[libc++] NFCI: Refactor allocator_traits

[ldionne]

Summary

The implementation had a lot of boilerplate and was more complicated than
necessary. This NFC refactoring introduces a few macros to reduce code
duplication, and uses a consistent style and formatting for the whole file.

[zoecarver]

Thanks for the cleanup, this is much improved!

libcxx/include/__memory/allocator_traits.h

159

If this is the only use of _LIBCPP_ALLOCATOR_TRAITS_HAS_XXX_TEMPLATE, is it better to just implement it here (and get rid of the macro)?

190

Let's put these at the bottom of the file. Otherwise, if someone tries to use them later on it might create a hard-to-fix error.

244

Could this be __allocator_traits_rebind_t?

272

Why the has_allocate_hint? Is that just for the deprecation warnings? Is it worth just taking the deprecation warnings so we could have just one implementation? For compile-time if nothing else. Those SFINAE checks are slow.

295

I don't really understand why we have two implementations here. This isn't a constexpr context, so they both do the same thing.

[ldionne]

Address Zoe's comments. Note that this is meant to be a NFC commit, so I don't
think we should incorporate additional changes beyond the refactorign to those
changes.

libcxx/include/__memory/allocator_traits.h

272

Do you mean that you'd prefer doing the check for whether a.allocate(n, hint) is supported inline in the template declaration?

If you're asking why we call allocate with a hint at all, it's because we must. If a user-defined allocator does something special when called with a hint, we need to maintain that.

295

construct_at isn't provided before C++20.

[zoecarver]

libcxx/include/__memory/allocator_traits.h

272

Oops, I didn't see that we're only passing the hint argument in one of these. Never mind then :)

295

I was thinking the opposite: always construct with placement new. Is there a way (that isn't UB) for a user to know the difference?

(This is also a super small thing, no need to address it, I know this is a bit out-of-scope.)

[zoecarver]

Thanks again for the cleanup!

[ldionne]

Thanks for the review Zoe!

libcxx/include/__memory/allocator_traits.h

295

The issue is that placement-new doesn't work inside constexpr, but construct_at does. This function is marked as constexpr in C++20 and above (_LIBCPP_CONSTEXPR_AFTER_CXX17).

libcxx/include/__memory/allocator_traits.h

 #pragma GCC system_header
 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-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 {};
-
-namespace __pointer_type_imp
-{
-
-template <class _Tp, class _Dp, bool = __has_pointer_type<_Dp>::value>
-struct __pointer_type
-{
-    typedef _LIBCPP_NODEBUG_TYPE typename _Dp::pointer type;
+#define _LIBCPP_ALLOCATOR_TRAITS_HAS_XXX(NAME, PROPERTY)                \
+    template <class _Tp, class = void> struct NAME : false_type { };    \
+    template <class _Tp>               struct NAME<_Tp, typename __void_t<typename _Tp:: PROPERTY >::type> : true_type { }
+
+// __pointer
+_LIBCPP_ALLOCATOR_TRAITS_HAS_XXX(__has_pointer, pointer);
+template <class _Tp, class _Alloc,
+          class _RawAlloc = typename remove_reference<_Alloc>::type,
+          bool = __has_pointer<_RawAlloc>::value>
+struct __pointer {
+    using type _LIBCPP_NODEBUG_TYPE = typename _RawAlloc::pointer;
+};
+template <class _Tp, class _Alloc, class _RawAlloc>
+struct __pointer<_Tp, _Alloc, _RawAlloc, false> {
+    using type _LIBCPP_NODEBUG_TYPE = _Tp*;
+};
+
+// __const_pointer
+_LIBCPP_ALLOCATOR_TRAITS_HAS_XXX(__has_const_pointer, const_pointer);
+template <class _Tp, class _Ptr, class _Alloc,
+          bool = __has_const_pointer<_Alloc>::value>
+struct __const_pointer {
+    using type _LIBCPP_NODEBUG_TYPE = typename _Alloc::const_pointer;
 };
-
-template <class _Tp, class _Dp>
-struct __pointer_type<_Tp, _Dp, false>
-{
-    typedef _LIBCPP_NODEBUG_TYPE _Tp* type;
-};
-
-}  // __pointer_type_imp
-
-template <class _Tp, class _Dp>
-struct __pointer_type
-{
-    typedef _LIBCPP_NODEBUG_TYPE typename __pointer_type_imp::__pointer_type<_Tp, typename remove_reference<_Dp>::type>::type type;
-};
-
-template <class _Tp, class = void>
-struct __has_const_pointer : false_type {};
-
-template <class _Tp>
-struct __has_const_pointer<_Tp,
-            typename __void_t<typename _Tp::const_pointer>::type> : true_type {};
-
-template <class _Tp, class _Ptr, class _Alloc, bool = __has_const_pointer<_Alloc>::value>
-struct __const_pointer
-{
-    typedef _LIBCPP_NODEBUG_TYPE typename _Alloc::const_pointer type;
-};
-
 template <class _Tp, class _Ptr, class _Alloc>
-struct __const_pointer<_Tp, _Ptr, _Alloc, false>
-{
-#ifndef _LIBCPP_CXX03_LANG
-    typedef _LIBCPP_NODEBUG_TYPE typename pointer_traits<_Ptr>::template rebind<const _Tp> type;
+struct __const_pointer<_Tp, _Ptr, _Alloc, false> {
+#ifdef _LIBCPP_CXX03_LANG
+    using type = typename pointer_traits<_Ptr>::template rebind<const _Tp>::other;
 #else
-    typedef typename pointer_traits<_Ptr>::template rebind<const _Tp>::other type;
+    using type _LIBCPP_NODEBUG_TYPE = typename pointer_traits<_Ptr>::template rebind<const _Tp>;
 #endif
 };
 
