[libc++] Add missing C++20 [time.point.arithmetic] (#143165)

This was part of https://wg21.link/p0355r7, but apparently never
implemented.

---------

Co-authored-by: MarcoFalke <*~=`'#}+{/-|&$^_@721217.xyz>
Co-authored-by: Hristo Hristov <zingam@outlook.com>

libcxx/include/__chrono/time_point.h

@@ -58,6 +58,19 @@ public:
 
   // arithmetic
 
+#if _LIBCPP_STD_VER >= 20
+  _LIBCPP_HIDE_FROM_ABI constexpr time_point& operator++() {
+    ++__d_;
+    return *this;
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr time_point operator++(int) { return time_point{__d_++}; }
+  _LIBCPP_HIDE_FROM_ABI constexpr time_point& operator--() {
+    --__d_;
+    return *this;
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr time_point operator--(int) { return time_point{__d_--}; }
+#endif // _LIBCPP_STD_VER >= 20
+
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 time_point& operator+=(const duration& __d) {
     __d_ += __d;
     return *this;

libcxx/include/chrono

@@ -132,6 +132,11 @@ public:
 
     // arithmetic
 
+    constexpr time_point& operator++();    // C++20
+    constexpr time_point  operator++(int); // C++20
+    constexpr time_point& operator--();    // C++20
+    constexpr time_point  operator--(int); // C++20
+
     time_point& operator+=(const duration& d); // constexpr in C++17
     time_point& operator-=(const duration& d); // constexpr in C++17
 

libcxx/test/std/time/time.point/time.point.arithmetic/op_++.pass.cpp

@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+// REQUIRES: std-at-least-c++20
+
+// <chrono>
+
+// time_point
+
+// constexpr time_point& operator++();
+
+#include <cassert>
+#include <chrono>
+
+#include "test_macros.h"
+
+constexpr bool test() {
+  using Clock    = std::chrono::system_clock;
+  using Duration = std::chrono::milliseconds;
+  std::chrono::time_point<Clock, Duration> t{Duration{5}};
+  std::chrono::time_point<Clock, Duration>& tref{++t};
+  assert(&tref == &t);
+  assert(tref.time_since_epoch() == Duration{6});
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/time/time.point/time.point.arithmetic/op_++int.pass.cpp

@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+// REQUIRES: std-at-least-c++20
+
+// <chrono>
+
+// time_point
+
+// constexpr time_point operator++(int);
+
+#include <cassert>
+#include <chrono>
+
+#include "test_macros.h"
+
+constexpr bool test() {
+  using Clock    = std::chrono::system_clock;
+  using Duration = std::chrono::milliseconds;
+  std::chrono::time_point<Clock, Duration> t1{Duration{3}};
+  std::chrono::time_point<Clock, Duration> t2{t1++};
+  assert(t1.time_since_epoch() == Duration{4});
+  assert(t2.time_since_epoch() == Duration{3});
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/time/time.point/time.point.arithmetic/op_--.pass.cpp

@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+// REQUIRES: std-at-least-c++20
+
+// <chrono>
+
+// time_point
+
+// constexpr time_point& operator--();
+
+#include <cassert>
+#include <chrono>
+
+#include "test_macros.h"
+
+constexpr bool test() {
+  using Clock    = std::chrono::system_clock;
+  using Duration = std::chrono::milliseconds;
+  std::chrono::time_point<Clock, Duration> t{Duration{5}};
+  std::chrono::time_point<Clock, Duration>& tref{--t};
+  assert(&tref == &t);
+  assert(tref.time_since_epoch() == Duration{4});
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/time/time.point/time.point.arithmetic/op_--int.pass.cpp

@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+// REQUIRES: std-at-least-c++20
+
+// <chrono>
+
+// time_point
+
+// constexpr time_point operator--(int);
+
+#include <cassert>
+#include <chrono>
+
+#include "test_macros.h"
+
+constexpr bool test() {
+  using Clock    = std::chrono::system_clock;
+  using Duration = std::chrono::milliseconds;
+  std::chrono::time_point<Clock, Duration> t1{Duration{3}};
+  std::chrono::time_point<Clock, Duration> t2{t1--};
+  assert(t1.time_since_epoch() == Duration{2});
+  assert(t2.time_since_epoch() == Duration{3});
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}