Implement C++20's P0784 (More constexpr containers)

[ldionne]

Summary

This commit adds std::construct_at, and marks various members of std::allocator_traits and std::allocator as constexpr. It also adds tests and turns the existing tests into hybrid constexpr/runtime tests.

Thanks to Richard Smith for initial work on this, and to Michael Park for D69803, D69132 and D69134, which are superseded by this patch.

[rsmith]

Moved some changes from the vector patch to here.

[rsmith]

Undo adding too many files to this change.

[mclow.lists]

Thanks for doing this, Richard.
A few things:

• Needs tests (as you said)
• The config macro stuff is all wrong (we build that file from a template)
• Use __libcpp_is_constant_evaluated instead of __builtin_is_constant_evaluated because it doesn't have to be #ifdefed. (see r364873 and r364884)
• If we're in a consteval block, we shouldn't be testing for exceptions being disabled, right?

[ldionne]

Complete implementation of P0784 with tests

[rsmith]

The mechanism by which this interacts with Clang looks good to me. I've not done any detailed analysis to check all the functions made constexpr by P0784 are handled by this patch, though.

libcxx/include/version

254

Should this be conditioned on compiler support being available?

libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/allocate_hint.pass.cpp

87

I really like this approach to testing this change :)

[ldionne]

In D68364#2280187, @rsmith wrote:

The mechanism by which this interacts with Clang looks good to me.

Excellent.

I've not done any detailed analysis to check all the functions made constexpr by P0784 are handled by this patch, though.

I spent like 4-5 hours making sure everything was to the spec :-). The only missing part is the ranges declarations.

libcxx/include/version

254

So.. I've decided not to do that in this patch so far.

The support for constexpr allocation was checked into Clang about a year ago, right? I actually expect this to be a slightly contentious point, but I'd like to assume that we're using a reasonably recent Clang. I don't see a strong point for being able to use new libc++ headers with an old Clang anyway, since vendors usually release the two together. IOW, supporting this would add complexity for virtually no benefit. I do agree it's a slightly more aggressive stance than we've had so far, but this sort of reasonable assumption makes it so much easier to write stuff for libc++.

libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/allocate_hint.pass.cpp

87

Thanks! That's how we do for all constexpr-ifications.

[rsmith]

libcxx/include/version

254

OK, just a few thoughts then I'm going to bow out of this; this seems like a policy decision for the libc++ maintainers to make.

In favor of dropping support for new libc++ + old clang: we generally don't permit version skew between different components of LLVM. It seems reasonable to expect all wanted parts of a particular LLVM release to be built together.

Against dropping support for new libc++ + old clang: we do support installing more than one version of LLVM (and in particular more than one version of Clang) on the same system, but because libc++ defaults to being installed in /usr/include/c++/v1, we don't seem to encourage installing more than one version of libc++, so -- even assuming we only support the *newest* version of libc++ going into /usr/include/c++/v1 -- new versions of libc++ need to work with old versions of Clang.

I think (largely by accident) Clang will prefer a libc++ installed into /usr/lib/clang/$VER/include over one from /usr/include/c++/v1. If we switched to installing libc++ there, I don't see any technical barrier to version-locking them, though I'm not sure what story that leaves for use of libc++ with GCC and other compilers. It seems worth noting that this is exactly what libstdc++ does in order to need to support only one version of GCC.

[ldionne]

libcxx/include/version

254

OK, just a few thoughts then I'm going to bow out of this; this seems like a policy decision for the libc++ maintainers to make.

Hear hear!

I think (largely by accident) Clang will prefer a libc++ installed into /usr/lib/clang/$VER/include over one from /usr/include/c++/v1. If we switched to installing libc++ there, I don't see any technical barrier to version-locking them, though I'm not sure what story that leaves for use of libc++ with GCC and other compilers. It seems worth noting that this is exactly what libstdc++ does in order to need to support only one version of GCC.

I think it would be great to do that. Honestly, this is a huge simplification and makes implementing new features much easier. Also, I think it's reasonable to support not-trunk compilers, like 6 months old or something like that. But not several years back.

One last question: do you know what controls where the libc++ headers are installed as part of the LLVM distribution?

[rsmith]

libcxx/include/version

254

Install paths are set in libcxx/include/CMakeLists.txt for the headers and in libcxx/src/CMakeLists.txt for the libraries (search for install() based on cmake variables LIBCXX_INSTALL_HEADER_PREFIX, LIBCXX_INSTALL_PREFIX, and LIBCXX_INSTALL_LIBRARY_DIR.

It would probably make sense to put the libc++ headers into somewhere like /usr/lib/clang/$VER/include/c++ instead of directly in /usr/lib/clang/$VER/include; that'll need some changes to the clang driver to make sure we look there. But of course that's doable if the two are version-locked :)

[ldionne]

libcxx/include/new

243

This breaks GCC (as of GCC 9). I don't know what mechanism GCC uses to tie into constexpr allocation, so I don't know what the fix is.

@jwakely Can you throw some hints at me?

libcxx/include/version

254

Oh, I thought it was more complicated than that for the LLVM distribution. Ok, thanks for the info.

[ldionne]

Add some tests from D69134.

[jwakely]

libcxx/include/new

243

GCC 9 doesn't support constexpr allocation at all.

[ldionne]

I'll go ahead and ship this even though I know it will cause problems on GCC 9, if GCC 10 supports constexpr allocation.

libcxx/include/new

243

Does GCC 10 support it?

[jwakely]

libcxx/include/new

243

GCC 9 doesn't support constexpr allocation at all. You can use __cpp_constexpr_dynamic_alloc to test for the feature.

[jwakely]

libcxx/include/new

243

Yes

[leonardchan]

Hi. Our GCC builders are failing with

[182/12433] CXX kernel-x64-gcc/obj/kernel/lib/ktl/ktl.dummy-new.cc.o
FAILED: kernel-x64-gcc/obj/kernel/lib/ktl/ktl.dummy-new.cc.o
/b/s/w/ir/k/fuchsia/prebuilt/third_party/goma/linux-x64/gomacc  ../../prebuilt/third_party/gcc/linux-x64/bin/x86_64-elf-g++ -MD -MF kernel-x64-gcc/obj/kernel/lib/ktl/ktl.dummy-new.cc.o.d -o kernel-x64...
In file included from ../../zircon/kernel/lib/ktl/dummy-new.cc:7:
../../prebuilt/third_party/clang/linux-x64/include/c++/v1/new: In function 'constexpr void* std::__2::__libcpp_allocate(size_t, size_t)':
../../prebuilt/third_party/clang/linux-x64/include/c++/v1/new:252:24: error: call to non-'constexpr' function 'void* operator new(size_t)'
  252 |   return ::operator new(__size);
      |          ~~~~~~~~~~~~~~^~~~~~~~
In file included from ../../zircon/kernel/lib/ktl/dummy-new.cc:7:
../../prebuilt/third_party/clang/linux-x64/include/c++/v1/new:186:66: note: 'void* operator new(size_t)' declared here
  186 | _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new(std::size_t __sz) _THROW_BAD_ALLOC;
      |                                                                  ^~~~~~~~

I presume it's because _LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE is defined from

#if !__has_builtin(__builtin_operator_new) || !__has_builtin(__builtin_operator_delete)
#define _LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE
#endif

Is there a recommended way for working around this? We're using GCC 10.2.1. Thanks.

[jwakely]

In D68364#2293971, @leonardchan wrote:

Is there a recommended way for working around this? We're using GCC 10.2.1. Thanks.

I don't think your implementation is valid. I think P0784 only allows new-expressions and calls to std::allocator::allocate in constexpr functions, not calls to operator new.

Can you use something like if (__builtin_is_constant_evaluated()) return new unsigned char[n]; and the equivalent in the corresponding deallocation function?

[ldionne]

In D68364#2294569, @jwakely wrote:

In D68364#2293971, @leonardchan wrote:

Is there a recommended way for working around this? We're using GCC 10.2.1. Thanks.

I don't think your implementation is valid. I think P0784 only allows new-expressions and calls to std::allocator::allocate in constexpr functions, not calls to operator new.

Hmm, right. It's actually impossible to implement __libcpp_allocate in a constexpr-friendly manner, since it's just allocating bytes.

Can you use something like if (__builtin_is_constant_evaluated()) return new unsigned char[n]; and the equivalent in the corresponding deallocation function?

That doesn't work, since we can't use unsigned chars as storage for arbitrary Ts in constexpr.

I fixed it by marking __libcpp_allocate as non-constexpr and calling operator new in allocator<T>::allocate, which seems to work on GCC.

[rsmith]

In D68364#2299336, @ldionne wrote:

In D68364#2294569, @jwakely wrote:

I don't think your implementation is valid. I think P0784 only allows new-expressions and calls to std::allocator::allocate in constexpr functions, not calls to operator new.

Hmm, right. It's actually impossible to implement __libcpp_allocate in a constexpr-friendly manner, since it's just allocating bytes.

Right, we need to rely on implementation magic. I wrote up a document proposing how to do this a year or so ago, and shared it with some GCC folks (I don't remember whether it was mailed to the GCC mailing list or just to Jason or what) suggesting that we follow option 3 of that doc -- in particular, this permits calls to operator new "transitively within a member of std::allocator<T>" (emphasis mine), precisely so that stdlib implementations can pull out helper functions as libc++ does, and don't need to branch on is_constant_evaluated() for this at all. That said, this is all vendor extension territory, and GCC is of course under no obligation to follow my proposal, and indeed seems not to have followed this detail of it. So be it.

But... there's no question of "valid" here :)

[yroux]

Hi Louis,

ARM and AArch64 libcxx bots are still broken after this commit (host
compiler is clang-8), logs are available here:
http://lab.llvm.org:8011/builders/libcxx-libcxxabi-libunwind-aarch64-linux/builds/3143

Cheers,
Yvan

libcxx/docs/FeatureTestMacroTable.rst

.. table:: Current Status
     ``__cpp_lib_bind_front``                          *unimplemented*  
     ------------------------------------------------- -----------------
     ``__cpp_lib_bit_cast``                            *unimplemented*  
     ------------------------------------------------- -----------------
     ``__cpp_lib_char8_t``                             ``201811L``      
     ------------------------------------------------- -----------------
     ``__cpp_lib_concepts``                            *unimplemented*  
     ------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_dynamic_alloc``             ``201907L``      
+    ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_misc``                      *unimplemented*  
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_swap_algorithms``           *unimplemented*  
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_utility``                   ``201811L``      
     ------------------------------------------------- -----------------
     ``__cpp_lib_destroying_delete``                   ``201806L``      
     ------------------------------------------------- -----------------

libcxx/include/memory

struct allocator_traits
     typedef Alloc::propagate_on_container_swap
           | false_type                   propagate_on_container_swap;
     typedef Alloc::is_always_equal
           | is_empty                     is_always_equal;
 
     template <class T> using rebind_alloc  = Alloc::rebind<T>::other | Alloc<T, Args...>;
     template <class T> using rebind_traits = allocator_traits<rebind_alloc<T>>;
 
