[libc++][TZDB] Implements time_zone::to_local.

libcxx/include/__chrono/time_zone.h

@@ -129,6 +129,24 @@ class _LIBCPP_AVAILABILITY_TZDB time_zone {
     return {};
   }
 
+  template <class _Duration>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI local_time<common_type_t<_Duration, seconds>>
+  to_local(const sys_time<_Duration>& __time) const {
+    using _Dp = common_type_t<_Duration, seconds>;
+
+    sys_info __info = get_info(__time);
+
+    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
+        __info.offset >= chrono::seconds{0} || __time.time_since_epoch() >= _Dp::min() - __info.offset,
+        "cannot convert the system time; it would be before the minimum local clock value");
+
+    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
+        __info.offset <= chrono::seconds{0} || __time.time_since_epoch() <= _Dp::max() - __info.offset,
+        "cannot convert the system time; it would be after the maximum local clock value");
+
+    return local_time<_Dp>{__time.time_since_epoch() + __info.offset};
+  }
+
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI const __impl& __implementation() const noexcept { return *__impl_; }
 
 private:

libcxx/include/chrono

@@ -778,6 +778,10 @@ class time_zone {
   template<class Duration>
   sys_time<common_type_t<Duration, seconds>>
     to_sys(const local_time<Duration>& tp, choose z) const;
+
+  template<class Duration>
+  local_time<common_type_t<Duration, seconds>>
+    to_local(const sys_time<Duration>& tp) const;
 };
 bool operator==(const time_zone& x, const time_zone& y) noexcept;                // C++20
 strong_ordering operator<=>(const time_zone& x, const time_zone& y) noexcept;    // C++20

libcxx/test/libcxx/time/time.zone/time.zone.timezone/time.zone.members/assert.to_local.pass.cpp

@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// REQUIRES: has-unix-headers
+// REQUIRES: libcpp-hardening-mode={{extensive|debug}}
+// XFAIL: libcpp-hardening-mode=debug && availability-verbose_abort-missing
+
+// XFAIL: libcpp-has-no-experimental-tzdb
+// XFAIL: availability-tzdb-missing
+
+// <chrono>
+
+// template <class _Duration>
+// local_time<common_type_t<Duration, seconds>>
+//   to_local(const sys_time<Duration>& tp) const;
+
+#include <chrono>
+
+#include "check_assertion.h"
+
+// Tests values that cannot be converted. To make sure the test is does not depend on changes
+// in the database it uses a time zone with a fixed offset.
+int main(int, char**) {
+  TEST_LIBCPP_ASSERT_FAILURE(std::chrono::locate_zone("Etc/GMT+1")->to_local(std::chrono::sys_seconds::min()),
+                             "cannot convert the system time; it would be before the minimum local clock value");
+
+  // TODO TZDB look why std::chrono::sys_seconds::max()  fails
+  TEST_LIBCPP_ASSERT_FAILURE(
+      std::chrono::locate_zone("Etc/GMT-1")->to_local(std::chrono::sys_seconds::max() - std::chrono::seconds(1)),
+      "cannot convert the system time; it would be after the maximum local clock value");
+
+  return 0;
+}

libcxx/test/std/time/time.zone/time.zone.timezone/time.zone.members/to_local.pass.cpp

@@ -0,0 +1,68 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-filesystem, no-localization, no-tzdb
+
+// XFAIL: libcpp-has-no-experimental-tzdb
+// XFAIL: availability-tzdb-missing
+
+// <chrono>
+
+// class time_zone;
+
+// template <class _Duration>
+// local_time<common_type_t<Duration, seconds>>
+//   to_local(const sys_time<Duration>& tp) const;
+
+#include <chrono>
+#include <format>
+#include <cassert>
+#include <string_view>
+
+#include "test_macros.h"
+#include "assert_macros.h"
+#include "concat_macros.h"
+
+int main(int, char**) {
+  // To make sure the test does not depend on changes in the database it uses a
+  // time zone with a fixed offset.
+  using namespace std::literals::chrono_literals;
+
+  const std::chrono::time_zone* tz = std::chrono::locate_zone("Etc/GMT+1");
+
+  assert(tz->to_local(std::chrono::sys_time<std::chrono::nanoseconds>{-1ns}) ==
+         std::chrono::local_time<std::chrono::nanoseconds>{-1ns - 1h});
+
+  assert(tz->to_local(std::chrono::sys_time<std::chrono::microseconds>{0us}) ==
+         std::chrono::local_time<std::chrono::microseconds>{0us - 1h});
+
+  assert(tz->to_local(
+             std::chrono::sys_time<std::chrono::seconds>{std::chrono::sys_days{std::chrono::January / 1 / -21970}}) ==
+         std::chrono::local_time<std::chrono::seconds>{
+             (std::chrono::sys_days{std::chrono::January / 1 / -21970}).time_since_epoch() - 1h});
+
+  assert(
+      tz->to_local(std::chrono::sys_time<std::chrono::days>{std::chrono::sys_days{std::chrono::January / 1 / 21970}}) ==
+      std::chrono::local_time<std::chrono::seconds>{
+          (std::chrono::sys_days{std::chrono::January / 1 / 21970}).time_since_epoch() - 1h});
+
+  assert(tz->to_local(std::chrono::sys_time<std::chrono::weeks>{}) ==
+         std::chrono::local_time<std::chrono::seconds>{
+             (std::chrono::sys_days{std::chrono::January / 1 / 1970}).time_since_epoch() - 1h});
+
+  assert(tz->to_local(std::chrono::sys_time<std::chrono::months>{}) ==
+         std::chrono::local_time<std::chrono::seconds>{
+             (std::chrono::sys_days{std::chrono::January / 1 / 1970}).time_since_epoch() - 1h});
+
+  assert(tz->to_local(std::chrono::sys_time<std::chrono::years>{}) ==
+         std::chrono::local_time<std::chrono::seconds>{
+             (std::chrono::sys_days{std::chrono::January / 1 / 1970}).time_since_epoch() - 1h});
+
+  return 0;
+}