-template <class _Tp, class = void>
-struct __has_void_pointer : false_type {};
-
-template <class _Tp>
-struct __has_void_pointer<_Tp,
-               typename __void_t<typename _Tp::void_pointer>::type> : true_type {};
-
-template <class _Ptr, class _Alloc, bool = __has_void_pointer<_Alloc>::value>
-struct __void_pointer
-{
-    typedef _LIBCPP_NODEBUG_TYPE typename _Alloc::void_pointer type;
+// __void_pointer
+_LIBCPP_ALLOCATOR_TRAITS_HAS_XXX(__has_void_pointer, void_pointer);
+template <class _Ptr, class _Alloc,
+          bool = __has_void_pointer<_Alloc>::value>
+struct __void_pointer {
+    using type _LIBCPP_NODEBUG_TYPE = typename _Alloc::void_pointer;
 };
-
 template <class _Ptr, class _Alloc>
-struct __void_pointer<_Ptr, _Alloc, false>
-{
-#ifndef _LIBCPP_CXX03_LANG
-    typedef _LIBCPP_NODEBUG_TYPE typename pointer_traits<_Ptr>::template rebind<void> type;
+struct __void_pointer<_Ptr, _Alloc, false> {
+#ifdef _LIBCPP_CXX03_LANG
+    using type _LIBCPP_NODEBUG_TYPE = typename pointer_traits<_Ptr>::template rebind<void>::other;
 #else
-    typedef _LIBCPP_NODEBUG_TYPE typename pointer_traits<_Ptr>::template rebind<void>::other type;
+    using type _LIBCPP_NODEBUG_TYPE = typename pointer_traits<_Ptr>::template rebind<void>;
 #endif
 };
 
-template <class _Tp, class = void>
-struct __has_const_void_pointer : false_type {};
-
-template <class _Tp>
-struct __has_const_void_pointer<_Tp,
-            typename __void_t<typename _Tp::const_void_pointer>::type> : true_type {};
-
-template <class _Ptr, class _Alloc, bool = __has_const_void_pointer<_Alloc>::value>
-struct __const_void_pointer
-{
-    typedef _LIBCPP_NODEBUG_TYPE typename _Alloc::const_void_pointer type;
+// __const_void_pointer
+_LIBCPP_ALLOCATOR_TRAITS_HAS_XXX(__has_const_void_pointer, const_void_pointer);
+template <class _Ptr, class _Alloc,
+          bool = __has_const_void_pointer<_Alloc>::value>
+struct __const_void_pointer {
+    using type _LIBCPP_NODEBUG_TYPE = typename _Alloc::const_void_pointer;
 };
-
 template <class _Ptr, class _Alloc>
-struct __const_void_pointer<_Ptr, _Alloc, false>
-{
-#ifndef _LIBCPP_CXX03_LANG
-    typedef _LIBCPP_NODEBUG_TYPE typename pointer_traits<_Ptr>::template rebind<const void> type;
+struct __const_void_pointer<_Ptr, _Alloc, false> {
+#ifdef _LIBCPP_CXX03_LANG
+    using type _LIBCPP_NODEBUG_TYPE = typename pointer_traits<_Ptr>::template rebind<const void>::other;
 #else
-    typedef _LIBCPP_NODEBUG_TYPE typename pointer_traits<_Ptr>::template rebind<const void>::other type;
+    using type _LIBCPP_NODEBUG_TYPE = typename pointer_traits<_Ptr>::template rebind<const void>;
 #endif
 };
 