-    static pointer allocate(allocator_type& a, size_type n);                          // [[nodiscard]] in C++20
-    static pointer allocate(allocator_type& a, size_type n, const_void_pointer hint); // [[nodiscard]] in C++20
+    static pointer allocate(allocator_type& a, size_type n);                          // constexpr and [[nodiscard]] in C++20
+    static pointer allocate(allocator_type& a, size_type n, const_void_pointer hint); // constexpr and [[nodiscard]] in C++20
 
-    static void deallocate(allocator_type& a, pointer p, size_type n) noexcept;
+    static void deallocate(allocator_type& a, pointer p, size_type n) noexcept; // constexpr in C++20
 
     template <class T, class... Args>
-        static void construct(allocator_type& a, T* p, Args&&... args);
+    static void construct(allocator_type& a, T* p, Args&&... args); // constexpr in C++20
 
     template <class T>
-        static void destroy(allocator_type& a, T* p);
+    static void destroy(allocator_type& a, T* p); // constexpr in C++20
 
-    static size_type max_size(const allocator_type& a); // noexcept in C++14
-
-    static allocator_type
-        select_on_container_copy_construction(const allocator_type& a);
+    static size_type max_size(const allocator_type& a); // noexcept in C++14, constexpr in C++20
+    static allocator_type select_on_container_copy_construction(const allocator_type& a); // constexpr in C++20
 };
 
 template <>
 class allocator<void> // deprecated in C++17, removed in C++20
 {
 public:
     typedef void*                                 pointer;
     typedef const void*                           const_pointer;
public:
 
     typedef true_type propagate_on_container_move_assignment;
     typedef true_type is_always_equal;
 
     constexpr allocator() noexcept;                      // constexpr in C++20
     constexpr allocator(const allocator&) noexcept;      // constexpr in C++20
     template <class U>
       constexpr allocator(const allocator<U>&) noexcept; // constexpr in C++20
-    ~allocator();
+    ~allocator();                                        // constexpr in C++20
     pointer address(reference x) const noexcept;             // deprecated in C++17, removed in C++20
     const_pointer address(const_reference x) const noexcept; // deprecated in C++17, removed in C++20
     T* allocate(size_t n, const void* hint);          // deprecated in C++17, removed in C++20
-    T* allocate(size_t n);
-    void deallocate(T* p, size_t n) noexcept;
+    T* allocate(size_t n);                              // constexpr in C++20
+    void deallocate(T* p, size_t n) noexcept;           // constexpr in C++20
     size_type max_size() const noexcept;              // deprecated in C++17, removed in C++20
     template<class U, class... Args>
         void construct(U* p, Args&&... args);         // deprecated in C++17, removed in C++20
     template <class U>
         void destroy(U* p);                           // deprecated in C++17, removed in C++20
 };
 
 template <class T, class U>
-bool operator==(const allocator<T>&, const allocator<U>&) noexcept;
+bool operator==(const allocator<T>&, const allocator<U>&) noexcept; // constexpr in C++20
 
 template <class T, class U>
-bool operator!=(const allocator<T>&, const allocator<U>&) noexcept;
+bool operator!=(const allocator<T>&, const allocator<U>&) noexcept; // constexpr in C++20
 
 template <class OutputIterator, class T>
 class raw_storage_iterator
     : public iterator<output_iterator_tag,
                       T,                               // purposefully not C++03
                       ptrdiff_t,                       // purposefully not C++03
                       T*,                              // purposefully not C++03
                       raw_storage_iterator&>           // purposefully not C++03

 
 template <class ForwardIterator, class T>
 void uninitialized_fill(ForwardIterator first, ForwardIterator last, const T& x);
 
 template <class ForwardIterator, class Size, class T>
 ForwardIterator
 uninitialized_fill_n(ForwardIterator first, Size n, const T& x);
 
+template <class T, class ...Args>
+constexpr T* construct_at(T* location, Args&& ...args); // since C++20
+
 template <class T>
-void destroy_at(T* location);
+void destroy_at(T* location); // constexpr in C++20
 
 template <class ForwardIterator>
- void destroy(ForwardIterator first, ForwardIterator last);
+void destroy(ForwardIterator first, ForwardIterator last); // constexpr in C++20
 
 template <class ForwardIterator, class Size>
- ForwardIterator destroy_n(ForwardIterator first, Size n);
+ForwardIterator destroy_n(ForwardIterator first, Size n); // constexpr in C++20
 
 template <class InputIterator, class ForwardIterator>
  ForwardIterator uninitialized_move(InputIterator first, InputIterator last, ForwardIterator result);
 
 template <class InputIterator, class Size, class ForwardIterator>
  pair<InputIterator,ForwardIterator> uninitialized_move_n(InputIterator first, Size n, ForwardIterator result);
 
 template <class ForwardIterator>

 struct __rebind_pointer {
 #ifndef _LIBCPP_CXX03_LANG
     typedef typename pointer_traits<_From>::template rebind<_To>        type;
 #else
     typedef typename pointer_traits<_From>::template rebind<_To>::other type;
 #endif
 };
 
+// construct_at
+
+#if _LIBCPP_STD_VER > 17
+
+template<class _Tp>
+_LIBCPP_CONSTEXPR_AFTER_CXX17 void* __voidify(_Tp& __ptr) noexcept {
+    return const_cast<void*>(static_cast<const volatile void*>(_VSTD::addressof(__ptr)));
+}
+
+template<class _Tp, class ..._Args, class = decltype(
+    ::new (_VSTD::declval<void*>()) _Tp(_VSTD::declval<_Args>()...)
+)>
+_LIBCPP_INLINE_VISIBILITY
+constexpr _Tp* construct_at(_Tp* __location, _Args&& ...__args) {
+    _LIBCPP_ASSERT(__location, "null pointer given to construct_at");
+    return ::new (_VSTD::__voidify(*__location)) _Tp(_VSTD::forward<_Args>(__args)...);
+}
+
+#endif
+
+// destroy_at
+
+#if _LIBCPP_STD_VER > 14
+
+template <class _Tp>
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+void destroy_at(_Tp* __loc) {
+    _LIBCPP_ASSERT(__loc, "null pointer given to destroy_at");
+    __loc->~_Tp();
+}
+
+#endif
+
 // allocator_traits
 
 template <class _Tp, class = void>
 struct __has_pointer_type : false_type {};
 
 template <class _Tp>
 struct __has_pointer_type<_Tp,
           typename __void_t<typename _Tp::pointer>::type> : true_type {};
#ifndef _LIBCPP_CXX03_LANG
     template <class _Tp> using rebind_traits = allocator_traits<rebind_alloc<_Tp> >;
 #else  // _LIBCPP_CXX03_LANG
     template <class _Tp> struct rebind_alloc
         {typedef typename __allocator_traits_rebind<allocator_type, _Tp>::type other;};
     template <class _Tp> struct rebind_traits
         {typedef allocator_traits<typename rebind_alloc<_Tp>::other> other;};
 #endif  // _LIBCPP_CXX03_LANG
 
-    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static pointer allocate(allocator_type& __a, size_type __n)
         {return __a.allocate(__n);}
-    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static pointer allocate(allocator_type& __a, size_type __n, const_void_pointer __hint)
         {return __allocate(__a, __n, __hint,
             __has_allocate_hint<allocator_type, size_type, const_void_pointer>());}
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static void deallocate(allocator_type& __a, pointer __p, size_type __n) _NOEXCEPT
         {__a.deallocate(__p, __n);}
 
     template <class _Tp, class... _Args>
-        _LIBCPP_INLINE_VISIBILITY
+        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
         static void construct(allocator_type& __a, _Tp* __p, _Args&&... __args)
             {__construct(__has_construct<allocator_type, _Tp*, _Args...>(),
                          __a, __p, _VSTD::forward<_Args>(__args)...);}
 
     template <class _Tp>
-        _LIBCPP_INLINE_VISIBILITY
+        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
         static void destroy(allocator_type& __a, _Tp* __p)
             {__destroy(__has_destroy<allocator_type, _Tp*>(), __a, __p);}
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static size_type max_size(const allocator_type& __a) _NOEXCEPT
         {return __max_size(__has_max_size<const allocator_type>(), __a);}
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static allocator_type
         select_on_container_copy_construction(const allocator_type& __a)
             {return __select_on_container_copy_construction(
                 __has_select_on_container_copy_construction<const allocator_type>(),
                 __a);}
 
     template <class _Ptr>
         _LIBCPP_INLINE_VISIBILITY
template <class _Tp>
             ptrdiff_t _Np = __end1 - __begin1;
             __end2 -= _Np;
             if (_Np > 0)
                 _VSTD::memcpy(__end2, __begin1, _Np * sizeof(_Tp));
         }
 
 private:
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static pointer __allocate(allocator_type& __a, size_type __n,
         const_void_pointer __hint, true_type)
         {
             _LIBCPP_SUPPRESS_DEPRECATED_PUSH
             return __a.allocate(__n, __hint);
             _LIBCPP_SUPPRESS_DEPRECATED_POP
         }
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static pointer __allocate(allocator_type& __a, size_type __n,
         const_void_pointer, false_type)
         {return __a.allocate(__n);}
 
     template <class _Tp, class... _Args>
-        _LIBCPP_INLINE_VISIBILITY
+        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
         static void __construct(true_type, allocator_type& __a, _Tp* __p, _Args&&... __args)
             {
                 _LIBCPP_SUPPRESS_DEPRECATED_PUSH
                 __a.construct(__p, _VSTD::forward<_Args>(__args)...);
                 _LIBCPP_SUPPRESS_DEPRECATED_POP
             }
 
     template <class _Tp, class... _Args>
-        _LIBCPP_INLINE_VISIBILITY
+        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
         static void __construct(false_type, allocator_type&, _Tp* __p, _Args&&... __args)
             {
+#if _LIBCPP_STD_VER > 17
+                _VSTD::construct_at(__p, _VSTD::forward<_Args>(__args)...);
+#else
                 ::new ((void*)__p) _Tp(_VSTD::forward<_Args>(__args)...);
+#endif
             }
 
     template <class _Tp>
-        _LIBCPP_INLINE_VISIBILITY
+        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
         static void __destroy(true_type, allocator_type& __a, _Tp* __p)
             {
                 _LIBCPP_SUPPRESS_DEPRECATED_PUSH
                 __a.destroy(__p);
                 _LIBCPP_SUPPRESS_DEPRECATED_POP
             }
     template <class _Tp>
-        _LIBCPP_INLINE_VISIBILITY
+        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
         static void __destroy(false_type, allocator_type&, _Tp* __p)
             {
+#if _LIBCPP_STD_VER > 17
+                _VSTD::destroy_at(__p);
+#else
                 __p->~_Tp();
+#endif
             }
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static size_type __max_size(true_type, const allocator_type& __a) _NOEXCEPT
             {
                 _LIBCPP_SUPPRESS_DEPRECATED_PUSH
                 return __a.max_size();
                 _LIBCPP_SUPPRESS_DEPRECATED_POP
             }
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static size_type __max_size(false_type, const allocator_type&) _NOEXCEPT
             {return numeric_limits<size_type>::max() / sizeof(value_type);}
 
