[llvm-branch-commits] [libcxx] release/20.x: [libc++][TZDB] Fixes mapping of nonexisting time. (#127330) (PR #127531)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/127531

Backport 941f7cbf5a3e7aa9f36b002dc22cfdb4ff50fea8

Requested by: @mordante

>From 2b96bbb64e0de22f9ac41aca6a3249cfbe8e8fa0 Mon Sep 17 00:00:00 2001
From: Mark de Wever <koraq at xs4all.nl>
Date: Mon, 17 Feb 2025 19:08:07 +0100
Subject: [PATCH] [libc++][TZDB] Fixes mapping of nonexisting time. (#127330)

All non-existing local times in a contiguous range should map to the
same time point. This fixes a bug, were the times inside the range were
mapped to the wrong time.

Fixes: #113654
(cherry picked from commit 941f7cbf5a3e7aa9f36b002dc22cfdb4ff50fea8)
---
 libcxx/include/__chrono/time_zone.h             |  8 ++++++--
 .../time.zone.members/to_sys_choose.pass.cpp    | 17 +++++++++++++++--
 2 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/libcxx/include/__chrono/time_zone.h b/libcxx/include/__chrono/time_zone.h
index ab5c22eceaaf1..d18d59d2736bf 100644
--- a/libcxx/include/__chrono/time_zone.h
+++ b/libcxx/include/__chrono/time_zone.h
@@ -103,10 +103,14 @@ class _LIBCPP_AVAILABILITY_TZDB time_zone {
   to_sys(const local_time<_Duration>& __time, choose __z) const {
     local_info __info = get_info(__time);
     switch (__info.result) {
-    case local_info::unique:
-    case local_info::nonexistent: // first and second are the same
+    case local_info::unique: // first and second are the same
       return sys_time<common_type_t<_Duration, seconds>>{__time.time_since_epoch() - __info.first.offset};
 
+    case local_info::nonexistent:
+      // first and second are the same
+      // All non-existing values are converted to the same time.
+      return sys_time<common_type_t<_Duration, seconds>>{__info.first.end};
+
     case local_info::ambiguous:
       switch (__z) {
       case choose::earliest:
diff --git a/libcxx/test/std/time/time.zone/time.zone.timezone/time.zone.members/to_sys_choose.pass.cpp b/libcxx/test/std/time/time.zone/time.zone.timezone/time.zone.members/to_sys_choose.pass.cpp
index bad4ef352e9b9..1147c9fadf9ae 100644
--- a/libcxx/test/std/time/time.zone/time.zone.timezone/time.zone.members/to_sys_choose.pass.cpp
+++ b/libcxx/test/std/time/time.zone/time.zone.timezone/time.zone.members/to_sys_choose.pass.cpp
@@ -88,7 +88,7 @@ static void test_nonexistent() {
   // Pick an historic date where it's well known what the time zone rules were.
   // This makes it unlikely updates to the database change these rules.
   std::chrono::local_time<std::chrono::seconds> time{
-      (std::chrono::sys_days{std::chrono::March / 30 / 1986} + 2h + 30min).time_since_epoch()};
+      (std::chrono::sys_days{std::chrono::March / 30 / 1986} + 2h).time_since_epoch()};
 
   std::chrono::sys_seconds expected{time.time_since_epoch() - 1h};
 
@@ -100,6 +100,13 @@ static void test_nonexistent() {
   assert(tz->to_sys(time + 0us, std::chrono::choose::latest) == expected);
   assert(tz->to_sys(time + 0ms, std::chrono::choose::earliest) == expected);
   assert(tz->to_sys(time + 0s, std::chrono::choose::latest) == expected);
+
+  // The entire nonexisting hour should map to the same time.
+  // For nonexistant the value of std::chrono::choose has no effect.
+  assert(tz->to_sys(time + 1s, std::chrono::choose::earliest) == expected);
+  assert(tz->to_sys(time + 1min, std::chrono::choose::latest) == expected);
+  assert(tz->to_sys(time + 30min, std::chrono::choose::earliest) == expected);
+  assert(tz->to_sys(time + 59min + 59s, std::chrono::choose::latest) == expected);
 }
 
 // Tests ambiguous conversions.
@@ -120,7 +127,7 @@ static void test_ambiguous() {
   // Pick an historic date where it's well known what the time zone rules were.
   // This makes it unlikely updates to the database change these rules.
   std::chrono::local_time<std::chrono::seconds> time{
-      (std::chrono::sys_days{std::chrono::September / 28 / 1986} + 2h + 30min).time_since_epoch()};
+      (std::chrono::sys_days{std::chrono::September / 28 / 1986} + 2h).time_since_epoch()};
 
   std::chrono::sys_seconds earlier{time.time_since_epoch() - 2h};
   std::chrono::sys_seconds later{time.time_since_epoch() - 1h};
@@ -133,6 +140,12 @@ static void test_ambiguous() {
   assert(tz->to_sys(time + 0us, std::chrono::choose::latest) == later);
   assert(tz->to_sys(time + 0ms, std::chrono::choose::earliest) == earlier);
   assert(tz->to_sys(time + 0s, std::chrono::choose::latest) == later);
+
+  // Test times in the ambigious hour
+  assert(tz->to_sys(time + 1s, std::chrono::choose::earliest) == earlier + 1s);
+  assert(tz->to_sys(time + 1min, std::chrono::choose::latest) == later + 1min);
+  assert(tz->to_sys(time + 30min, std::chrono::choose::earliest) == earlier + 30min);
+  assert(tz->to_sys(time + 59min + 59s, std::chrono::choose::latest) == later + 59min + 59s);
 }
 
 // This test does the basic validations of this function. The library function