[libcxx] makes pair conditionally trivially copyable for C++20

`pair` can have trivial copy and move assignment operators
when both its members have trivial copy and move operators,
which opens `pair` up to being trivially copyable. This is
the first of three possible commits to bring users a more
robust implementation.

It's currently unclear how to implement this without using a
_requires-clause_, so the feature is restricted to C++20 for
now. There has also been some interest in moving this out of
the unstable ABI mode. Both of these changes are intended to
arrive in future pull requests, should their feasibility pan
out.

libcxx/include/__config

@@ -180,6 +180,9 @@
 // requires code not to make these assumptions.
 #    define _LIBCPP_ABI_USE_WRAP_ITER_IN_STD_ARRAY
 #    define _LIBCPP_ABI_USE_WRAP_ITER_IN_STD_STRING_VIEW
+// Allows std::pair's copy assignment and move assignment operators to be trivial
+// when both its element types' respective assignment operators are trivial.
+#    define _LIBCPP_ABI_PAIR_TRIVIALLY_COPYABLE
 #  elif _LIBCPP_ABI_VERSION == 1
 #    if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))
 // Enable compiling copies of now inline methods into the dylib to support

libcxx/include/__utility/pair.h

@@ -11,6 +11,7 @@
 
 #include <__compare/common_comparison_category.h>
 #include <__compare/synth_three_way.h>
+#include <__concepts/assignable.h>
 #include <__concepts/different_from.h>
 #include <__config>
 #include <__fwd/array.h>
@@ -26,6 +27,7 @@
 #include <__type_traits/common_type.h>
 #include <__type_traits/conditional.h>
 #include <__type_traits/decay.h>
+#include <__type_traits/enable_if.h>
 #include <__type_traits/integral_constant.h>
 #include <__type_traits/is_assignable.h>
 #include <__type_traits/is_constructible.h>
@@ -40,8 +42,11 @@
 #include <__type_traits/is_nothrow_copy_constructible.h>
 #include <__type_traits/is_nothrow_default_constructible.h>
 #include <__type_traits/is_nothrow_move_assignable.h>
+#include <__type_traits/is_object.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_swappable.h>
+#include <__type_traits/is_trivially_copy_assignable.h>
+#include <__type_traits/is_trivially_move_assignable.h>
 #include <__type_traits/nat.h>
 #include <__type_traits/remove_cvref.h>
 #include <__type_traits/unwrap_ref.h>
@@ -67,6 +72,14 @@ struct __non_trivially_copyable_base {
   __non_trivially_copyable_base(__non_trivially_copyable_base const&) _NOEXCEPT {}
 };
 
+#if _LIBCPP_STD_VER >= 20
+template <class _Tp>
+concept __trivially_copy_assignable_object = is_trivially_copy_assignable_v<_Tp> && is_object_v<_Tp>;
+
+template <class _Tp>
+concept __trivially_move_assignable_object = is_trivially_move_assignable_v<_Tp> && is_object_v<_Tp>;
+#endif
+
 #if _LIBCPP_STD_VER >= 23
 template <class _Tp>
 struct __is_specialization_of_subrange : false_type {};
@@ -236,6 +249,28 @@ struct _LIBCPP_TEMPLATE_VIS pair
              typename __make_tuple_indices<sizeof...(_Args1)>::type(),
              typename __make_tuple_indices<sizeof...(_Args2) >::type()) {}
 
+#  if _LIBCPP_STD_VER >= 20 && defined(_LIBCPP_ABI_PAIR_TRIVIALLY_COPYABLE)
+  _LIBCPP_HIDE_FROM_ABI pair& operator=(pair const& __p) = default;
+  _LIBCPP_HIDE_FROM_ABI pair& operator=(pair&& __p) = default;
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair& operator=(pair const& __p)
+      _NOEXCEPT_(is_nothrow_copy_assignable<first_type>::value&& is_nothrow_copy_assignable<second_type>::value)
+    requires is_copy_assignable_v<first_type> && is_copy_assignable_v<second_type> && (!(__trivially_copy_assignable_object<first_type> && __trivially_copy_assignable_object<second_type>))
+  {
+    first  = __p.first;
+    second = __p.second;
+    return *this;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair& operator=(pair&& __p)
+      _NOEXCEPT_(is_nothrow_move_assignable<first_type>::value&& is_nothrow_move_assignable<second_type>::value)
+    requires is_move_assignable_v<first_type> && is_move_assignable_v<second_type> && (!(__trivially_move_assignable_object<first_type> && __trivially_move_assignable_object<second_type>))
+  {
+    first  = std::forward<first_type>(__p.first);
+    second = std::forward<second_type>(__p.second);
+    return *this;
+  }
+#  else
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair&
   operator=(__conditional_t< is_copy_assignable<first_type>::value && is_copy_assignable<second_type>::value,
                              pair,
@@ -254,6 +289,7 @@ struct _LIBCPP_TEMPLATE_VIS pair
     second = std::forward<second_type>(__p.second);
     return *this;
   }
+#  endif
 
   template <
       class _U1,

libcxx/test/std/utilities/utility/pairs/pairs.pair/assign_pair.pass.cpp

@@ -88,6 +88,16 @@ TEST_CONSTEXPR_CXX20 bool test() {
     assert(&p.second == &inc_obj);
   }
 