-template <class _Tp, class = void>
-struct __has_size_type : false_type {};
-
-template <class _Tp>
-struct __has_size_type<_Tp,
-               typename __void_t<typename _Tp::size_type>::type> : true_type {};
-
+// __size_type
+_LIBCPP_ALLOCATOR_TRAITS_HAS_XXX(__has_size_type, size_type);
 template <class _Alloc, class _DiffType, bool = __has_size_type<_Alloc>::value>
-struct __size_type
-{
-    typedef _LIBCPP_NODEBUG_TYPE typename make_unsigned<_DiffType>::type type;
-};
-
+struct __size_type : make_unsigned<_DiffType> { };
 template <class _Alloc, class _DiffType>
-struct __size_type<_Alloc, _DiffType, true>
-{
-    typedef _LIBCPP_NODEBUG_TYPE typename _Alloc::size_type type;
+struct __size_type<_Alloc, _DiffType, true> {
+    using type _LIBCPP_NODEBUG_TYPE = typename _Alloc::size_type;
 };
 
-template <class _Tp, class = void>
-struct __has_propagate_on_container_copy_assignment : false_type {};
-
-template <class _Tp>
-struct __has_propagate_on_container_copy_assignment<_Tp,
-    typename __void_t<typename _Tp::propagate_on_container_copy_assignment>::type>
-        : true_type {};
-
-template <class _Alloc, bool = __has_propagate_on_container_copy_assignment<_Alloc>::value>
-struct __propagate_on_container_copy_assignment
-{
-    typedef _LIBCPP_NODEBUG_TYPE false_type type;
+// __alloc_traits_difference_type
+_LIBCPP_ALLOCATOR_TRAITS_HAS_XXX(__has_alloc_traits_difference_type, difference_type);
+template <class _Alloc, class _Ptr, bool = __has_alloc_traits_difference_type<_Alloc>::value>
+struct __alloc_traits_difference_type {
+    using type _LIBCPP_NODEBUG_TYPE = typename pointer_traits<_Ptr>::difference_type;
+};
+template <class _Alloc, class _Ptr>
+struct __alloc_traits_difference_type<_Alloc, _Ptr, true> {
+    using type _LIBCPP_NODEBUG_TYPE = typename _Alloc::difference_type;
 };
 
+// __propagate_on_container_copy_assignment
+_LIBCPP_ALLOCATOR_TRAITS_HAS_XXX(__has_propagate_on_container_copy_assignment, propagate_on_container_copy_assignment);
+template <class _Alloc, bool = __has_propagate_on_container_copy_assignment<_Alloc>::value>
+struct __propagate_on_container_copy_assignment : false_type { };
 template <class _Alloc>
-struct __propagate_on_container_copy_assignment<_Alloc, true>
-{
-    typedef _LIBCPP_NODEBUG_TYPE typename _Alloc::propagate_on_container_copy_assignment type;
+struct __propagate_on_container_copy_assignment<_Alloc, true> {
+    using type _LIBCPP_NODEBUG_TYPE = typename _Alloc::propagate_on_container_copy_assignment;
 };
 
-template <class _Tp, class = void>
-struct __has_propagate_on_container_move_assignment : false_type {};
-
-template <class _Tp>
-struct __has_propagate_on_container_move_assignment<_Tp,
-           typename __void_t<typename _Tp::propagate_on_container_move_assignment>::type>
-               : true_type {};
-
+// __propagate_on_container_move_assignment
+_LIBCPP_ALLOCATOR_TRAITS_HAS_XXX(__has_propagate_on_container_move_assignment, propagate_on_container_move_assignment);
 template <class _Alloc, bool = __has_propagate_on_container_move_assignment<_Alloc>::value>
-struct __propagate_on_container_move_assignment
-{
-    typedef false_type type;
-};
-
+struct __propagate_on_container_move_assignment : false_type { };
 template <class _Alloc>
-struct __propagate_on_container_move_assignment<_Alloc, true>
-{
-    typedef _LIBCPP_NODEBUG_TYPE typename _Alloc::propagate_on_container_move_assignment type;
+struct __propagate_on_container_move_assignment<_Alloc, true> {
+    using type _LIBCPP_NODEBUG_TYPE = typename _Alloc::propagate_on_container_move_assignment;
 };
 