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static allocator_type
         __select_on_container_copy_construction(true_type, const allocator_type& __a)
             {return __a.select_on_container_copy_construction();}
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static allocator_type
         __select_on_container_copy_construction(false_type, const allocator_type& __a)
             {return __a;}
 };
 
 template <class _Traits, class _Tp>
 struct __rebind_alloc_helper
 {
#if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS)
     _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_INLINE_VISIBILITY
     pointer address(reference __x) const _NOEXCEPT
         {return _VSTD::addressof(__x);}
     _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_INLINE_VISIBILITY
     const_pointer address(const_reference __x) const _NOEXCEPT
         {return _VSTD::addressof(__x);}
 #endif
 
-    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _Tp* allocate(size_t __n)
+    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    _Tp* allocate(size_t __n)
         {
-        // TODO(mpark): Replace with `allocator_traits<allocator>::max_size(*this)`.
-        if (__n > (size_t(~0) / sizeof(_Tp)))
+        if (__n > allocator_traits<allocator>::max_size(*this))
             __throw_length_error("allocator<T>::allocate(size_t n)"
                                  " 'n' exceeds maximum supported size");
         return static_cast<_Tp*>(_VSTD::__libcpp_allocate(__n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp)));
         }
 
 #if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS)
     _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_DEPRECATED_IN_CXX17
     _Tp* allocate(size_t __n, const void*) { return allocate(__n); }
 #endif
 
-    _LIBCPP_INLINE_VISIBILITY void deallocate(_Tp* __p, size_t __n) _NOEXCEPT
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    void deallocate(_Tp* __p, size_t __n) _NOEXCEPT
         {_VSTD::__libcpp_deallocate((void*)__p, __n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp));}
 
 #if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS)
     _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_INLINE_VISIBILITY size_type max_size() const _NOEXCEPT
         {return size_type(~0) / sizeof(_Tp);}
 
     template <class _Up, class... _Args>
         _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_INLINE_VISIBILITY
#endif
     allocator(const allocator<_Up>&) _NOEXCEPT {}
 
 #if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS)
     _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_INLINE_VISIBILITY
     const_pointer address(const_reference __x) const _NOEXCEPT
         {return _VSTD::addressof(__x);}
 #endif
 
-    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY const _Tp* allocate(size_t __n)
+    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    const _Tp* allocate(size_t __n)
     {
-        // TODO(mpark): Replace with `allocator_traits<allocator>::max_size(*this)`.
-        if (__n > (size_t(~0) / sizeof(_Tp)))
+        if (__n > allocator_traits<allocator>::max_size(*this))
             __throw_length_error("allocator<const T>::allocate(size_t n)"
                                  " 'n' exceeds maximum supported size");
         return static_cast<const _Tp*>(_VSTD::__libcpp_allocate(__n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp)));
     }
 
 #if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS)
     _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_DEPRECATED_IN_CXX17
     const _Tp* allocate(size_t __n, const void*) { return allocate(__n); }
 #endif
 
-    _LIBCPP_INLINE_VISIBILITY void deallocate(const _Tp* __p, size_t __n)
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    void deallocate(const _Tp* __p, size_t __n)
         {_VSTD::__libcpp_deallocate((void*) const_cast<_Tp *>(__p), __n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp));}
 
 #if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS)
     _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_INLINE_VISIBILITY size_type max_size() const _NOEXCEPT
         {return size_type(~0) / sizeof(_Tp);}
 
     template <class _Up, class... _Args>
         _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_INLINE_VISIBILITY
         void
         construct(_Up* __p, _Args&&... __args)
         {
             ::new((void*)__p) _Up(_VSTD::forward<_Args>(__args)...);
         }
 
     _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_INLINE_VISIBILITY void destroy(pointer __p) {__p->~_Tp();}
 #endif
 };
 
 template <class _Tp, class _Up>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool operator==(const allocator<_Tp>&, const allocator<_Up>&) _NOEXCEPT {return true;}
 
 template <class _Tp, class _Up>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 bool operator!=(const allocator<_Tp>&, const allocator<_Up>&) _NOEXCEPT {return false;}
 
 template <class _OutputIterator, class _Tp>
 class _LIBCPP_TEMPLATE_VIS raw_storage_iterator
     : public iterator<output_iterator_tag,
                       _Tp,                                         // purposefully not C++03
                       ptrdiff_t,                                   // purposefully not C++03
                       _Tp*,                                        // purposefully not C++03
catch (...)
         throw;
     }
 #endif
     return __f;
 }
 
 #if _LIBCPP_STD_VER > 14
 
-template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
-void destroy_at(_Tp* __loc) {
-    _LIBCPP_ASSERT(__loc, "null pointer given to destroy_at");
-    __loc->~_Tp();
-}
-
 template <class _ForwardIterator>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 void destroy(_ForwardIterator __first, _ForwardIterator __last) {
     for (; __first != __last; ++__first)
         _VSTD::destroy_at(_VSTD::addressof(*__first));
 }
 
 template <class _ForwardIterator, class _Size>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _ForwardIterator destroy_n(_ForwardIterator __first, _Size __n) {
     for (; __n > 0; (void)++__first, --__n)
         _VSTD::destroy_at(_VSTD::addressof(*__first));
     return __first;
 }
 
 template <class _ForwardIterator>
 inline _LIBCPP_INLINE_VISIBILITY

libcxx/include/new

 _LIBCPP_CONSTEXPR inline _LIBCPP_INLINE_VISIBILITY bool __is_overaligned_for_new(size_t __align) _NOEXCEPT {
 #ifdef __STDCPP_DEFAULT_NEW_ALIGNMENT__
   return __align > __STDCPP_DEFAULT_NEW_ALIGNMENT__;
 #else
   return __align > alignment_of<max_align_t>::value;
 #endif
 }
 
-inline _LIBCPP_INLINE_VISIBILITY void *__libcpp_allocate(size_t __size, size_t __align) {
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+void *__libcpp_allocate(size_t __size, size_t __align) {
 #ifndef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
   if (__is_overaligned_for_new(__align)) {
     const align_val_t __align_val = static_cast<align_val_t>(__align);
 # ifdef _LIBCPP_HAS_NO_BUILTIN_OVERLOADED_OPERATOR_NEW_DELETE
     return ::operator new(__size, __align_val);
 # else
     return __builtin_operator_new(__size, __align_val);
 # endif
   }
 #else
   ((void)__align);
 #endif
 #ifdef _LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE
   return ::operator new(__size);
 #else
   return __builtin_operator_new(__size);
 #endif
 }
 
 struct _DeallocateCaller {
-  static inline _LIBCPP_INLINE_VISIBILITY
-  void __do_deallocate_handle_size_align(void *__ptr, size_t __size, size_t __align) {
-#if defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
-    ((void)__align);
-    return __do_deallocate_handle_size(__ptr, __size);
+  template <class _A1, class _A2>
+  _LIBCPP_CONSTEXPR_AFTER_CXX17
+  static inline void __do_call(void *__ptr, _A1 __a1, _A2 __a2) {
+#if defined(_LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE) || \
+    defined(_LIBCPP_HAS_NO_BUILTIN_OVERLOADED_OPERATOR_NEW_DELETE)
+    return ::operator delete(__ptr, __a1, __a2);
 #else
-    if (__is_overaligned_for_new(__align)) {
-      const align_val_t __align_val = static_cast<align_val_t>(__align);
-      return __do_deallocate_handle_size(__ptr, __size, __align_val);
-    } else {
-      return __do_deallocate_handle_size(__ptr, __size);
-    }
+    return __builtin_operator_delete(__ptr, __a1, __a2);
 #endif
   }
 
-  static inline _LIBCPP_INLINE_VISIBILITY
-  void __do_deallocate_handle_align(void *__ptr, size_t __align) {
-#if defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
-    ((void)__align);
-    return __do_call(__ptr);
+  template <class _A1>
+  _LIBCPP_CONSTEXPR_AFTER_CXX17
+  static inline void __do_call(void *__ptr, _A1 __a1) {
+#if defined(_LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE) || \
+    defined(_LIBCPP_HAS_NO_BUILTIN_OVERLOADED_OPERATOR_NEW_DELETE)
+    return ::operator delete(__ptr, __a1);
 #else
-    if (__is_overaligned_for_new(__align)) {
-      const align_val_t __align_val = static_cast<align_val_t>(__align);
-      return __do_call(__ptr, __align_val);
-    } else {
-      return __do_call(__ptr);
-    }
+    return __builtin_operator_delete(__ptr, __a1);
+#endif
+
+  _LIBCPP_CONSTEXPR_AFTER_CXX17
+  static inline void __do_call(void *__ptr) {
+#ifdef _LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE
+    return ::operator delete(__ptr);
+#else
+    return __builtin_operator_delete(__ptr);
 #endif
   }
 
- private:
+  _LIBCPP_CONSTEXPR_AFTER_CXX17
   static inline void __do_deallocate_handle_size(void *__ptr, size_t __size) {
 #ifdef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
     ((void)__size);
     return __do_call(__ptr);
 #else
     return __do_call(__ptr, __size);
 #endif
   }
 
 #ifndef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
+  _LIBCPP_CONSTEXPR_AFTER_CXX17
   static inline void __do_deallocate_handle_size(void *__ptr, size_t __size, align_val_t __align) {
 #ifdef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
     ((void)__size);
     return __do_call(__ptr, __align);
 #else
     return __do_call(__ptr, __size, __align);
 #endif
   }
 #endif
 
-private:
-  template <class _A1, class _A2>
-  static inline void __do_call(void *__ptr, _A1 __a1, _A2 __a2) {
-#if defined(_LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE) || \
-    defined(_LIBCPP_HAS_NO_BUILTIN_OVERLOADED_OPERATOR_NEW_DELETE)
-    return ::operator delete(__ptr, __a1, __a2);
+  static inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+  void __do_deallocate_handle_size_align(void *__ptr, size_t __size, size_t __align) {
+#if defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
+    ((void)__align);
+    return __do_deallocate_handle_size(__ptr, __size);
 #else
-    return __builtin_operator_delete(__ptr, __a1, __a2);
-#endif
+    if (__is_overaligned_for_new(__align)) {
+      const align_val_t __align_val = static_cast<align_val_t>(__align);
+      return __do_deallocate_handle_size(__ptr, __size, __align_val);
+    } else {
+      return __do_deallocate_handle_size(__ptr, __size);
     }
-
-  template <class _A1>
-  static inline void __do_call(void *__ptr, _A1 __a1) {
-#if defined(_LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE) || \
-    defined(_LIBCPP_HAS_NO_BUILTIN_OVERLOADED_OPERATOR_NEW_DELETE)
-    return ::operator delete(__ptr, __a1);
-#else
-    return __builtin_operator_delete(__ptr, __a1);
 #endif
   }
 
-  static inline void __do_call(void *__ptr) {
-#ifdef _LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE
-    return ::operator delete(__ptr);
+  static inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+  void __do_deallocate_handle_align(void *__ptr, size_t __align) {
+#if defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
+    ((void)__align);
+    return __do_call(__ptr);
 #else
-    return __builtin_operator_delete(__ptr);
+    if (__is_overaligned_for_new(__align)) {
+      const align_val_t __align_val = static_cast<align_val_t>(__align);
+      return __do_call(__ptr, __align_val);
+    } else {
+      return __do_call(__ptr);
+    }
 #endif
   }
 };
 
