[libc++] P2747R2: constexpr placement new (library part)

Paper link: https://wg21.link/P2747R2

The library changes affect direct `operator new` and `operator new[]`
calls even when the core language changes are absent. The approach of
Clang for "actual" placement new in constant evaluation doesn't depend
on the library changes.

The changes are not available for MS ABI because the `operator new` and
`operator new[]` are from VCRuntime's `<vcruntime_new.h>`.

Drive-by: formatting the whole `new.pass.cpp` and `new_array.pass.cpp`.

Author: frederick-vs-ja

libcxx/docs/FeatureTestMacroTable.rst

@@ -404,7 +404,7 @@ Status
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_bitset``                                       ``202306L``
     ---------------------------------------------------------- -----------------
-    ``__cpp_lib_constexpr_new``                                *unimplemented*
+    ``__cpp_lib_constexpr_new``                                ``202406L``
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_constrained_equality``                         *unimplemented*
     ---------------------------------------------------------- -----------------

libcxx/docs/ReleaseNotes/20.rst

@@ -38,7 +38,7 @@ What's New in Libc++ 20.0.0?
 Implemented Papers
 ------------------
 
-- TODO
+- P2747R2 - ``constexpr`` placement new  (`Github <https://github.com/llvm/llvm-project/issues/105427>`__)
 
 
 Improvements and New Features

libcxx/docs/Status/Cxx2cPapers.csv

@@ -63,7 +63,7 @@
 "`P2642R6 <https://wg21.link/P2642R6>`__","Padded ``mdspan`` layouts","2024-03 (Tokyo)","","",""
 "`P3029R1 <https://wg21.link/P3029R1>`__","Better ``mdspan``'s CTAD","2024-03 (Tokyo)","|Complete|","19.0",""
 "","","","","",""
-"`P2747R2 <https://wg21.link/P2747R2>`__","``constexpr`` placement new","2024-06 (St. Louis)","","",""
+"`P2747R2 <https://wg21.link/P2747R2>`__","``constexpr`` placement new","2024-06 (St. Louis)","|Complete|","20.0",""
 "`P2997R1 <https://wg21.link/P2997R1>`__","Removing the common reference requirement from the indirectly invocable concepts","2024-06 (St. Louis)","|Complete| [#note-P2997R1]_","19.0",""
 "`P2389R2 <https://wg21.link/P2389R2>`__","``dextents`` Index Type Parameter","2024-06 (St. Louis)","|Complete|","19.0",""
 "`P3168R2 <https://wg21.link/P3168R2>`__","Give ``std::optional`` Range Support","2024-06 (St. Louis)","","","|ranges|"

libcxx/include/__config

@@ -788,6 +788,12 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_CONSTEXPR_SINCE_CXX23
 #  endif
 
+#  if _LIBCPP_STD_VER >= 26
+#    define _LIBCPP_CONSTEXPR_SINCE_CXX26 constexpr
+#  else
+#    define _LIBCPP_CONSTEXPR_SINCE_CXX26
+#  endif
+
 #  ifndef _LIBCPP_WEAK
 #    define _LIBCPP_WEAK __attribute__((__weak__))
 #  endif

libcxx/include/new

@@ -242,8 +242,14 @@ _LIBCPP_OVERRIDABLE_FUNC_VIS void operator delete[](void* __p, std::size_t __sz,
 #    endif
 #  endif
 
-_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI void* operator new(std::size_t, void* __p) _NOEXCEPT { return __p; }
-_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI void* operator new[](std::size_t, void* __p) _NOEXCEPT { return __p; }
+_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void*
+operator new(std::size_t, void* __p) _NOEXCEPT {
+  return __p;
+}
+_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void*
+operator new[](std::size_t, void* __p) _NOEXCEPT {
+  return __p;
+}
 inline _LIBCPP_HIDE_FROM_ABI void operator delete(void*, void*) _NOEXCEPT {}
 inline _LIBCPP_HIDE_FROM_ABI void operator delete[](void*, void*) _NOEXCEPT {}
 

libcxx/include/version

@@ -510,7 +510,9 @@ __cpp_lib_void_t                                        201411L <type_traits>
 # undef  __cpp_lib_bind_front
 # define __cpp_lib_bind_front                           202306L
 # define __cpp_lib_bitset                               202306L
-// # define __cpp_lib_constexpr_new                        202406L
+# if !defined(_LIBCPP_ABI_VCRUNTIME)
+#   define __cpp_lib_constexpr_new                      202406L
+# endif
 // # define __cpp_lib_constrained_equality                 202403L
 // # define __cpp_lib_copyable_function                    202306L
 // # define __cpp_lib_debugging                            202311L

libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.placement/new.pass.cpp

@@ -15,19 +15,30 @@
 
 int A_constructed = 0;
 
-struct A
-{
-    A() {++A_constructed;}
-    ~A() {--A_constructed;}
+struct A {
+  A() { ++A_constructed; }
+  ~A() { --A_constructed; }
 };
 
-int main(int, char**)
-{
-    char buf[sizeof(A)];
+TEST_CONSTEXPR_OPERATOR_NEW void test_direct_call() {
+  assert(::operator new(sizeof(int), &A_constructed) == &A_constructed);
 
-    A* ap = new(buf) A;
-    assert((char*)ap == buf);
-    assert(A_constructed == 1);
+  char ch = '*';
+  assert(::operator new(1, &ch) == &ch);
+  assert(ch == '*');
+}
+
+#ifdef __cpp_lib_constexpr_new
+static_assert((test_direct_call(), true));
+#endif
+
+int main(int, char**) {
+  char buf[sizeof(A)];
+
+  A* ap = new (buf) A;
+  assert((char*)ap == buf);
+  assert(A_constructed == 1);
 