-template <class _Tp, class = void>
-struct __has_propagate_on_container_swap : false_type {};
-
-template <class _Tp>
-struct __has_propagate_on_container_swap<_Tp,
-           typename __void_t<typename _Tp::propagate_on_container_swap>::type>
-               : true_type {};
-
+// __propagate_on_container_swap
+_LIBCPP_ALLOCATOR_TRAITS_HAS_XXX(__has_propagate_on_container_swap, propagate_on_container_swap);
 template <class _Alloc, bool = __has_propagate_on_container_swap<_Alloc>::value>
-struct __propagate_on_container_swap
-{
-    typedef false_type type;
-};
-
+struct __propagate_on_container_swap : false_type { };
 template <class _Alloc>
-struct __propagate_on_container_swap<_Alloc, true>
-{
-    typedef _LIBCPP_NODEBUG_TYPE typename _Alloc::propagate_on_container_swap type;
+struct __propagate_on_container_swap<_Alloc, true> {
+    using type _LIBCPP_NODEBUG_TYPE = typename _Alloc::propagate_on_container_swap;
 };
 
-template <class _Tp, class = void>
-struct __has_is_always_equal : false_type {};
-
-template <class _Tp>
-struct __has_is_always_equal<_Tp,
-           typename __void_t<typename _Tp::is_always_equal>::type>
-               : true_type {};
-
+// __is_always_equal
+_LIBCPP_ALLOCATOR_TRAITS_HAS_XXX(__has_is_always_equal, is_always_equal);
 template <class _Alloc, bool = __has_is_always_equal<_Alloc>::value>
-struct __is_always_equal
-{
-    typedef _LIBCPP_NODEBUG_TYPE typename _VSTD::is_empty<_Alloc>::type type;
-};
-
+struct __is_always_equal : is_empty<_Alloc> { };
 template <class _Alloc>
-struct __is_always_equal<_Alloc, true>
-{
-    typedef _LIBCPP_NODEBUG_TYPE typename _Alloc::is_always_equal type;
+struct __is_always_equal<_Alloc, true> {
+    using type _LIBCPP_NODEBUG_TYPE = typename _Alloc::is_always_equal;
 };
 
-template <class _Tp, class _Up, bool = __has_rebind<_Tp, _Up>::value>
-struct __has_rebind_other
-{
-private:
-    struct __two {char __lx; char __lxx;};
-    template <class _Xp> static __two __test(...);
-    _LIBCPP_SUPPRESS_DEPRECATED_PUSH
-    template <class _Xp> static char __test(typename _Xp::template rebind<_Up>::other* = 0);
-    _LIBCPP_SUPPRESS_DEPRECATED_POP
-public:
-    static const bool value = sizeof(__test<_Tp>(0)) == 1;
-};
-
+// __allocator_traits_rebind
+template <class _Tp, class _Up, class = void>
+struct __has_rebind_other : false_type { };
 template <class _Tp, class _Up>
-struct __has_rebind_other<_Tp, _Up, false>
-{
-    static const bool value = false;
-};
+struct __has_rebind_other<_Tp, _Up, typename __void_t<
+    typename _Tp::template rebind<_Up>::other
+>::type> : true_type { };
 
 template <class _Tp, class _Up, bool = __has_rebind_other<_Tp, _Up>::value>
-struct __allocator_traits_rebind
-{
-    _LIBCPP_SUPPRESS_DEPRECATED_PUSH
-    typedef _LIBCPP_NODEBUG_TYPE typename _Tp::template rebind<_Up>::other type;
-    _LIBCPP_SUPPRESS_DEPRECATED_POP
+struct __allocator_traits_rebind {
+    using type _LIBCPP_NODEBUG_TYPE = typename _Tp::template rebind<_Up>::other;
 };
-
 template <template <class, class...> class _Alloc, class _Tp, class ..._Args, class _Up>
-struct __allocator_traits_rebind<_Alloc<_Tp, _Args...>, _Up, true>
-{
-    _LIBCPP_SUPPRESS_DEPRECATED_PUSH
-    typedef _LIBCPP_NODEBUG_TYPE typename _Alloc<_Tp, _Args...>::template rebind<_Up>::other type;
-    _LIBCPP_SUPPRESS_DEPRECATED_POP
+struct __allocator_traits_rebind<_Alloc<_Tp, _Args...>, _Up, true> {
+    using type _LIBCPP_NODEBUG_TYPE = typename _Alloc<_Tp, _Args...>::template rebind<_Up>::other;
 };
-
 template <template <class, class...> class _Alloc, class _Tp, class ..._Args, class _Up>
-struct __allocator_traits_rebind<_Alloc<_Tp, _Args...>, _Up, false>
-{
-    typedef _LIBCPP_NODEBUG_TYPE _Alloc<_Up, _Args...> type;
+struct __allocator_traits_rebind<_Alloc<_Tp, _Args...>, _Up, false> {
+    using type _LIBCPP_NODEBUG_TYPE = _Alloc<_Up, _Args...>;
 };
-
-#ifndef _LIBCPP_CXX03_LANG
-
-_LIBCPP_SUPPRESS_DEPRECATED_PUSH
-template <class _Alloc, class _SizeType, class _ConstVoidPtr>
-auto
-__has_allocate_hint_test(_Alloc&& __a, _SizeType&& __sz, _ConstVoidPtr&& __p)
-    -> decltype((void)__a.allocate(__sz, __p), true_type());
 _LIBCPP_SUPPRESS_DEPRECATED_POP
 