-inline _LIBCPP_INLINE_VISIBILITY void __libcpp_deallocate(void* __ptr, size_t __size, size_t __align) {
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+void __libcpp_deallocate(void* __ptr, size_t __size, size_t __align) {
   _DeallocateCaller::__do_deallocate_handle_size_align(__ptr, __size, __align);
 }
 
 inline _LIBCPP_INLINE_VISIBILITY void __libcpp_deallocate_unsized(void* __ptr, size_t __align) {
   _DeallocateCaller::__do_deallocate_handle_align(__ptr, __align);
 }
 
 template <class _Tp>

libcxx/include/version

 __cpp_lib_char8_t                                       201811L <atomic> <filesystem> <istream>
                                                                 <limits> <locale> <ostream>
                                                                 <string> <string_view>
 __cpp_lib_chrono                                        201611L <chrono>
 __cpp_lib_chrono_udls                                   201304L <chrono>
 __cpp_lib_clamp                                         201603L <algorithm>
 __cpp_lib_complex_udls                                  201309L <complex>
 __cpp_lib_concepts                                      201806L <concepts>
+__cpp_lib_constexpr_dynamic_alloc                       201907L <memory>
 __cpp_lib_constexpr_misc                                201811L <array> <functional> <iterator>
                                                                 <string_view> <tuple> <utility>
 __cpp_lib_constexpr_swap_algorithms                     201806L <algorithm>
 __cpp_lib_constexpr_utility                             201811L <utility>
 __cpp_lib_destroying_delete                             201806L <new>
 __cpp_lib_enable_shared_from_this                       201603L <memory>
 __cpp_lib_endian                                        201907L <bit>
 __cpp_lib_erase_if                                      202002L <string> <deque> <forward_list>

 #   define __cpp_lib_atomic_wait                        201907L
 # endif
 // # define __cpp_lib_bind_front                           201811L
 // # define __cpp_lib_bit_cast                             201806L
 # if !defined(_LIBCPP_NO_HAS_CHAR8_T)
 #   define __cpp_lib_char8_t                            201811L
 # endif
 // # define __cpp_lib_concepts                             201806L
+# define __cpp_lib_constexpr_dynamic_alloc              201907L
 // # define __cpp_lib_constexpr_misc                       201811L
 // # define __cpp_lib_constexpr_swap_algorithms            201806L
 # define __cpp_lib_constexpr_utility                    201811L
 # if _LIBCPP_STD_VER > 17 && defined(__cpp_impl_destroying_delete) && __cpp_impl_destroying_delete >= 201806L
 #   define __cpp_lib_destroying_delete                  201806L
 # endif
 # define __cpp_lib_endian                               201907L
 # define __cpp_lib_erase_if                             202002L

libcxx/test/std/containers/sequences/vector/vector.cons/copy.move_only.verify.cpp

struct move_only
     move_only() = default;
     move_only(move_only&&) = default;
     move_only& operator=(move_only&&) = default;
 };
 
 int main(int, char**)
 {
     std::vector<move_only> v;
-    std::vector<move_only> copy = v; // expected-error@memory:* {{call to implicitly-deleted copy constructor of 'move_only'}}
+    std::vector<move_only> copy = v; // expected-error-re@memory:* {{{{(no matching function for call to 'construct_at')|(call to implicitly-deleted copy constructor of 'move_only')}}}}
     return 0;
 }

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

 // <memory>
 
 // Test the feature test macros defined by <memory>
 
 /*  Constant                                      Value
     __cpp_lib_addressof_constexpr                 201603L [C++17]
     __cpp_lib_allocator_traits_is_always_equal    201411L [C++17]
     __cpp_lib_atomic_value_initialization         201911L [C++2a]
+    __cpp_lib_constexpr_dynamic_alloc             201907L [C++2a]
     __cpp_lib_enable_shared_from_this             201603L [C++17]
     __cpp_lib_make_unique                         201304L [C++14]
     __cpp_lib_ranges                              201811L [C++2a]
     __cpp_lib_raw_memory_algorithms               201606L [C++17]
     __cpp_lib_shared_ptr_arrays                   201611L [C++17]
     __cpp_lib_shared_ptr_weak_type                201606L [C++17]
 */
 

 # ifdef __cpp_lib_allocator_traits_is_always_equal
 #   error "__cpp_lib_allocator_traits_is_always_equal should not be defined before c++17"
 # endif
 
 # ifdef __cpp_lib_atomic_value_initialization
 #   error "__cpp_lib_atomic_value_initialization should not be defined before c++2a"
 # endif
 
+# ifdef __cpp_lib_constexpr_dynamic_alloc
+#   error "__cpp_lib_constexpr_dynamic_alloc should not be defined before c++2a"
+# endif
+
 # ifdef __cpp_lib_enable_shared_from_this
 #   error "__cpp_lib_enable_shared_from_this should not be defined before c++17"
 # endif
 
 # ifdef __cpp_lib_make_unique
 #   error "__cpp_lib_make_unique should not be defined before c++14"
 # endif
 

 # ifdef __cpp_lib_allocator_traits_is_always_equal
 #   error "__cpp_lib_allocator_traits_is_always_equal should not be defined before c++17"
 # endif
 
 # ifdef __cpp_lib_atomic_value_initialization
 #   error "__cpp_lib_atomic_value_initialization should not be defined before c++2a"
 # endif
 
+# ifdef __cpp_lib_constexpr_dynamic_alloc
+#   error "__cpp_lib_constexpr_dynamic_alloc should not be defined before c++2a"
+# endif
+
 # ifdef __cpp_lib_enable_shared_from_this
 #   error "__cpp_lib_enable_shared_from_this should not be defined before c++17"
 # endif
 
 # ifndef __cpp_lib_make_unique
 #   error "__cpp_lib_make_unique should be defined in c++14"
 # endif
 # if __cpp_lib_make_unique != 201304L

 # if __cpp_lib_allocator_traits_is_always_equal != 201411L
 #   error "__cpp_lib_allocator_traits_is_always_equal should have the value 201411L in c++17"
 # endif
 
 # ifdef __cpp_lib_atomic_value_initialization
 #   error "__cpp_lib_atomic_value_initialization should not be defined before c++2a"
 # endif
 
+# ifdef __cpp_lib_constexpr_dynamic_alloc
+#   error "__cpp_lib_constexpr_dynamic_alloc should not be defined before c++2a"
+# endif
+
 # ifndef __cpp_lib_enable_shared_from_this
 #   error "__cpp_lib_enable_shared_from_this should be defined in c++17"
 # endif
 # if __cpp_lib_enable_shared_from_this != 201603L
 #   error "__cpp_lib_enable_shared_from_this should have the value 201603L in c++17"
 # endif
 
 # ifndef __cpp_lib_make_unique

 #     error "__cpp_lib_atomic_value_initialization should have the value 201911L in c++2a"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_atomic_value_initialization
 #     error "__cpp_lib_atomic_value_initialization should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
+# ifndef __cpp_lib_constexpr_dynamic_alloc
+#   error "__cpp_lib_constexpr_dynamic_alloc should be defined in c++2a"
+# endif
+# if __cpp_lib_constexpr_dynamic_alloc != 201907L
+#   error "__cpp_lib_constexpr_dynamic_alloc should have the value 201907L in c++2a"
+# endif
+
 # ifndef __cpp_lib_enable_shared_from_this
 #   error "__cpp_lib_enable_shared_from_this should be defined in c++2a"
 # endif
 # if __cpp_lib_enable_shared_from_this != 201603L
 #   error "__cpp_lib_enable_shared_from_this should have the value 201603L in c++2a"
 # endif
 
 # ifndef __cpp_lib_make_unique

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

/*  Constant                                       Value
     __cpp_lib_boyer_moore_searcher                 201603L [C++17]
     __cpp_lib_byte                                 201603L [C++17]
     __cpp_lib_char8_t                              201811L [C++2a]
     __cpp_lib_chrono                               201611L [C++17]
     __cpp_lib_chrono_udls                          201304L [C++14]
     __cpp_lib_clamp                                201603L [C++17]
     __cpp_lib_complex_udls                         201309L [C++14]
     __cpp_lib_concepts                             201806L [C++2a]
+    __cpp_lib_constexpr_dynamic_alloc              201907L [C++2a]
     __cpp_lib_constexpr_misc                       201811L [C++2a]
     __cpp_lib_constexpr_swap_algorithms            201806L [C++2a]
     __cpp_lib_constexpr_utility                    201811L [C++2a]
     __cpp_lib_destroying_delete                    201806L [C++2a]
     __cpp_lib_enable_shared_from_this              201603L [C++17]
     __cpp_lib_endian                               201907L [C++2a]
     __cpp_lib_erase_if                             202002L [C++2a]
     __cpp_lib_exchange_function                    201304L [C++14]

 # ifdef __cpp_lib_complex_udls
 #   error "__cpp_lib_complex_udls should not be defined before c++14"
 # endif
 
 # ifdef __cpp_lib_concepts
 #   error "__cpp_lib_concepts should not be defined before c++2a"
 # endif
 
+# ifdef __cpp_lib_constexpr_dynamic_alloc
+#   error "__cpp_lib_constexpr_dynamic_alloc should not be defined before c++2a"
+# endif
+
 # ifdef __cpp_lib_constexpr_misc
 #   error "__cpp_lib_constexpr_misc should not be defined before c++2a"
 # endif
 
 # ifdef __cpp_lib_constexpr_swap_algorithms
 #   error "__cpp_lib_constexpr_swap_algorithms should not be defined before c++2a"
 # endif
 

 # if __cpp_lib_complex_udls != 201309L
 #   error "__cpp_lib_complex_udls should have the value 201309L in c++14"
 # endif
 
 # ifdef __cpp_lib_concepts
 #   error "__cpp_lib_concepts should not be defined before c++2a"
 # endif
 
+# ifdef __cpp_lib_constexpr_dynamic_alloc
+#   error "__cpp_lib_constexpr_dynamic_alloc should not be defined before c++2a"
+# endif
+
 # ifdef __cpp_lib_constexpr_misc
 #   error "__cpp_lib_constexpr_misc should not be defined before c++2a"
 # endif
 
 # ifdef __cpp_lib_constexpr_swap_algorithms
 #   error "__cpp_lib_constexpr_swap_algorithms should not be defined before c++2a"
 # endif
 

 # if __cpp_lib_complex_udls != 201309L
 #   error "__cpp_lib_complex_udls should have the value 201309L in c++17"
 # endif
 
 # ifdef __cpp_lib_concepts
 #   error "__cpp_lib_concepts should not be defined before c++2a"
 # endif
 
+# ifdef __cpp_lib_constexpr_dynamic_alloc
+#   error "__cpp_lib_constexpr_dynamic_alloc should not be defined before c++2a"
+# endif
+
 # ifdef __cpp_lib_constexpr_misc
 #   error "__cpp_lib_constexpr_misc should not be defined before c++2a"
 # endif
 
 # ifdef __cpp_lib_constexpr_swap_algorithms
 #   error "__cpp_lib_constexpr_swap_algorithms should not be defined before c++2a"
 # endif
 

