[libc++] Implement LWG4169: std::atomic<T>'s default constructor should be constrained (#131950)

Drive-by: Rename `constexpr_noexcept.compile.pass.cpp` to
`ctor.default.pass.cpp` as test coverage is added to it, and compile and
run it in all modes.

Author: frederick-vs-ja

libcxx/docs/Status/Cxx26Issues.csv

@@ -108,7 +108,7 @@
 "`LWG4154 <https://wg21.link/LWG4154>`__","The Mandates for ``std::packaged_task``'s constructor from a callable entity should consider decaying","2024-11 (Wrocław)","","","`#118364 <https://github.com/llvm/llvm-project/issues/118364>`__",""
 "`LWG4157 <https://wg21.link/LWG4157>`__","The resolution of LWG3465 was damaged by P2167R3","2024-11 (Wrocław)","|Complete|","20","`#118365 <https://github.com/llvm/llvm-project/issues/118365>`__",""
 "`LWG4164 <https://wg21.link/LWG4164>`__","Missing guarantees for ``forward_list`` modifiers","2024-11 (Wrocław)","|Complete|","21","`#118366 <https://github.com/llvm/llvm-project/issues/118366>`__",""
-"`LWG4169 <https://wg21.link/LWG4169>`__","``std::atomic<T>``'s default constructor should be constrained","2024-11 (Wrocław)","","","`#118367 <https://github.com/llvm/llvm-project/issues/118367>`__",""
+"`LWG4169 <https://wg21.link/LWG4169>`__","``std::atomic<T>``'s default constructor should be constrained","2024-11 (Wrocław)","|Complete|","24","`#118367 <https://github.com/llvm/llvm-project/issues/118367>`__",""
 "`LWG4170 <https://wg21.link/LWG4170>`__","``contiguous_iterator`` should require ``to_address(I{})``","2024-11 (Wrocław)","","","`#118368 <https://github.com/llvm/llvm-project/issues/118368>`__",""
 "","","","","","",""
 "`LWG3578 <https://wg21.link/LWG3578>`__","Iterator SCARYness in the context of associative container merging","2025-02 (Hagenberg)","","","`#127859 <https://github.com/llvm/llvm-project/issues/127859>`__",""

libcxx/include/__atomic/atomic.h

@@ -22,6 +22,7 @@
 #include <__cstddef/ptrdiff_t.h>
 #include <__memory/addressof.h>
 #include <__type_traits/enable_if.h>
+#include <__type_traits/is_constructible.h>
 #include <__type_traits/is_floating_point.h>
 #include <__type_traits/is_function.h>
 #include <__type_traits/is_integral.h>
@@ -258,7 +259,9 @@ struct atomic : public __atomic_base<typename __check_atomic_mandates<_Tp>::type
   using __base _LIBCPP_NODEBUG = __atomic_base<_Tp>;
 
 #if _LIBCPP_STD_VER >= 20
-  _LIBCPP_HIDE_FROM_ABI atomic() = default;
+  _LIBCPP_HIDE_FROM_ABI atomic()
+    requires is_default_constructible_v<_Tp>
+  = default;
 #else
   _LIBCPP_HIDE_FROM_ABI atomic() _NOEXCEPT = default;
 #endif

libcxx/test/std/atomics/atomics.types.generic/constexpr_noexcept.compile.pass.cpp

@@ -0,0 +1,71 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <atomic>
+
+// atomic() noexcept = default;                                        // until C++20
+// constexpr atomic() noexcept(is_nothrow_default_constructible_v<T>); // since C++20
+
+// Constraints: is_default_constructible_v<T> is true. (since C++20)
+
+#include <atomic>
+#include <type_traits>
+
+#include "test_macros.h"
+
+template <typename T>
+void test() {
+#if TEST_STD_VER >= 11
+  constexpr T a{};
+  (void)a;
+#  if TEST_STD_VER >= 20
+  [[maybe_unused]] constexpr T b;
+#  endif
+#else
+  T a;
+  (void)a;
+#endif
+  static_assert(std::is_nothrow_constructible<T>::value, "");
+  ASSERT_NOEXCEPT(T{});
+}
+
+struct throwing {
+  throwing() {}
+};
+
+struct trivial {
+  int a;
+};
+
+struct not_default_constructible {
+  explicit not_default_constructible(int) {}
+};
+
+int main(int, char**) {
+  test<std::atomic<bool> >();
+  test<std::atomic<int> >();
+  test<std::atomic<int*> >();
+  test<std::atomic<trivial> >();
+  test<std::atomic_flag>();
+
+#if TEST_STD_VER >= 20
+  static_assert(!std::is_nothrow_constructible_v<std::atomic<throwing>>);
+  ASSERT_NOT_NOEXCEPT(std::atomic<throwing>{});
+
+  static_assert(!std::is_default_constructible_v<std::atomic<not_default_constructible>>);
+#else
+  static_assert(std::is_nothrow_constructible<std::atomic<throwing> >::value, "");
+
+  ASSERT_NOEXCEPT(std::atomic<throwing>{});
+#  ifndef TEST_COMPILER_GCC
+  static_assert(std::is_default_constructible<std::atomic<not_default_constructible> >::value, "");
+#  endif
+#endif
+
+  return 0;
+}