-template <class _Alloc, class _SizeType, class _ConstVoidPtr>
-auto
-__has_allocate_hint_test(const _Alloc& __a, _SizeType&& __sz, _ConstVoidPtr&& __p)
-    -> false_type;
+template<class _Alloc, class _Tp>
+using __allocator_traits_rebind_t = typename __allocator_traits_rebind<_Alloc, _Tp>::type;
 
-template <class _Alloc, class _SizeType, class _ConstVoidPtr>
-struct __has_allocate_hint
-    : decltype(_VSTD::__has_allocate_hint_test(declval<_Alloc>(),
-                                               declval<_SizeType>(),
-                                               declval<_ConstVoidPtr>()))
-{
-};
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 
-#else  // _LIBCPP_CXX03_LANG
+// __has_allocate_hint
+template <class _Alloc, class _SizeType, class _ConstVoidPtr, class = void>
+struct __has_allocate_hint : false_type { };
 
 template <class _Alloc, class _SizeType, class _ConstVoidPtr>
-struct __has_allocate_hint
-    : true_type
-{
-};
-
-#endif  // _LIBCPP_CXX03_LANG
-
-_LIBCPP_SUPPRESS_DEPRECATED_PUSH
-template <class _Alloc, class ..._Args,
-    class = decltype(_VSTD::declval<_Alloc>().construct(_VSTD::declval<_Args>()...))>
-static true_type __test_has_construct(int);
-_LIBCPP_SUPPRESS_DEPRECATED_POP
-
-template <class _Alloc, class...>
-static false_type __test_has_construct(...);
+struct __has_allocate_hint<_Alloc, _SizeType, _ConstVoidPtr, decltype(
+    (void)declval<_Alloc>().allocate(declval<_SizeType>(), declval<_ConstVoidPtr>())
+)> : true_type { };
+
+// __has_construct
+template <class, class _Alloc, class ..._Args>
+struct __has_construct_impl : false_type { };
 
 template <class _Alloc, class ..._Args>
-struct __has_construct : decltype(__test_has_construct<_Alloc, _Args...>(0)) {};
-
-#if !defined(_LIBCPP_CXX03_LANG)
-
-_LIBCPP_SUPPRESS_DEPRECATED_PUSH
-template <class _Alloc, class _Pointer>
-auto
-__has_destroy_test(_Alloc&& __a, _Pointer&& __p)
-    -> decltype(__a.destroy(__p), true_type());
-_LIBCPP_SUPPRESS_DEPRECATED_POP
-
-template <class _Alloc, class _Pointer>
-auto
-__has_destroy_test(const _Alloc& __a, _Pointer&& __p)
-    -> false_type;
-
-template <class _Alloc, class _Pointer>
-struct __has_destroy
-    : decltype(_VSTD::__has_destroy_test(declval<_Alloc>(),
-                                         declval<_Pointer>()))
-{
-};
-
-_LIBCPP_SUPPRESS_DEPRECATED_PUSH
-template <class _Alloc>
-auto
-__has_max_size_test(_Alloc&& __a)
-    -> decltype(__a.max_size(), true_type());
-_LIBCPP_SUPPRESS_DEPRECATED_POP
-
-template <class _Alloc>
-auto
-__has_max_size_test(const volatile _Alloc& __a)
-    -> false_type;
-
-template <class _Alloc>
-struct __has_max_size
-    : decltype(_VSTD::__has_max_size_test(declval<_Alloc&>()))
-{
-};
+struct __has_construct_impl<decltype(
+    (void)declval<_Alloc>().construct(declval<_Args>()...)
+), _Alloc, _Args...> : true_type { };
 