 #     error "__cpp_lib_concepts should have the value 201806L in c++2a"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_concepts
 #     error "__cpp_lib_concepts should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
+# ifndef __cpp_lib_constexpr_dynamic_alloc
+#   error "__cpp_lib_constexpr_dynamic_alloc should be defined in c++2a"
+# endif
+# if __cpp_lib_constexpr_dynamic_alloc != 201907L
+#   error "__cpp_lib_constexpr_dynamic_alloc should have the value 201907L in c++2a"
+# endif
+
 # if !defined(_LIBCPP_VERSION)
 #   ifndef __cpp_lib_constexpr_misc
 #     error "__cpp_lib_constexpr_misc should be defined in c++2a"
 #   endif
 #   if __cpp_lib_constexpr_misc != 201811L
 #     error "__cpp_lib_constexpr_misc should have the value 201811L in c++2a"
 #   endif
 # else // _LIBCPP_VERSION

libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/allocate.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // <memory>
 
 // template <class Alloc>
 // struct allocator_traits
 // {
-//     static pointer allocate(allocator_type& a, size_type n);
+//     static constexpr pointer allocate(allocator_type& a, size_type n);
 //     ...
 // };
 
 #include <memory>
 #include <cstdint>
 #include <cassert>
 
 #include "test_macros.h"
 #include "incomplete_type_helper.h"
 
 template <class T>
 struct A
 {
     typedef T value_type;
 
-    value_type* allocate(std::size_t n)
+    TEST_CONSTEXPR_CXX20 A() {}
+
+    TEST_CONSTEXPR_CXX20 value_type* allocate(std::size_t n)
     {
         assert(n == 10);
-        return reinterpret_cast<value_type*>(static_cast<std::uintptr_t>(0xDEADBEEF));
+        return &storage;
     }
+
+    value_type storage;
 };
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
     {
         A<int> a;
-    assert(std::allocator_traits<A<int> >::allocate(a, 10) == reinterpret_cast<int*>(static_cast<std::uintptr_t>(0xDEADBEEF)));
+        assert(std::allocator_traits<A<int> >::allocate(a, 10) == &a.storage);
     }
     {
-    typedef IncompleteHolder* VT;
-    typedef A<VT> Alloc;
+        typedef A<IncompleteHolder*> Alloc;
         Alloc a;
-    assert(std::allocator_traits<Alloc >::allocate(a, 10) == reinterpret_cast<VT*>(static_cast<std::uintptr_t>(0xDEADBEEF)));
+        assert(std::allocator_traits<Alloc>::allocate(a, 10) == &a.storage);
     }
 
+    return true;
+}
+
+int main(int, char**)
+{
+    test();
+#if TEST_STD_VER > 17
+    static_assert(test());
+#endif
     return 0;
 }

libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/allocate.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // <memory>
 
 // template <class Alloc>
 // struct allocator_traits
 // {
-//     static pointer allocate(allocator_type& a, size_type n);
+//     static constexpr pointer allocate(allocator_type& a, size_type n);
 //     ...
 // };
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
 #include <memory>
 #include <cstdint>
 #include <cassert>

libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/allocate_hint.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // <memory>
 
 // template <class Alloc>
 // struct allocator_traits
 // {
-//     static pointer allocate(allocator_type& a, size_type n, const_void_pointer hint);
+//     static constexpr pointer allocate(allocator_type& a, size_type n, const_void_pointer hint);
 //     ...
 // };
 
 #include <memory>
 #include <cstdint>
 #include <cassert>
 
 #include "test_macros.h"
 #include "incomplete_type_helper.h"
 
 template <class T>
 struct A
 {
     typedef T value_type;
 
-    value_type* allocate(std::size_t n)
+    TEST_CONSTEXPR_CXX20 A() {}
+
+    TEST_CONSTEXPR_CXX20 value_type* allocate(std::size_t n)
     {
         assert(n == 10);
-        return reinterpret_cast<value_type*>(static_cast<std::uintptr_t>(0xDEADBEEF));
+        return &storage;
     }
+
+    value_type storage;
 };
 
 template <class T>
 struct B
 {
     typedef T value_type;
 
-    value_type* allocate(std::size_t n)
-    {
-        assert(n == 12);
-        return reinterpret_cast<value_type*>(static_cast<std::uintptr_t>(0xEEADBEEF));
-    }
-    value_type* allocate(std::size_t n, const void* p)
+    TEST_CONSTEXPR_CXX20 value_type* allocate(std::size_t n, const void* p)
     {
         assert(n == 11);
-        assert(p == 0);
-        return reinterpret_cast<value_type*>(static_cast<std::uintptr_t>(0xFEADBEEF));
+        assert(p == nullptr);
+        return &storage;
     }
-};
 
+    value_type storage;
+};
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
 #if TEST_STD_VER >= 11
     {
         A<int> a;
-    assert(std::allocator_traits<A<int> >::allocate(a, 10, nullptr) == reinterpret_cast<int*>(static_cast<std::uintptr_t>(0xDEADBEEF)));
+        assert(std::allocator_traits<A<int> >::allocate(a, 10, nullptr) == &a.storage);
     }
     {
-    typedef IncompleteHolder* VT;
-    typedef A<VT> Alloc;
+        typedef A<IncompleteHolder*> Alloc;
         Alloc a;
-    assert(std::allocator_traits<Alloc >::allocate(a, 10, nullptr) == reinterpret_cast<VT*>(static_cast<std::uintptr_t>(0xDEADBEEF)));
+        assert(std::allocator_traits<Alloc>::allocate(a, 10, nullptr) == &a.storage);
     }
 #endif
     {
         B<int> b;
-    assert(std::allocator_traits<B<int> >::allocate(b, 11, nullptr) == reinterpret_cast<int*>(static_cast<std::uintptr_t>(0xFEADBEEF)));
+        assert(std::allocator_traits<B<int> >::allocate(b, 11, nullptr) == &b.storage);
     }
     {
-    typedef IncompleteHolder* VT;
-    typedef B<VT> Alloc;
+        typedef B<IncompleteHolder*> Alloc;
         Alloc b;
-    assert(std::allocator_traits<Alloc >::allocate(b, 11, nullptr) == reinterpret_cast<VT*>(static_cast<std::uintptr_t>(0xFEADBEEF)));
+        assert(std::allocator_traits<Alloc>::allocate(b, 11, nullptr) == &b.storage);
     }
 
+    return true;
+}
+
+
+int main(int, char**)
+{
+    test();
+#if TEST_STD_VER > 17
+    static_assert(test());
+#endif
     return 0;
 }

libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/construct.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // <memory>
 
 // template <class Alloc>
 // struct allocator_traits
 // {
 //     template <class Ptr, class... Args>
-//         static void construct(allocator_type& a, Ptr p, Args&&... args);
+//     static constexpr void construct(allocator_type& a, Ptr p, Args&&... args);
 //     ...
 // };
 
 #include <memory>
 #include <new>
 #include <type_traits>
 #include <cassert>
 
 #include "test_macros.h"
 #include "incomplete_type_helper.h"
 
 template <class T>
 struct A
 {
     typedef T value_type;
 
 };
 
-int b_construct = 0;
-
 template <class T>
 struct B
 {
     typedef T value_type;
 
+    TEST_CONSTEXPR_CXX20 B(int& count) : count(count) {}
+
 #if TEST_STD_VER >= 11
     template <class U, class ...Args>
-    void construct(U* p, Args&& ...args)
+    TEST_CONSTEXPR_CXX20 void construct(U* p, Args&& ...args)
     {
-        ++b_construct;
+        ++count;
+#if TEST_STD_VER > 17
+        std::construct_at(p, std::forward<Args>(args)...);
+#else
         ::new ((void*)p) U(std::forward<Args>(args)...);
+#endif
     }
 #endif
+
+    int& count;
 };
 
 struct A0
 {
-    static int count;
-    A0() {++count;}
+    TEST_CONSTEXPR_CXX20 A0(int* count) {++*count;}
 };
 
-int A0::count = 0;
-
 struct A1
 {
-    static int count;
-    A1(char c)
+    TEST_CONSTEXPR_CXX20 A1(int* count, char c)
     {
         assert(c == 'c');
-        ++count;
+        ++*count;
     }
 };
 
-int A1::count = 0;
-
 struct A2
 {
-    static int count;
-    A2(char c, int i)
+    TEST_CONSTEXPR_CXX20 A2(int* count, char c, int i)
     {
         assert(c == 'd');
         assert(i == 5);
-        ++count;
+        ++*count;
     }
 };
 
-int A2::count = 0;
-
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
     {
-        A0::count = 0;
-        A<int> a;
-        std::aligned_storage<sizeof(A0)>::type a0;
-        assert(A0::count == 0);
-        std::allocator_traits<A<int> >::construct(a, (A0*)&a0);
-        assert(A0::count == 1);
-    }
-    {
-        A1::count = 0;
-        A<int> a;
-        std::aligned_storage<sizeof(A1)>::type a1;
-        assert(A1::count == 0);
-        std::allocator_traits<A<int> >::construct(a, (A1*)&a1, 'c');
-        assert(A1::count == 1);
-    }
-    {
-        A2::count = 0;
-        A<int> a;
-        std::aligned_storage<sizeof(A2)>::type a2;
-        assert(A2::count == 0);
-        std::allocator_traits<A<int> >::construct(a, (A2*)&a2, 'd', 5);
-        assert(A2::count == 1);
+        int A0_count = 0;
+        A<A0> a;
+        std::allocator<A0> alloc;
+        A0* a0 = alloc.allocate(1);
+        assert(A0_count == 0);
+        std::allocator_traits<A<A0> >::construct(a, a0, &A0_count);
+        assert(A0_count == 1);
+        alloc.deallocate(a0, 1);
+    }
+    {
+        int A1_count = 0;
+        A<A1> a;
+        std::allocator<A1> alloc;
+        A1* a1 = alloc.allocate(1);
+        assert(A1_count == 0);
+        std::allocator_traits<A<A1> >::construct(a, a1, &A1_count, 'c');
+        assert(A1_count == 1);
+        alloc.deallocate(a1, 1);
+    }
+    {
+        int A2_count = 0;
+        A<A2> a;
+        std::allocator<A2> alloc;
+        A2* a2 = alloc.allocate(1);
+        assert(A2_count == 0);
+        std::allocator_traits<A<A2> >::construct(a, a2, &A2_count, 'd', 5);
+        assert(A2_count == 1);
+        alloc.deallocate(a2, 1);
     }
     {
       typedef IncompleteHolder* VT;
       typedef A<VT> Alloc;
       Alloc a;
-      std::aligned_storage<sizeof(VT)>::type store;
-      std::allocator_traits<Alloc>::construct(a, (VT*)&store, nullptr);
+      std::allocator<VT> alloc;
+      VT* vt = alloc.allocate(1);
+      std::allocator_traits<Alloc>::construct(a, vt, nullptr);
+      alloc.deallocate(vt, 1);
     }
+
 #if TEST_STD_VER >= 11
     {
-        A0::count = 0;
-        b_construct = 0;
-        B<int> b;
-        std::aligned_storage<sizeof(A0)>::type a0;
-        assert(A0::count == 0);
+        int A0_count = 0;
+        int b_construct = 0;
+        B<A0> b(b_construct);
+        std::allocator<A0> alloc;
+        A0* a0 = alloc.allocate(1);
+        assert(A0_count == 0);
         assert(b_construct == 0);
-        std::allocator_traits<B<int> >::construct(b, (A0*)&a0);
-        assert(A0::count == 1);
+        std::allocator_traits<B<A0> >::construct(b, a0, &A0_count);
+        assert(A0_count == 1);
         assert(b_construct == 1);
+        alloc.deallocate(a0, 1);
     }
     {
-        A1::count = 0;
-        b_construct = 0;
-        B<int> b;
-        std::aligned_storage<sizeof(A1)>::type a1;
-        assert(A1::count == 0);
+        int A1_count = 0;
+        int b_construct = 0;
+        B<A1> b(b_construct);
+        std::allocator<A1> alloc;
+        A1* a1 = alloc.allocate(1);
+        assert(A1_count == 0);
         assert(b_construct == 0);
-        std::allocator_traits<B<int> >::construct(b, (A1*)&a1, 'c');
-        assert(A1::count == 1);
+        std::allocator_traits<B<A1> >::construct(b, a1, &A1_count, 'c');
+        assert(A1_count == 1);
         assert(b_construct == 1);
+        alloc.deallocate(a1, 1);
     }
     {
-        A2::count = 0;
-        b_construct = 0;
-        B<int> b;
-        std::aligned_storage<sizeof(A2)>::type a2;
-        assert(A2::count == 0);
+        int A2_count = 0;
+        int b_construct = 0;
+        B<A2> b(b_construct);
+        std::allocator<A2> alloc;
+        A2* a2 = alloc.allocate(1);
+        assert(A2_count == 0);
         assert(b_construct == 0);
-        std::allocator_traits<B<int> >::construct(b, (A2*)&a2, 'd', 5);
-        assert(A2::count == 1);
+        std::allocator_traits<B<A2> >::construct(b, a2, &A2_count, 'd', 5);
+        assert(A2_count == 1);
         assert(b_construct == 1);
+        alloc.deallocate(a2, 1);
     }
 #endif
 
