[libc++] P2747R2: constexpr placement new (library part) (#105768)

This patch implements https://wg21.link/P2747R2.

The library changes affect direct `operator new` and `operator new[]`
calls even when the core language changes are absent.

The changes are not available for MS ABI because the `operator new` and
`operator new[]` are from VCRuntime's `<vcruntime_new.h>`. A feature
request was submitted for that [1].

As a drive-by change, the patch reformatted the whole `new.pass.cpp` and
`new_array.pass.cpp` tests.

Closes #105427

[1]: https://developercommunity.visualstudio.com/t/constexpr-for-placement-operator-newope/10730304.

Author: frederick-vs-ja

libcxx/docs/FeatureTestMacroTable.rst

@@ -406,7 +406,7 @@ 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

@@ -38,6 +38,7 @@ What's New in Libc++ 20.0.0?
 Implemented Papers
 ------------------
 
+- P2747R2: ``constexpr`` placement new (`Github <https://github.com/llvm/llvm-project/issues/105427>`__)
 - P2609R3: Relaxing Ranges Just A Smidge (`Github <https://github.com/llvm/llvm-project/issues/105253>`__)
 - P2985R0: A type trait for detecting virtual base classes (`Github <https://github.com/llvm/llvm-project/issues/105432>`__)
 

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

@@ -79,8 +79,8 @@ void  operator delete[](void* ptr, const std::nothrow_t&) noexcept;     // repla
 void  operator delete[](void* ptr, std::align_val_t alignment,
                         const std::nothrow_t&) noexcept;                // replaceable, C++17
 
-void* operator new  (std::size_t size, void* ptr) noexcept;             // nodiscard in C++20
-void* operator new[](std::size_t size, void* ptr) noexcept;             // nodiscard in C++20
+void* operator new  (std::size_t size, void* ptr) noexcept;             // nodiscard in C++20, constexpr since C++26
+void* operator new[](std::size_t size, void* ptr) noexcept;             // nodiscard in C++20, constexpr since C++26
 void  operator delete  (void* ptr, void*) noexcept;
 void  operator delete[](void* ptr, void*) noexcept;
 
@@ -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

@@ -513,7 +513,9 @@ __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

@@ -6555,16 +6555,16 @@
 #   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",