-template <class _Alloc>
-auto
-__has_select_on_container_copy_construction_test(_Alloc&& __a)
-    -> decltype(__a.select_on_container_copy_construction(), true_type());
-
-template <class _Alloc>
-auto
-__has_select_on_container_copy_construction_test(const volatile _Alloc& __a)
-    -> false_type;
-
-template <class _Alloc>
-struct __has_select_on_container_copy_construction
-    : decltype(_VSTD::__has_select_on_container_copy_construction_test(declval<_Alloc&>()))
-{
-};
-
-#else  // _LIBCPP_CXX03_LANG
+template <class _Alloc, class ..._Args>
+struct __has_construct : __has_construct_impl<void, _Alloc, _Args...> { };
 
+// __has_destroy
 template <class _Alloc, class _Pointer, class = void>
-struct __has_destroy : false_type {};
+struct __has_destroy : false_type { };
 
 template <class _Alloc, class _Pointer>
-struct __has_destroy<_Alloc, _Pointer, typename __void_t<
-    decltype(_VSTD::declval<_Alloc>().destroy(_VSTD::declval<_Pointer>()))
->::type> : true_type {};
+struct __has_destroy<_Alloc, _Pointer, decltype(
+    (void)declval<_Alloc>().destroy(declval<_Pointer>())
+)> : true_type { };
 
-template <class _Alloc>
-struct __has_max_size
-    : true_type
-{
-};
+// __has_max_size
+template <class _Alloc, class = void>
+struct __has_max_size : false_type { };
 
 template <class _Alloc>
-struct __has_select_on_container_copy_construction
-    : false_type
-{
-};
-
-#endif  // _LIBCPP_CXX03_LANG
-
-template <class _Alloc, class _Ptr, bool = __has_difference_type<_Alloc>::value>
-struct __alloc_traits_difference_type
-{
-    typedef _LIBCPP_NODEBUG_TYPE typename pointer_traits<_Ptr>::difference_type type;
-};
-
-template <class _Alloc, class _Ptr>
-struct __alloc_traits_difference_type<_Alloc, _Ptr, true>
-{
-    typedef _LIBCPP_NODEBUG_TYPE typename _Alloc::difference_type type;
-};
-
-template <class _Tp>
-struct __is_default_allocator : false_type {};
+struct __has_max_size<_Alloc, decltype(
+    (void)declval<_Alloc&>().max_size()
+)> : true_type { };
 
-template <class _Tp>
-struct __is_default_allocator<_VSTD::allocator<_Tp> > : true_type {};
-
-
-
-template <class _Alloc,
-    bool = __has_construct<_Alloc, typename _Alloc::value_type*,  typename _Alloc::value_type&&>::value && !__is_default_allocator<_Alloc>::value
-    >
-struct __is_cpp17_move_insertable;
-template <class _Alloc>
-struct __is_cpp17_move_insertable<_Alloc, true> : true_type {};
-template <class _Alloc>
-struct __is_cpp17_move_insertable<_Alloc, false> : is_move_constructible<typename _Alloc::value_type> {};
+// __has_select_on_container_copy_construction
+template <class _Alloc, class = void>
+struct __has_select_on_container_copy_construction : false_type { };
 
-template <class _Alloc,
-    bool = __has_construct<_Alloc, typename _Alloc::value_type*, const typename _Alloc::value_type&>::value && !__is_default_allocator<_Alloc>::value
-    >
-struct __is_cpp17_copy_insertable;
 template <class _Alloc>
-struct __is_cpp17_copy_insertable<_Alloc, true> : __is_cpp17_move_insertable<_Alloc> {};
-template <class _Alloc>
-struct __is_cpp17_copy_insertable<_Alloc, false> : integral_constant<bool,
-    is_copy_constructible<typename _Alloc::value_type>::value &&
-    __is_cpp17_move_insertable<_Alloc>::value>
-  {};
-
+struct __has_select_on_container_copy_construction<_Alloc, decltype(
+    (void)declval<_Alloc>().select_on_container_copy_construction()
+)> : true_type { };
 