+    return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test());
+#endif
+
     return 0;
 }

libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/deallocate.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // <memory>
 
 // template <class Alloc>
 // struct allocator_traits
 // {
-//     static void deallocate(allocator_type& a, pointer p, size_type n);
+//     static constexpr void deallocate(allocator_type& a, pointer p, size_type n);
 //     ...
 // };
 
 #include <memory>
-#include <cstdint>
 #include <cassert>
+#include <cstddef>
 
 #include "test_macros.h"
 #include "incomplete_type_helper.h"
 
-int called = 0;
-
 template <class T>
 struct A
 {
     typedef T value_type;
 
-    void deallocate(value_type* p, std::size_t n)
+    TEST_CONSTEXPR_CXX20 A(int& called) : called(called) {}
+
+    TEST_CONSTEXPR_CXX20 void deallocate(value_type* p, std::size_t n)
     {
-        assert(p == reinterpret_cast<value_type*>(static_cast<std::uintptr_t>(0xDEADBEEF)));
+        assert(p == &storage);
         assert(n == 10);
         ++called;
     }
+
+    int& called;
+
+    value_type storage;
 };
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
     {
-    A<int> a;
-    std::allocator_traits<A<int> >::deallocate(a, reinterpret_cast<int*>(static_cast<std::uintptr_t>(0xDEADBEEF)), 10);
+        int called = 0;
+        A<int> a(called);
+        std::allocator_traits<A<int> >::deallocate(a, &a.storage, 10);
         assert(called == 1);
     }
-  called = 0;
     {
-    typedef IncompleteHolder* VT;
-    typedef A<VT> Alloc;
-    Alloc a;
-    std::allocator_traits<Alloc >::deallocate(a, reinterpret_cast<VT*>(static_cast<std::uintptr_t>(0xDEADBEEF)), 10);
+        int called = 0;
+        typedef A<IncompleteHolder*> Alloc;
+        Alloc a(called);
+        std::allocator_traits<Alloc>::deallocate(a, &a.storage, 10);
         assert(called == 1);
     }
 
+    return true;
+}
+
+int main(int, char**)
+{
+    test();
+#if TEST_STD_VER > 17
+    static_assert(test());
+#endif
     return 0;
 }

libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/destroy.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // <memory>
 
 // template <class Alloc>
 // struct allocator_traits
 // {
 //     template <class Ptr>
-//         static void destroy(allocator_type& a, Ptr p);
+//     static constexpr void destroy(allocator_type& a, Ptr p);
 //     ...
 // };
 
 #include <memory>
-#include <new>
-#include <type_traits>
 #include <cassert>
+#include <cstddef>
 
 #include "test_macros.h"
 #include "incomplete_type_helper.h"
 
 template <class T>
-struct A
+struct NoDestroy
 {
     typedef T value_type;
 
-};
+    TEST_CONSTEXPR_CXX20 T* allocate(std::size_t n)
+    {
+        return std::allocator<T>().allocate(n);
+    }
 
-int b_destroy = 0;
+    TEST_CONSTEXPR_CXX20 void deallocate(T* p, std::size_t n)
+    {
+        return std::allocator<T>().deallocate(p, n);
+    }
+};
 
 template <class T>
-struct B
+struct CountDestroy
 {
+    TEST_CONSTEXPR explicit CountDestroy(int* counter)
+        : counter_(counter)
+    { }
+
     typedef T value_type;
 
+    TEST_CONSTEXPR_CXX20 T* allocate(std::size_t n)
+    {
+        return std::allocator<T>().allocate(n);
+    }
+
+    TEST_CONSTEXPR_CXX20 void deallocate(T* p, std::size_t n)
+    {
+        return std::allocator<T>().deallocate(p, n);
+    }
+
     template <class U>
-    void destroy(U* p)
+    TEST_CONSTEXPR_CXX20 void destroy(U* p)
     {
-        ++b_destroy;
+        ++*counter_;
         p->~U();
     }
+
+    int* counter_;
 };
 
-struct A0
+struct CountDestructor
 {
-    static int count;
-    ~A0() {++count;}
+    TEST_CONSTEXPR explicit CountDestructor(int* counter)
+        : counter_(counter)
+    { }
+
+    TEST_CONSTEXPR_CXX20 ~CountDestructor() { ++*counter_; }
+
+    int* counter_;
 };
 
-int A0::count = 0;
+TEST_CONSTEXPR_CXX20 bool test()
+{
+    {
+        typedef NoDestroy<CountDestructor> Alloc;
+        int destructors = 0;
+        Alloc alloc;
+        CountDestructor* pool = std::allocator_traits<Alloc>::allocate(alloc, 1);
 
-int main(int, char**)
+        std::allocator_traits<Alloc>::construct(alloc, pool, &destructors);
+        assert(destructors == 0);
+
+        std::allocator_traits<Alloc>::destroy(alloc, pool);
+        assert(destructors == 1);
+
+        std::allocator_traits<Alloc>::deallocate(alloc, pool, 1);
+    }
     {
+        typedef IncompleteHolder* T;
+        typedef NoDestroy<T> Alloc;
+        Alloc alloc;
+        T* pool = std::allocator_traits<Alloc>::allocate(alloc, 1);
+        std::allocator_traits<Alloc>::construct(alloc, pool, nullptr);
+        std::allocator_traits<Alloc>::destroy(alloc, pool);
+        std::allocator_traits<Alloc>::deallocate(alloc, pool, 1);
+    }
     {
-        A0::count = 0;
-        A<int> a;
-        std::aligned_storage<sizeof(A0)>::type a0;
-        std::allocator_traits<A<int> >::construct(a, (A0*)&a0);
-        assert(A0::count == 0);
-        std::allocator_traits<A<int> >::destroy(a, (A0*)&a0);
-        assert(A0::count == 1);
-    }
-    {
-      typedef IncompleteHolder* VT;
-      typedef A<VT> Alloc;
-      Alloc a;
-      std::aligned_storage<sizeof(VT)>::type store;
-      std::allocator_traits<Alloc>::destroy(a, (VT*)&store);
-    }
-#if defined(_LIBCPP_VERSION) || TEST_STD_VER >= 11
-    {
-        A0::count = 0;
-        b_destroy = 0;
-        B<int> b;
-        std::aligned_storage<sizeof(A0)>::type a0;
-        std::allocator_traits<B<int> >::construct(b, (A0*)&a0);
-        assert(A0::count == 0);
-        assert(b_destroy == 0);
-        std::allocator_traits<B<int> >::destroy(b, (A0*)&a0);
-        assert(A0::count == 1);
-        assert(b_destroy == 1);
-    }
-#endif
+        typedef CountDestroy<CountDestructor> Alloc;
+        int destroys_called = 0;
+        int destructors_called = 0;
+        Alloc alloc(&destroys_called);
+
+        CountDestructor* pool = std::allocator_traits<Alloc>::allocate(alloc, 1);
+        std::allocator_traits<Alloc>::construct(alloc, pool, &destructors_called);
+        assert(destroys_called == 0);
+        assert(destructors_called == 0);
+
+        std::allocator_traits<Alloc>::destroy(alloc, pool);
+        assert(destroys_called == 1);
+        assert(destructors_called == 1);
+
+        std::allocator_traits<Alloc>::deallocate(alloc, pool, 1);
+    }
+    return true;
 
+int main(int, char**)
+{
+    test();
+#if TEST_STD_VER > 17
+    static_assert(test());
+#endif
     return 0;
 }

libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/max_size.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // <memory>
 
 // template <class Alloc>
 // struct allocator_traits
 // {
-//     static size_type max_size(const allocator_type& a) noexcept;
+//     static constexpr size_type max_size(const allocator_type& a) noexcept;
 //     ...
 // };
 
 #include <limits>
 #include <memory>
 #include <new>
 #include <type_traits>
 #include <cassert>
 
 #include "test_macros.h"
 #include "incomplete_type_helper.h"
 
 template <class T>
 struct A
 {
     typedef T value_type;
 
 };
 
 template <class T>
 struct B
 {
     typedef T value_type;
 
-    size_t max_size() const
+    TEST_CONSTEXPR_CXX20 size_t max_size() const
     {
         return 100;
     }
 };
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
     {
         B<int> b;
         assert(std::allocator_traits<B<int> >::max_size(b) == 100);
     }
     {
         const B<int> b = {};
         assert(std::allocator_traits<B<int> >::max_size(b) == 100);
#if TEST_STD_VER >= 11
                std::numeric_limits<std::size_t>::max() / sizeof(int));
     }
     {
         std::allocator<int> a;
         static_assert(noexcept(std::allocator_traits<std::allocator<int>>::max_size(a)) == true, "");
     }
 #endif
 
+    return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test());
+#endif
+
     return 0;
 }

libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/select_on_container_copy_construction.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // <memory>
 
 // template <class Alloc>
 // struct allocator_traits
 // {
-//     static allocator_type
+//     static constexpr allocator_type
 //         select_on_container_copy_construction(const allocator_type& a);
 //     ...
 // };
 