+#if TEST_STD_VER >= 20 && defined(_LIBCPP_ABI_PAIR_TRIVIALLY_COPYABLE)
+  static_assert(std::is_trivially_copy_assignable<std::pair<int, int>>::value, "");
+#else
+  static_assert(!std::is_trivially_copy_assignable<std::pair<int, int>>::value, "");
+#endif
+
+  static_assert(!std::is_trivially_copy_assignable<std::pair<CountAssign, int>>::value, "");
+  static_assert(!std::is_trivially_copy_assignable<std::pair<int, CountAssign>>::value, "");
+  static_assert(!std::is_trivially_copy_assignable<std::pair<CountAssign, CountAssign>>::value, "");
+
   return true;
 }
 

libcxx/test/std/utilities/utility/pairs/pairs.pair/assign_pair_cxx03.pass.cpp

@@ -62,7 +62,9 @@ int main(int, char**)
       assert(p.second == 'x');
     }
 
-  return 0;
+    static_assert(!std::is_trivially_copy_assignable<std::pair<int, int> >::value, "");
+
+    return 0;
 }
 
 struct Incomplete {};

libcxx/test/std/utilities/utility/pairs/pairs.pair/assign_rv_pair.pass.cpp

@@ -130,6 +130,17 @@ TEST_CONSTEXPR_CXX20 bool test() {
     static_assert(!std::is_move_assignable<P5>::value, "");
     static_assert(!std::is_move_assignable<P6>::value, "");
   }
+
+#if TEST_STD_VER >= 20 && defined(_LIBCPP_ABI_PAIR_TRIVIALLY_COPYABLE)
+  static_assert(std::is_trivially_move_assignable<std::pair<int, int>>::value, "");
+#else
+  static_assert(!std::is_trivially_move_assignable<std::pair<int, int>>::value, "");
+#endif
+
+  static_assert(!std::is_trivially_move_assignable<std::pair<CountAssign, int>>::value, "");
+  static_assert(!std::is_trivially_move_assignable<std::pair<int, CountAssign>>::value, "");
+  static_assert(!std::is_trivially_move_assignable<std::pair<CountAssign, CountAssign>>::value, "");
+
   return true;
 }
 

libcxx/test/std/utilities/utility/pairs/pairs.pair/trivially_copyable.compile.pass.cpp

@@ -0,0 +1,45 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <utility>
+
+// template <class T1, class T2> struct pair
+
+// Checks that `std::pair` is actually trivially copyable.
+
+#include <type_traits>
+#include <utility>
+
+#if defined(_LIBCPP_ABI_PAIR_TRIVIALLY_COPYABLE)
+static_assert(std::is_trivially_copyable_v<std::pair<int, int>>);
+static_assert(std::is_trivially_copyable_v<std::pair<int, int const>>);
+static_assert(std::is_trivially_copyable_v<std::pair<int const, int>>);
+static_assert(std::is_trivially_copyable_v<std::pair<int const, int const>>);
+
+static_assert(!std::is_trivially_copyable_v<std::pair<int, int&>>);
+static_assert(!std::is_trivially_copyable_v<std::pair<int&, int>>);
+static_assert(!std::is_trivially_copyable_v<std::pair<int&, int&>>);
+
+enum E {};
+static_assert(std::is_trivially_copyable_v<std::pair<E, E>>);
+static_assert(std::is_trivially_copyable_v<std::pair<E, E const>>);
+static_assert(std::is_trivially_copyable_v<std::pair<E const, E>>);
+static_assert(std::is_trivially_copyable_v<std::pair<E const, E const>>);
+
+static_assert(std::is_trivially_copyable_v<std::pair<E, int>>);
+static_assert(std::is_trivially_copyable_v<std::pair<E, int const>>);
+static_assert(std::is_trivially_copyable_v<std::pair<E const, int>>);
+static_assert(std::is_trivially_copyable_v<std::pair<E const, int const>>);
+
+struct S {};
+static_assert(std::is_trivially_copyable_v<std::pair<S, S>>);
+static_assert(std::is_trivially_copyable_v<std::pair<S, int>>);
+static_assert(std::is_trivially_copyable_v<std::pair<S, E>>);
+#endif