+_LIBCPP_SUPPRESS_DEPRECATED_POP
 
 template <class _Alloc>
 struct _LIBCPP_TEMPLATE_VIS allocator_traits
 {
-    typedef _Alloc                              allocator_type;
-    typedef typename allocator_type::value_type value_type;
-
-    typedef typename __pointer_type<value_type, allocator_type>::type pointer;
-    typedef typename __const_pointer<value_type, pointer, allocator_type>::type const_pointer;
-    typedef typename __void_pointer<pointer, allocator_type>::type void_pointer;
-    typedef typename __const_void_pointer<pointer, allocator_type>::type const_void_pointer;
-
-    typedef typename __alloc_traits_difference_type<allocator_type, pointer>::type difference_type;
-    typedef typename __size_type<allocator_type, difference_type>::type size_type;
-
-    typedef typename __propagate_on_container_copy_assignment<allocator_type>::type
-                     propagate_on_container_copy_assignment;
-    typedef typename __propagate_on_container_move_assignment<allocator_type>::type
-                     propagate_on_container_move_assignment;
-    typedef typename __propagate_on_container_swap<allocator_type>::type
-                     propagate_on_container_swap;
-    typedef typename __is_always_equal<allocator_type>::type
-                     is_always_equal;
+    using allocator_type = _Alloc;
+    using value_type = typename allocator_type::value_type;
+    using pointer = typename __pointer<value_type, allocator_type>::type;
+    using const_pointer = typename __const_pointer<value_type, pointer, allocator_type>::type;
+    using void_pointer = typename __void_pointer<pointer, allocator_type>::type;
+    using const_void_pointer = typename __const_void_pointer<pointer, allocator_type>::type;
+    using difference_type = typename __alloc_traits_difference_type<allocator_type, pointer>::type;
+    using size_type = typename __size_type<allocator_type, difference_type>::type;
+    using propagate_on_container_copy_assignment = typename __propagate_on_container_copy_assignment<allocator_type>::type;
+    using propagate_on_container_move_assignment = typename __propagate_on_container_move_assignment<allocator_type>::type;
+    using propagate_on_container_swap = typename __propagate_on_container_swap<allocator_type>::type;
+    using is_always_equal = typename __is_always_equal<allocator_type>::type;
 
 #ifndef _LIBCPP_CXX03_LANG
-    template <class _Tp> using rebind_alloc =
-                  typename __allocator_traits_rebind<allocator_type, _Tp>::type;
-    template <class _Tp> using rebind_traits = allocator_traits<rebind_alloc<_Tp> >;
+    template <class _Tp>
+    using rebind_alloc = __allocator_traits_rebind_t<allocator_type, _Tp>;
+    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;};
+    template <class _Tp>
+    struct rebind_alloc {
+        using other = __allocator_traits_rebind_t<allocator_type, _Tp>;
+    };
+    template <class _Tp>
+    struct rebind_traits {
+        using other = allocator_traits<typename rebind_alloc<_Tp>::other>;
+    };
 #endif  // _LIBCPP_CXX03_LANG
 
     _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_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_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_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_CONSTEXPR_AFTER_CXX17
-        static void destroy(allocator_type& __a, _Tp* __p)
-            {__destroy(__has_destroy<allocator_type, _Tp*>(), __a, __p);}
-
-    _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_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);}
+    static pointer allocate(allocator_type& __a, size_type __n) {
+        return __a.allocate(__n);
+    }
 
-private:
-    _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
-        }
+    template <class _Ap = _Alloc, class =
+        _EnableIf<__has_allocate_hint<_Ap, size_type, const_void_pointer>::value> >
+    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    static pointer allocate(allocator_type& __a, size_type __n, const_void_pointer __hint) {
+    template <class _Ap = _Alloc, class = void, class =
+        _EnableIf<!__has_allocate_hint<_Ap, size_type, const_void_pointer>::value> >
+    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
+    static pointer allocate(allocator_type& __a, size_type __n, const_void_pointer) {
+        return __a.allocate(__n);
+    }
+
     _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);}
+    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_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_CONSTEXPR_AFTER_CXX17
-        static void __construct(false_type, allocator_type&, _Tp* __p, _Args&&... __args)
-            {
+    template <class _Tp, class... _Args, class =
+        _EnableIf<__has_construct<allocator_type, _Tp*, _Args...>::value> >
+    static void construct(allocator_type& __a, _Tp* __p, _Args&&... __args) {
+    template <class _Tp, class... _Args, class = void, class =
+        _EnableIf<!__has_construct<allocator_type, _Tp*, _Args...>::value> >
+    static void construct(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_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_CONSTEXPR_AFTER_CXX17
-        static void __destroy(false_type, allocator_type&, _Tp* __p)
-            {
+    template <class _Tp, class =
+        _EnableIf<__has_destroy<allocator_type, _Tp*>::value> >
+    static void destroy(allocator_type& __a, _Tp* __p) {
+    template <class _Tp, class = void, class =
+        _EnableIf<!__has_destroy<allocator_type, _Tp*>::value> >
+    static void destroy(allocator_type&, _Tp* __p) {
 #if _LIBCPP_STD_VER > 17
-                _VSTD::destroy_at(__p);
 #else
-                __p->~_Tp();
 #endif
-            }
 