 #include <memory>
 #include <new>
 #include <type_traits>
 #include <cassert>
 
 #include "test_macros.h"
 #include "incomplete_type_helper.h"
 
 template <class T>
 struct A
 {
     typedef T value_type;
     int id;
-    explicit A(int i = 0) : id(i) {}
+    TEST_CONSTEXPR_CXX20 explicit A(int i = 0) : id(i) {}
 
 };
 
 template <class T>
 struct B
 {
     typedef T value_type;
 
     int id;
-    explicit B(int i = 0) : id(i) {}
+    TEST_CONSTEXPR_CXX20 explicit B(int i = 0) : id(i) {}
 
-    B select_on_container_copy_construction() const
+    TEST_CONSTEXPR_CXX20 B select_on_container_copy_construction() const
     {
         return B(100);
     }
 };
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
     {
         A<int> a;
         assert(std::allocator_traits<A<int> >::select_on_container_copy_construction(a).id == 0);
     }
     {
         const A<int> a(0);
         assert(std::allocator_traits<A<int> >::select_on_container_copy_construction(a).id == 0);
#if TEST_STD_VER >= 11
         assert(std::allocator_traits<B<int> >::select_on_container_copy_construction(b).id == 100);
     }
     {
         const B<int> b(0);
         assert(std::allocator_traits<B<int> >::select_on_container_copy_construction(b).id == 100);
     }
 #endif
 
+    return true;
+}
+
+int main(int, char**)
+{
+    test();
+#if TEST_STD_VER > 17
+    static_assert(test());
+#endif
     return 0;
 }

libcxx/test/std/utilities/memory/default.allocator/allocator.dtor.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
-// <memory>
+// UNSUPPORTED: c++03, c++11, c++14, c++17
 
-// allocator:
-
-// template <class T1, class T2>
-//   bool
-//   operator==(const allocator<T1>&, const allocator<T2>&) throw();
-//
-// template <class T1, class T2>
-//   bool
-//   operator!=(const allocator<T1>&, const allocator<T2>&) throw();
+// template <class T>
+// constexpr allocator<T>::~allocator();
 
 #include <memory>
-#include <cassert>
 
-#include "test_macros.h"
+
+template <typename T>
+constexpr bool test() {
+    std::allocator<T> alloc;
+    (void)alloc;
+
+    // destructor called here
+    return true;
+}
 
 int main(int, char**)
 {
-    std::allocator<int> a1;
-    std::allocator<int> a2;
-    assert(a1 == a2);
-    assert(!(a1 != a2));
+    test<int>();
+    test<int const>();
+
+    static_assert(test<int>());
+    static_assert(test<int const>());
 
     return 0;
 }

libcxx/test/std/utilities/memory/default.allocator/allocator.globals/eq.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // <memory>
 
 // allocator:
 
 // template <class T1, class T2>
-//   bool
+//   constexpr bool
 //   operator==(const allocator<T1>&, const allocator<T2>&) throw();
 //
 // template <class T1, class T2>
-//   bool
+//   constexpr bool
 //   operator!=(const allocator<T1>&, const allocator<T2>&) throw();
 
 #include <memory>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
     std::allocator<int> a1;
     std::allocator<int> a2;
     assert(a1 == a2);
     assert(!(a1 != a2));
 
+    return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test());
+#endif
+
     return 0;
 }

libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.constexpr.size.verify.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <memory>
+
+// allocator:
+// constexpr T* allocate(size_type n);
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+#include <memory>
+#include <cassert>
+
+#include "test_macros.h"
+
+template <typename T>
+constexpr bool test()
+{
+    typedef std::allocator<T> A;
+    typedef std::allocator_traits<A> AT;
+    A a;
+    TEST_IGNORE_NODISCARD a.allocate(AT::max_size(a) + 1);           // just barely too large
+    TEST_IGNORE_NODISCARD a.allocate(AT::max_size(a) * 2);           // significantly too large
+    TEST_IGNORE_NODISCARD a.allocate(((size_t) -1) / sizeof(T) + 1); // multiply will overflow
+    TEST_IGNORE_NODISCARD a.allocate((size_t) -1);                   // way too large
+
+    return true;
+}
+
+int main(int, char**)
+{
+    static_assert(test<double>()); // expected-error {{static_assert expression is not an integral constant expression}}
+    LIBCPP_STATIC_ASSERT(test<const double>()); // expected-error {{static_assert expression is not an integral constant expression}}
+    return 0;
+}

libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // <memory>
 
 // allocator:
-// T* allocate(size_t n);
+// constexpr T* allocate(size_t n);
 
 #include <memory>
 #include <cassert>
 #include <cstddef>       // for std::max_align_t
 
 #include "test_macros.h"
 #include "count_new.h"
 
const bool ExpectAligned = IsOverAlignedType && UsingAlignedNew;
     assert(globalMemCounter.checkOutstandingNewEq(0));
     assert(globalMemCounter.checkDeleteCalledEq(1));
     assert(globalMemCounter.checkAlignedDeleteCalledEq(ExpectAligned));
     assert(globalMemCounter.checkLastDeleteAlignEq(ExpectAligned ? Align : 0));
     assert(T::constructed == 0);
   }
 }
 
+#if TEST_STD_VER > 17
+template <size_t Align>
+constexpr bool test_aligned_constexpr() {
+    typedef AlignedType<Align> T;
+    std::allocator<T> a;
+    T* ap = a.allocate(3);
+    a.deallocate(ap, 3);
+
+    return true;
+}
+#endif
+
 int main(int, char**) {
     test_aligned<1>();
     test_aligned<2>();
     test_aligned<4>();
     test_aligned<8>();
     test_aligned<16>();
     test_aligned<MaxAligned>();
     test_aligned<OverAligned>();
     test_aligned<OverAligned * 2>();
 
+#if TEST_STD_VER > 17
+    static_assert(test_aligned_constexpr<1>());
+    static_assert(test_aligned_constexpr<2>());
+    static_assert(test_aligned_constexpr<4>());
+    static_assert(test_aligned_constexpr<8>());
+    static_assert(test_aligned_constexpr<16>());
+    static_assert(test_aligned_constexpr<MaxAligned>());
+    static_assert(test_aligned_constexpr<OverAligned>());
+    static_assert(test_aligned_constexpr<OverAligned * 2>());
+#endif
+
   return 0;
 }

libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.size.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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: no-exceptions
 // <memory>
 
 // allocator:
-// T* allocate(size_t n);
+// constexpr T* allocate(size_t n);
 
 #include <memory>
 #include <cassert>
 
 #include "test_macros.h"
 
 template <typename T>
 void test_max(size_t count)

libcxx/test/std/utilities/memory/default.allocator/allocator_types.pass.cpp

 
 #include <memory>
 #include <type_traits>
 #include <cstddef>
 
 #include "test_macros.h"
 
 template <typename T, typename U>
-void check()
+TEST_CONSTEXPR_CXX20 bool test()
 {
     static_assert((std::is_same<typename std::allocator<T>::size_type, std::size_t>::value), "");
     static_assert((std::is_same<typename std::allocator<T>::difference_type, std::ptrdiff_t>::value), "");
     static_assert((std::is_same<typename std::allocator<T>::value_type, T>::value), "");
     static_assert((std::is_same<typename std::allocator<T>::propagate_on_container_move_assignment, std::true_type>::value), "");
     static_assert((std::is_same<typename std::allocator<T>::is_always_equal, std::true_type>::value), "");
 
     std::allocator<T> a;
     std::allocator<T> a2 = a;
     a2 = a;
     std::allocator<U> a3 = a2;
     (void)a3;
+
+    return true;
 }
 
 int main(int, char**)
 {
-    check<char, int>();
-    check<char const, int const>();
+    test<char, int>();
+    test<char const, int const>();
+#if TEST_STD_VER > 17
+    static_assert(test<char, int>());
+    static_assert(test<char const, int const>());
+#endif
     return 0;
 }

libcxx/test/std/utilities/memory/specialized.algorithms/specialized.construct/construct_at.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <memory>
+
+// template <class T, class ...Args>
+// constexpr T* construct_at(T* location, Args&& ...args);
+
+#include <memory>
+#include <cassert>
+
+
+struct Foo {
+    int a;
+    char b;
+    double c;
+    constexpr Foo() { }
+    constexpr Foo(int a, char b, double c) : a(a), b(b), c(c) { }
+    constexpr Foo(int a, char b, double c, int* count) : Foo(a, b, c) { *count += 1; }
+    constexpr bool operator==(Foo const& other) const {
+        return a == other.a && b == other.b && c == other.c;
+    }
+};
+
+constexpr bool test()
+{
+    {
+        int ints[1] = {0};
+        int* res = std::construct_at(&ints[0], 42);
+        assert(res == &ints[0]);
+        assert(*res == 42);
+    }
+
+    {
+        Foo foos[1] = {};
+        int count = 0;
+        Foo* res = std::construct_at(&foos[0], 42, 'x', 123.89, &count);
+        assert(res == &foos[0]);
+        assert(*res == Foo(42, 'x', 123.89));
+        assert(count == 1);
+    }
+
+    return true;
+}
+
+// Make sure std::construct_at SFINAEs out based on the validity of calling
+// the constructor, instead of hard-erroring.
+template <typename T, typename = decltype(
+    std::construct_at((T*)nullptr, 1, 2) // missing arguments for Foo(...)
+)>
+constexpr bool test_sfinae(int) { return false; }
+template <typename T>
+constexpr bool test_sfinae(...) { return true; }
+static_assert(test_sfinae<Foo>(int()));
+
+int main(int, char**)
+{
+    test();
+    static_assert(test());
+    return 0;
+}

libcxx/test/std/utilities/memory/specialized.algorithms/specialized.destroy/destroy.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <memory>
 
 // template <class ForwardIt>
-// void destroy(ForwardIt, ForwardIt);
+// constexpr void destroy(ForwardIt, ForwardIt);
 
 #include <memory>
-#include <cstdlib>
 #include <cassert>
 
 #include "test_macros.h"
 #include "test_iterators.h"
 
 struct Counted {
-  static int count;
-  static void reset() { count = 0; }
-  Counted() { ++count; }
-  Counted(Counted const&) { ++count; }
-  ~Counted() { --count; }
+    int* counter_;
+    TEST_CONSTEXPR Counted(int* counter) : counter_(counter) { ++*counter_; }
+    TEST_CONSTEXPR Counted(Counted const& other) : counter_(other.counter_) { ++*counter_; }
+    TEST_CONSTEXPR_CXX20 ~Counted() { --*counter_; }
     friend void operator&(Counted) = delete;
 };
-int Counted::count = 0;
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
-    using It = forward_iterator<Counted*>;
-    const int N = 5;
-    alignas(Counted) char pool[sizeof(Counted)*N] = {};
-    Counted* p = (Counted*)pool;
-    std::uninitialized_fill(p, p+N, Counted());
-    assert(Counted::count == 5);
-    std::destroy(p, p+1);
-    p += 1;
-    assert(Counted::count == 4);
-    std::destroy(It(p), It(p + 4));
-    assert(Counted::count == 0);
+    using Alloc = std::allocator<Counted>;
+    int counter = 0;
+    int const N = 5;
+    Alloc alloc;
+    Counted* pool = std::allocator_traits<Alloc>::allocate(alloc, N);
+
+    for (Counted* p = pool; p != pool + N; ++p)
+        std::allocator_traits<Alloc>::construct(alloc, p, &counter);
+    assert(counter == 5);
+
+    std::destroy(pool, pool + 1);
+    assert(counter == 4);
+
+    std::destroy(forward_iterator<Counted*>(pool + 1), forward_iterator<Counted*>(pool + 5));
+    assert(counter == 0);
+
+    std::allocator_traits<Alloc>::deallocate(alloc, pool, N);
 
