/chrono b/libcxx/include/chrono
index 82e99a31bcc9f..fd6689c81c0c1 100644
--- a/libcxx/include/chrono
+++ b/libcxx/include/chrono
@@ -277,7 +277,9 @@ template <class Rep, class Period>
     constexpr duration<Rep, Period> abs(duration<Rep, Period> d);  // C++17
 
 // Clocks
-
+/// The system_clock represents wall-clock time based on the system-wide realtime clock.
+/// The representation for it is a signed representation, so it can express times before and after the epoch
+/// like: (1970-01-01 00:00:00 UTC).
 class system_clock
 {
 public:
@@ -306,6 +308,8 @@ template<class charT, class traits>                    // C++20
     operator<<(basic_ostream<charT, traits>& os, const sys_days& dp);
 
 // [time.clock.utc], class utc_clock
+/// The utc_clock represents Coordinated Universal Time (UTC)
+/// It uses a signed representation just like system_clock and represents time before and after its epoch
 class utc_clock {                                      // C++20
 public:
     using rep                       = a signed arithmetic type;
@@ -342,6 +346,8 @@ template<class Duration>                                // C++20
 
 
 // [time.clock.tai], class tai_clock
+/// The tai_clock uses Coordinated Universal Time (UTC).
+/// It uses a signed representation and can express time both before and after its epoch.
 class tai_clock {                                      // C++20
 public:
     using rep                       = a signed arithmetic type;
@@ -369,6 +375,8 @@ template<class charT, class traits, class Duration>     // C++20
     operator<<(basic_ostream<charT, traits>& os, const tai_time<Duration>& t);
 
 // [time.clock.gps], class gps_clock
+/// The gps_clock represents International Atomic Time (TAI). It uses a signed
+/// representation and can express time both before and after its epoch.    
 class gps_clock {                                      // C++20
 public:
     using rep                       = a signed arithmetic type;
@@ -395,6 +403,8 @@ template<class charT, class traits, class Duration>     // C++20
   basic_ostream<charT, traits>&
     operator<<(basic_ostream<charT, traits>& os, const gps_time<Duration>& t);
 
+/// The class file_clock is used for file timestamps. It uses a signed representation
+/// and can express time both before and after its epoch.
 class file_clock                                        // C++20
 {
 public:
@@ -420,6 +430,8 @@ template<class charT, class traits, class Duration>     // C++20
   basic_ostream<charT, traits>&
     operator<<(basic_ostream<charT, traits>& os, const file_time<Duration>& tp);
 
+/// The class steady_clock is monotonic and it does not support negative time points.
+/// Wheter it supports time before its epoch is unspecified.
 class steady_clock
 {
 public:
@@ -431,7 +443,7 @@ public:
 
     static time_point now() noexcept;
 };
-
+// high_resolution_clock is an alias for the steady_clock and has the same behavior.
 typedef steady_clock high_resolution_clock;
 
 // [time.clock.local] local time           // C++20

[frederick-vs-ja]

I believe we need to update https://github.com/llvm/llvm-project/blob/main/libcxx/docs/Status/Cxx20Issues.csv.

libc++'s duration::rep is nanoseconds so I think it can represent negative time points. According to the list in LWG3318, I think we don't need to mention signedness except for steady_clock and high_resolution_clock.

Also, it's also possibly redundant for steady_clock and high_resolution_clock. The standard says that the rep types are unspecified, and users can check the signedness by rep::min() < rep::zero(). Note that libc++ is already saying nanoseconds is the duration type of steady_clock in the comments.