+  test_direct_call();
   return 0;
 }

libcxx/test/std/language.support/support.dynamic/new.delete/new.delete.placement/new_array.pass.cpp

@@ -15,23 +15,34 @@
 
 int A_constructed = 0;
 
-struct A
-{
-    A() {++A_constructed;}
-    ~A() {--A_constructed;}
+struct A {
+  A() { ++A_constructed; }
+  ~A() { --A_constructed; }
 };
 
-int main(int, char**)
-{
-    const std::size_t Size = 3;
-    // placement new might require additional space.
-    const std::size_t ExtraSize = 64;
-    char buf[Size*sizeof(A) + ExtraSize];
+TEST_CONSTEXPR_OPERATOR_NEW void test_direct_call() {
+  assert(::operator new[](sizeof(int), &A_constructed) == &A_constructed);
 
-    A* ap = new(buf) A[Size];
-    assert((char*)ap >= buf);
-    assert((char*)ap < (buf + ExtraSize));
-    assert(A_constructed == Size);
+  char ch = '*';
+  assert(::operator new[](1, &ch) == &ch);
+  assert(ch == '*');
+}
+
+#ifdef __cpp_lib_constexpr_new
+static_assert((test_direct_call(), true));
+#endif
+
+int main(int, char**) {
+  const std::size_t Size = 3;
+  // placement new might require additional space.
+  const std::size_t ExtraSize = 64;
+  char buf[Size * sizeof(A) + ExtraSize];
+
+  A* ap = new (buf) A[Size];
+  assert((char*)ap >= buf);
+  assert((char*)ap < (buf + ExtraSize));
+  assert(A_constructed == Size);
 
+  test_direct_call();
   return 0;
 }

libcxx/test/std/language.support/support.limits/support.limits.general/new.version.compile.pass.cpp

@@ -171,16 +171,16 @@
 
 #elif TEST_STD_VER > 23
 
-# if !defined(_LIBCPP_VERSION)
+# if !defined(_LIBCPP_ABI_VCRUNTIME)
 #   ifndef __cpp_lib_constexpr_new
 #     error "__cpp_lib_constexpr_new should be defined in c++26"
 #   endif
 #   if __cpp_lib_constexpr_new != 202406L
 #     error "__cpp_lib_constexpr_new should have the value 202406L in c++26"
 #   endif
-# else // _LIBCPP_VERSION
+# else
 #   ifdef __cpp_lib_constexpr_new
-#     error "__cpp_lib_constexpr_new should not be defined because it is unimplemented in libc++!"
+#     error "__cpp_lib_constexpr_new should not be defined when the requirement '!defined(_LIBCPP_ABI_VCRUNTIME)' is not met!"
 #   endif
 # endif
 

libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp

@@ -6554,16 +6554,16 @@
 #   error "__cpp_lib_constexpr_memory should have the value 202202L in c++26"
 # endif
 
-# if !defined(_LIBCPP_VERSION)
+# if !defined(_LIBCPP_ABI_VCRUNTIME)
 #   ifndef __cpp_lib_constexpr_new
 #     error "__cpp_lib_constexpr_new should be defined in c++26"
 #   endif
 #   if __cpp_lib_constexpr_new != 202406L
 #     error "__cpp_lib_constexpr_new should have the value 202406L in c++26"
 #   endif
-# else // _LIBCPP_VERSION
+# else
 #   ifdef __cpp_lib_constexpr_new
-#     error "__cpp_lib_constexpr_new should not be defined because it is unimplemented in libc++!"
+#     error "__cpp_lib_constexpr_new should not be defined when the requirement '!defined(_LIBCPP_ABI_VCRUNTIME)' is not met!"
 #   endif
 # endif
 

libcxx/test/support/test_macros.h

@@ -184,6 +184,12 @@
 #  define TEST_CONSTEXPR_CXX23
 #endif
 
+#if TEST_STD_VER >= 26
+#  define TEST_CONSTEXPR_CXX26 constexpr
+#else
+#  define TEST_CONSTEXPR_CXX26
+#endif
+
 #define TEST_ALIGNAS_TYPE(...) TEST_ALIGNAS(TEST_ALIGNOF(__VA_ARGS__))
 
 #if !TEST_HAS_FEATURE(cxx_rtti) && !defined(__cpp_rtti) \
@@ -497,4 +503,12 @@ inline Tp const& DoNotOptimize(Tp const& value) {
 #  define TEST_HAS_EXPLICIT_THIS_PARAMETER
 #endif
 
+// Placement `operator new`/`operator new[]` are not yet constexpr in C++26
+// when using MS ABI, because they are from <vcruntime_new.h>.
+#if defined(__cpp_lib_constexpr_new) && __cpp_lib_constexpr_new >= 202406L
+#  define TEST_CONSTEXPR_OPERATOR_NEW constexpr
+#else
+#  define TEST_CONSTEXPR_OPERATOR_NEW
+#endif
+
 #endif // SUPPORT_TEST_MACROS_HPP

libcxx/utils/generate_feature_test_macro_components.py

@@ -363,7 +363,8 @@ def add_version_header(tc):
             "name": "__cpp_lib_constexpr_new",
             "values": {"c++26": 202406},  # P2747R2 constexpr placement new
             "headers": ["new"],
-            "unimplemented": True,
+            "test_suite_guard": "!defined(_LIBCPP_ABI_VCRUNTIME)",
+            "libcxx_guard": "!defined(_LIBCPP_ABI_VCRUNTIME)",
         },
         {
             "name": "__cpp_lib_constexpr_numeric",