+    return true;
+}
+
+int main(int, char**)
+{
+    test();
+#if TEST_STD_VER > 17
+    static_assert(test());
+#endif
     return 0;
 }

libcxx/test/std/utilities/memory/specialized.algorithms/specialized.destroy/destroy_at.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <memory>
 
-// template <class _Tp>
-// void destroy_at(_Tp*);
+// template <class T>
+// constexpr void destroy_at(T*);
 
 #include <memory>
-#include <cstdlib>
 #include <cassert>
 
 #include "test_macros.h"
 
 struct Counted {
-  static int count;
-  static void reset() { count = 0; }
-  Counted() { ++count; }
-  Counted(Counted const&) { ++count; }
-  ~Counted() { --count; }
+    int* counter_;
+    TEST_CONSTEXPR Counted(int* counter) : counter_(counter) { ++*counter_; }
+    TEST_CONSTEXPR_CXX20 ~Counted() { --*counter_; }
     friend void operator&(Counted) = delete;
 };
-int Counted::count = 0;
 
-struct VCounted {
-  static int count;
-  static void reset() { count = 0; }
-  VCounted() { ++count; }
-  VCounted(VCounted const&) { ++count; }
-  virtual ~VCounted() { --count; }
-  friend void operator&(VCounted) = delete;
+struct VirtualCounted {
+    int* counter_;
+    TEST_CONSTEXPR VirtualCounted(int* counter) : counter_(counter) { ++*counter_; }
+    TEST_CONSTEXPR_CXX20 virtual ~VirtualCounted() { --*counter_; }
+    friend void operator&(VirtualCounted) = delete;
 };
-int VCounted::count = 0;
 
-struct DCounted : VCounted {
-    friend void operator&(DCounted) = delete;
+struct DerivedCounted : VirtualCounted {
+    TEST_CONSTEXPR DerivedCounted(int* counter) : VirtualCounted(counter) { }
+    friend void operator&(DerivedCounted) = delete;
 };
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
     {
-    void* mem1 = std::malloc(sizeof(Counted));
-    void* mem2 = std::malloc(sizeof(Counted));
-    assert(mem1 && mem2);
-    assert(Counted::count == 0);
-    Counted* ptr1 = ::new(mem1) Counted();
-    Counted* ptr2 = ::new(mem2) Counted();
-    assert(Counted::count == 2);
+        using Alloc = std::allocator<Counted>;
+        Alloc alloc;
+        Counted* ptr1 = std::allocator_traits<Alloc>::allocate(alloc, 1);
+        Counted* ptr2 = std::allocator_traits<Alloc>::allocate(alloc, 1);
+
+        int counter = 0;
+        std::allocator_traits<Alloc>::construct(alloc, ptr1, &counter);
+        std::allocator_traits<Alloc>::construct(alloc, ptr2, &counter);
+        assert(counter == 2);
+
         std::destroy_at(ptr1);
-    assert(Counted::count == 1);
+        assert(counter == 1);
+
         std::destroy_at(ptr2);
-    assert(Counted::count == 0);
-    std::free(mem1);
-    std::free(mem2);
+        assert(counter == 0);
+
+        std::allocator_traits<Alloc>::deallocate(alloc, ptr1, 1);
+        std::allocator_traits<Alloc>::deallocate(alloc, ptr2, 1);
     }
     {
-    void* mem1 = std::malloc(sizeof(DCounted));
-    void* mem2 = std::malloc(sizeof(DCounted));
-    assert(mem1 && mem2);
-    assert(DCounted::count == 0);
-    DCounted* ptr1 = ::new(mem1) DCounted();
-    DCounted* ptr2 = ::new(mem2) DCounted();
-    assert(DCounted::count == 2);
-    assert(VCounted::count == 2);
+        using Alloc = std::allocator<DerivedCounted>;
+        Alloc alloc;
+        DerivedCounted* ptr1 = std::allocator_traits<Alloc>::allocate(alloc, 1);
+        DerivedCounted* ptr2 = std::allocator_traits<Alloc>::allocate(alloc, 1);
+
+        int counter = 0;
+        std::allocator_traits<Alloc>::construct(alloc, ptr1, &counter);
+        std::allocator_traits<Alloc>::construct(alloc, ptr2, &counter);
+        assert(counter == 2);
+
         std::destroy_at(ptr1);
-    assert(VCounted::count == 1);
+        assert(counter == 1);
+
         std::destroy_at(ptr2);
-    assert(VCounted::count == 0);
-    std::free(mem1);
-    std::free(mem2);
-    }
+        assert(counter == 0);
+
+        std::allocator_traits<Alloc>::deallocate(alloc, ptr1, 1);
+        std::allocator_traits<Alloc>::deallocate(alloc, ptr2, 1);
+    }
+
+    return true;
 
+int main(int, char**)
+{
+    test();
+#if TEST_STD_VER > 17
+    static_assert(test());
+#endif
     return 0;
 }

libcxx/test/std/utilities/memory/specialized.algorithms/specialized.destroy/destroy_n.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 
 // <memory>
 
 // template <class ForwardIt, class Size>
-// ForwardIt destroy_n(ForwardIt, Size s);
+// constexpr ForwardIt destroy_n(ForwardIt, Size s);
 
 #include <memory>
-#include <cstdlib>
 #include <cassert>
 
 #include "test_macros.h"
 #include "test_iterators.h"
 
 struct Counted {
-  static int count;
-  static void reset() { count = 0; }
-  Counted() { ++count; }
-  Counted(Counted const&) { ++count; }
-  ~Counted() { --count; }
+    int* counter_;
+    TEST_CONSTEXPR Counted(int* counter) : counter_(counter) { ++*counter_; }
+    TEST_CONSTEXPR Counted(Counted const& other) : counter_(other.counter_) { ++*counter_; }
+    TEST_CONSTEXPR_CXX20 ~Counted() { --*counter_; }
     friend void operator&(Counted) = delete;
 };
-int Counted::count = 0;
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
-    using It = forward_iterator<Counted*>;
-    const int N = 5;
-    alignas(Counted) char pool[sizeof(Counted)*N] = {};
-    Counted* p = (Counted*)pool;
-    std::uninitialized_fill(p, p+N, Counted());
-    assert(Counted::count == 5);
-    Counted* np = std::destroy_n(p, 1);
-    assert(np == p+1);
-    assert(Counted::count == 4);
-    p += 1;
-    It it = std::destroy_n(It(p), 4);
-    assert(it == It(p+4));
-    assert(Counted::count == 0);
+    using Alloc = std::allocator<Counted>;
+    int counter = 0;
+    int const N = 5;
+    Alloc alloc;
+    Counted* pool = std::allocator_traits<Alloc>::allocate(alloc, N);
+
+    for (Counted* p = pool; p != pool + N; ++p)
+        std::allocator_traits<Alloc>::construct(alloc, p, &counter);
+    assert(counter == 5);
+
+    Counted* np = std::destroy_n(pool, 1);
+    assert(np == pool + 1);
+    assert(counter == 4);
+
+    forward_iterator<Counted*> it = std::destroy_n(forward_iterator<Counted*>(pool + 1), 4);
+    assert(it == forward_iterator<Counted*>(pool + 5));
+    assert(counter == 0);
+
+    std::allocator_traits<Alloc>::deallocate(alloc, pool, N);
+
+    return true;
+}
 
+int main(int, char**)
+{
+    test();
+#if TEST_STD_VER > 17
+    static_assert(test());
+#endif
     return 0;
 }

libcxx/utils/generate_feature_test_macro_components.py

feature_test_macros = sorted([ add_version_header(x) for x in [
    "values": {
      "c++2a": int(201911),
    },
    "headers": ["atomic", "memory"],
    "unimplemented": True,
    "depends": "!defined(_LIBCPP_HAS_NO_THREADS)",
    "internal_depends": "!defined(_LIBCPP_HAS_NO_THREADS)",
    },
+   {"name": "__cpp_lib_constexpr_dynamic_alloc",
+    "values": {
+      "c++2a": int(201907)
+    },
+    "headers": ["memory"]
+   },
 ]], key=lambda tc: tc["name"])
 
 def get_std_dialects():
   std_dialects = ['c++14', 'c++17', 'c++2a']
   return list(std_dialects)
 
 def get_first_std(d):
     for s in get_std_dialects():

libcxx/www/cxx2a_status.html

-->
    <tr><td></td><td></td><td></td><td></td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0325">P0325</a></td><td>LWG</td><td>to_array from LFTS with updates</td><td>Cologne</td><td>Complete</td><td>10.0</td></tr>
   <tr><td><a href="https://wg21.link/P0408">P0408</a></td><td>LWG</td><td>Efficient Access to basic_stringbuf ’s Buffer</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0466">P0466</a></td><td>LWG</td><td>Layout-compatibility and Pointer-interconvertibility Traits</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0553">P0553</a></td><td>LWG</td><td>Bit operations</td><td>Cologne</td><td>Complete</td><td>9.0</td></tr>
   <tr><td><a href="https://wg21.link/P0631">P0631</a></td><td>LWG</td><td>Math Constants</td><td>Cologne</td><td>Complete</td><td>11.0</td></tr>
   <tr><td><a href="https://wg21.link/P0645">P0645</a></td><td>LWG</td><td>Text Formatting</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0660">P0660</a></td><td>LWG</td><td>Stop Token and Joining Thread, Rev 10</td><td>Cologne</td><td></td><td></td></tr>
-  <tr><td><a href="https://wg21.link/P0784">P0784</a></td><td>CWG</td><td>More constexpr containers</td><td>Cologne</td><td></td><td></td></tr>
+  <tr><td><a href="https://wg21.link/P0784">P0784</a></td><td>CWG</td><td>More constexpr containers</td><td>Cologne</td><td>Complete</td><td>12.0</td></tr>
   <tr><td><a href="https://wg21.link/P0980">P0980</a></td><td>LWG</td><td>Making std::string constexpr</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P1004">P1004</a></td><td>LWG</td><td>Making std::vector constexpr</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P1035">P1035</a></td><td>LWG</td><td>Input Range Adaptors</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P1065">P1065</a></td><td>LWG</td><td>Constexpr INVOKE</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P1135">P1135</a></td><td>LWG</td><td>The C++20 Synchronization Library</td><td>Cologne</td><td>Complete</td><td></td></tr>
   <tr><td><a href="https://wg21.link/P1207">P1207</a></td><td>LWG</td><td>Movability of Single-pass Iterators</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P1208">P1208</a></td><td>LWG</td><td>Adopt source_location for C++20</td><td>Cologne</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P1355">P1355</a></td><td>LWG</td><td>Exposing a narrow contract for ceil2</td><td>Cologne</td><td>Complete</td><td>9.0</td></tr>