+    template <class _Ap = _Alloc, class =
+        _EnableIf<__has_max_size<const _Ap>::value> >
     _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
-            }
-
+    static size_type max_size(const allocator_type& __a) _NOEXCEPT {
+    template <class _Ap = _Alloc, class = void, class =
+        _EnableIf<!__has_max_size<const _Ap>::value> >
     _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);}
+    static size_type max_size(const allocator_type&) _NOEXCEPT {
+        return numeric_limits<size_type>::max() / sizeof(value_type);
+    }
 
+    template <class _Ap = _Alloc, class =
+        _EnableIf<__has_select_on_container_copy_construction<const _Ap>::value> >
     _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();}
+    static allocator_type select_on_container_copy_construction(const allocator_type& __a) {
+        return __a.select_on_container_copy_construction();
+    }
+    template <class _Ap = _Alloc, class = void, class =
+        _EnableIf<!__has_select_on_container_copy_construction<const _Ap>::value> >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-    static allocator_type
-        __select_on_container_copy_construction(false_type, const allocator_type& __a)
-            {return __a;}
+    static allocator_type select_on_container_copy_construction(const allocator_type& __a) {
+        return __a;
+    }
 };
 
 template <class _Traits, class _Tp>
-struct __rebind_alloc_helper
-{
+struct __rebind_alloc_helper {
 #ifndef _LIBCPP_CXX03_LANG
-    typedef _LIBCPP_NODEBUG_TYPE typename _Traits::template rebind_alloc<_Tp>        type;
+    using type _LIBCPP_NODEBUG_TYPE = typename _Traits::template rebind_alloc<_Tp>;
 #else
-    typedef typename _Traits::template rebind_alloc<_Tp>::other type;
+    using type = typename _Traits::template rebind_alloc<_Tp>::other;
 #endif
 };
 
+// __is_default_allocator
+template <class _Tp>
+struct __is_default_allocator : false_type { };
+
+template <class _Tp>
+struct __is_default_allocator<_VSTD::allocator<_Tp> > : true_type { };
+
+// __is_cpp17_move_insertable
+template <class _Alloc, class = void>
+struct __is_cpp17_move_insertable
+    : is_move_constructible<typename _Alloc::value_type>
+{ };
+
+template <class _Alloc>
+struct __is_cpp17_move_insertable<_Alloc, _EnableIf<
+    !__is_default_allocator<_Alloc>::value &&
+    __has_construct<_Alloc, typename _Alloc::value_type*, typename _Alloc::value_type&&>::value
+> > : true_type { };
+
+// __is_cpp17_copy_insertable
+template <class _Alloc, class = void>
+struct __is_cpp17_copy_insertable
+    : integral_constant<bool,
+        is_copy_constructible<typename _Alloc::value_type>::value &&
+        __is_cpp17_move_insertable<_Alloc>::value
+    >
+{ };
+
+template <class _Alloc>
+struct __is_cpp17_copy_insertable<_Alloc, _EnableIf<
+    !__is_default_allocator<_Alloc>::value &&
+    __has_construct<_Alloc, typename _Alloc::value_type*, const typename _Alloc::value_type&>::value
+> >
+    : __is_cpp17_move_insertable<_Alloc>
+{ };
+
+#undef _LIBCPP_ALLOCATOR_TRAITS_HAS_XXX
+
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif  // _LIBCPP___MEMORY_ALLOCATOR_TRAITS_H

libcxx/include/memory

 #  define _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI
 #endif
 
 template <class _Tp, class _Dp = default_delete<_Tp> >
 class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS unique_ptr {
 public:
   typedef _Tp element_type;
   typedef _Dp deleter_type;
-  typedef _LIBCPP_NODEBUG_TYPE typename __pointer_type<_Tp, deleter_type>::type pointer;
+  typedef _LIBCPP_NODEBUG_TYPE typename __pointer<_Tp, deleter_type>::type pointer;
 
   static_assert(!is_rvalue_reference<deleter_type>::value,
                 "the specified deleter type cannot be an rvalue reference");
 
 private:
   __compressed_pair<pointer, deleter_type> __ptr_;
 
   struct __nat { int __for_bool_; };

 };
 
 
 template <class _Tp, class _Dp>
 class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS unique_ptr<_Tp[], _Dp> {
 public:
   typedef _Tp element_type;
   typedef _Dp deleter_type;
-  typedef typename __pointer_type<_Tp, deleter_type>::type pointer;
+  typedef typename __pointer<_Tp, deleter_type>::type pointer;
 
 private:
   __compressed_pair<pointer, deleter_type> __ptr_;
 
   template <class _From>
   struct _CheckArrayPointerConversion : is_same<_From, pointer> {};
 
   template <class _FromElem>