[libc++][chrono] Adds operator<=> for day.

Since the calendar classes were introduced in C++20 there's no need to
keep the old comparison operators.

This commit does the day calender class, the other calendar classes will
be in a followup commit.

Implements parts of:
- P1614R2 The mothership has landed

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D128603

libcxx/include/__chrono/day.h

@@ -12,6 +12,7 @@
 
 #include <__chrono/duration.h>
 #include <__config>
+#include <compare>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -45,25 +46,9 @@ _LIBCPP_HIDE_FROM_ABI inline constexpr
 bool operator==(const day& __lhs, const day& __rhs) noexcept
 { return static_cast<unsigned>(__lhs) == static_cast<unsigned>(__rhs); }
 
-_LIBCPP_HIDE_FROM_ABI inline constexpr
-bool operator!=(const day& __lhs, const day& __rhs) noexcept
-{ return !(__lhs == __rhs); }
-
-_LIBCPP_HIDE_FROM_ABI inline constexpr
-bool operator< (const day& __lhs, const day& __rhs) noexcept
-{ return static_cast<unsigned>(__lhs) <  static_cast<unsigned>(__rhs); }
-
-_LIBCPP_HIDE_FROM_ABI inline constexpr
-bool operator> (const day& __lhs, const day& __rhs) noexcept
-{ return __rhs < __lhs; }
-
-_LIBCPP_HIDE_FROM_ABI inline constexpr
-bool operator<=(const day& __lhs, const day& __rhs) noexcept
-{ return !(__rhs < __lhs);}
-
-_LIBCPP_HIDE_FROM_ABI inline constexpr
-bool operator>=(const day& __lhs, const day& __rhs) noexcept
-{ return !(__lhs < __rhs); }
+_LIBCPP_HIDE_FROM_ABI constexpr strong_ordering operator<=>(const day& __lhs, const day& __rhs) noexcept {
+  return static_cast<unsigned>(__lhs) <=> static_cast<unsigned>(__rhs);
+}
 
 _LIBCPP_HIDE_FROM_ABI inline constexpr
 day operator+ (const day& __lhs, const days& __rhs) noexcept

libcxx/include/chrono

@@ -13,6 +13,8 @@
 /*
     chrono synopsis
 
+#include <compare> // C++20
+
 namespace std
 {
 namespace chrono
@@ -325,11 +327,7 @@ struct last_spec;
 
 class day;
 constexpr bool operator==(const day& x, const day& y) noexcept;
-constexpr bool operator!=(const day& x, const day& y) noexcept;
-constexpr bool operator< (const day& x, const day& y) noexcept;
-constexpr bool operator> (const day& x, const day& y) noexcept;
-constexpr bool operator<=(const day& x, const day& y) noexcept;
-constexpr bool operator>=(const day& x, const day& y) noexcept;
+constexpr strong_ordering operator<=>(const day& x, const day& y) noexcept;
 constexpr day  operator+(const day&  x, const days& y) noexcept;
 constexpr day  operator+(const days& x, const day&  y) noexcept;
 constexpr day  operator-(const day&  x, const days& y) noexcept;
@@ -715,9 +713,11 @@ constexpr chrono::year                                  operator ""y(unsigned lo
 #include <__chrono/year_month_day.h>
 #include <__chrono/year_month_weekday.h>
 #include <__config>
-#include <compare>
 #include <version>
 
+// standard-mandated includes
+#include <compare>
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif

libcxx/test/std/time/time.cal/time.cal.day/time.cal.day.nonmembers/comparisons.pass.cpp

@@ -12,9 +12,8 @@
 
 // constexpr bool operator==(const day& x, const day& y) noexcept;
 //   Returns: unsigned{x} == unsigned{y}.
-// constexpr bool operator<(const day& x, const day& y) noexcept;
-//   Returns: unsigned{x} < unsigned{y}.
-
+// constexpr strong_ordering operator<=>(const day& x, const day& y) noexcept;
+//   Returns: unsigned{x} <=> unsigned{y}.
 
 #include <chrono>
 #include <type_traits>
@@ -23,23 +22,30 @@
 #include "test_macros.h"
 #include "test_comparisons.h"
 
-int main(int, char**)
-{
-    using day = std::chrono::day;
+constexpr bool test() {
+  using day = std::chrono::day;
 
-    AssertComparisonsAreNoexcept<day>();
-    AssertComparisonsReturnBool<day>();
+  // Validate invalid values. The range [0, 255] is guaranteed to be allowed.
+  assert(testOrderValues<day>(0U, 0U));
+  assert(testOrderValues<day>(0U, 1U));
+  assert(testOrderValues<day>(254U, 255U));
+  assert(testOrderValues<day>(255U, 255U));
 
-    static_assert(testComparisonsValues<day>(0U, 0U), "");
-    static_assert(testComparisonsValues<day>(0U, 1U), "");
+  // Validate some valid values.
+  for (unsigned i = 1; i < 10; ++i)
+    for (unsigned j = 1; j < 10; ++j)
+      assert(testOrderValues<day>(i, j));
 
-    //  Some 'ok' values as well
-    static_assert(testComparisonsValues<day>( 5U,  5U), "");
-    static_assert(testComparisonsValues<day>( 5U, 10U), "");
+  return true;
+}
 
-    for (unsigned i = 1; i < 10; ++i)
-        for (unsigned j = 1; j < 10; ++j)
-            assert(testComparisonsValues<day>(i, j));
+int main(int, char**) {
+  using day = std::chrono::day;
+  AssertOrderAreNoexcept<day>();
+  AssertOrderReturn<std::strong_ordering, day>();
+
+  test();
+  static_assert(test());
 
   return 0;
 }