From a8e168e0c18bb7ac57361500cd98601160a107cc Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser@berlin.de>
Date: Tue, 25 Mar 2025 15:58:24 +0100
Subject: [PATCH] [libc++][NFC] Replace structs with variable templates in
 <__memory/allocator_traits.h> (#129237)

Variable templates are a bit easier on the compiler and improve the
readability of the code.
---
 libcxx/include/__flat_map/flat_map.h          |   1 +
 libcxx/include/__memory/allocator_traits.h    | 119 ++++++++----------
 .../__memory/uninitialized_algorithms.h       |  24 ++--
 libcxx/include/__vector/container_traits.h    |   2 +-
 libcxx/include/deque                          |   2 +-
 5 files changed, 64 insertions(+), 84 deletions(-)

diff --git a/libcxx/include/__flat_map/flat_map.h b/libcxx/include/__flat_map/flat_map.h
index a0594ed9dc41..f5abfd098528 100644
--- a/libcxx/include/__flat_map/flat_map.h
+++ b/libcxx/include/__flat_map/flat_map.h
@@ -33,6 +33,7 @@
 #include <__functional/invoke.h>
 #include <__functional/is_transparent.h>
 #include <__functional/operations.h>
+#include <__fwd/memory.h>
 #include <__fwd/vector.h>
 #include <__iterator/concepts.h>
 #include <__iterator/distance.h>
diff --git a/libcxx/include/__memory/allocator_traits.h b/libcxx/include/__memory/allocator_traits.h
index 3180f5e3d542..7e964b764701 100644
--- a/libcxx/include/__memory/allocator_traits.h
+++ b/libcxx/include/__memory/allocator_traits.h
@@ -151,13 +151,13 @@ using __is_always_equal _LIBCPP_NODEBUG =
 
 // __allocator_traits_rebind
 template <class _Tp, class _Up, class = void>
-struct __has_rebind_other : false_type {};
+inline const bool __has_rebind_other_v = false;
 template <class _Tp, class _Up>
-struct __has_rebind_other<_Tp, _Up, __void_t<typename _Tp::template rebind<_Up>::other> > : true_type {};
+inline const bool __has_rebind_other_v<_Tp, _Up, __void_t<typename _Tp::template rebind<_Up>::other> > = true;
 
-template <class _Tp, class _Up, bool = __has_rebind_other<_Tp, _Up>::value>
+template <class _Tp, class _Up, bool = __has_rebind_other_v<_Tp, _Up> >
 struct __allocator_traits_rebind {
-  static_assert(__has_rebind_other<_Tp, _Up>::value, "This allocator has to implement rebind");
+  static_assert(__has_rebind_other_v<_Tp, _Up>, "This allocator has to implement rebind");
   using type _LIBCPP_NODEBUG = typename _Tp::template rebind<_Up>::other;
 };
 template <template <class, class...> class _Alloc, class _Tp, class... _Args, class _Up>
@@ -175,53 +175,52 @@ using __allocator_traits_rebind_t _LIBCPP_NODEBUG = typename __allocator_traits_
 
 _LIBCPP_SUPPRESS_DEPRECATED_PUSH
 
-// __has_allocate_hint
+// __has_allocate_hint_v
 template <class _Alloc, class _SizeType, class _ConstVoidPtr, class = void>
-struct __has_allocate_hint : false_type {};
+inline const bool __has_allocate_hint_v = false;
 
 template <class _Alloc, class _SizeType, class _ConstVoidPtr>
-struct __has_allocate_hint<
+inline const bool __has_allocate_hint_v<
     _Alloc,
     _SizeType,
     _ConstVoidPtr,
-    decltype((void)std::declval<_Alloc>().allocate(std::declval<_SizeType>(), std::declval<_ConstVoidPtr>()))>
-    : true_type {};
+    decltype((void)std::declval<_Alloc>().allocate(std::declval<_SizeType>(), std::declval<_ConstVoidPtr>()))> = true;
 
-// __has_construct
+// __has_construct_v
 template <class, class _Alloc, class... _Args>
-struct __has_construct_impl : false_type {};
+inline const bool __has_construct_impl = false;
 
 template <class _Alloc, class... _Args>
-struct __has_construct_impl<decltype((void)std::declval<_Alloc>().construct(std::declval<_Args>()...)),
-                            _Alloc,
-                            _Args...> : true_type {};
+inline const bool
+    __has_construct_impl<decltype((void)std::declval<_Alloc>().construct(std::declval<_Args>()...)), _Alloc, _Args...> =
+        true;
 
 template <class _Alloc, class... _Args>
-struct __has_construct : __has_construct_impl<void, _Alloc, _Args...> {};
+inline const bool __has_construct_v = __has_construct_impl<void, _Alloc, _Args...>;
 
-// __has_destroy
+// __has_destroy_v
 template <class _Alloc, class _Pointer, class = void>
-struct __has_destroy : false_type {};
+inline const bool __has_destroy_v = false;
 
 template <class _Alloc, class _Pointer>
-struct __has_destroy<_Alloc, _Pointer, decltype((void)std::declval<_Alloc>().destroy(std::declval<_Pointer>()))>
-    : true_type {};
+inline const bool
+    __has_destroy_v<_Alloc, _Pointer, decltype((void)std::declval<_Alloc>().destroy(std::declval<_Pointer>()))> = true;
 
-// __has_max_size
+// __has_max_size_v
 template <class _Alloc, class = void>
-struct __has_max_size : false_type {};
+inline const bool __has_max_size_v = false;
 
 template <class _Alloc>
-struct __has_max_size<_Alloc, decltype((void)std::declval<_Alloc&>().max_size())> : true_type {};
+inline const bool __has_max_size_v<_Alloc, decltype((void)std::declval<_Alloc&>().max_size())> = true;
 
-// __has_select_on_container_copy_construction
+// __has_select_on_container_copy_construction_v
 template <class _Alloc, class = void>
-struct __has_select_on_container_copy_construction : false_type {};
+inline const bool __has_select_on_container_copy_construction_v = false;
 
 template <class _Alloc>
-struct __has_select_on_container_copy_construction<
+inline const bool __has_select_on_container_copy_construction_v<
     _Alloc,
-    decltype((void)std::declval<_Alloc>().select_on_container_copy_construction())> : true_type {};
+    decltype((void)std::declval<_Alloc>().select_on_container_copy_construction())> = true;
 
 _LIBCPP_SUPPRESS_DEPRECATED_POP
 
@@ -272,16 +271,14 @@ struct _LIBCPP_TEMPLATE_VIS allocator_traits {
     return __a.allocate(__n);
   }
 
-  template <class _Ap = _Alloc, __enable_if_t<__has_allocate_hint<_Ap, size_type, const_void_pointer>::value, int> = 0>
+  template <class _Ap = _Alloc, __enable_if_t<__has_allocate_hint_v<_Ap, size_type, const_void_pointer>, int> = 0>
   [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 static pointer
   allocate(allocator_type& __a, size_type __n, const_void_pointer __hint) {
     _LIBCPP_SUPPRESS_DEPRECATED_PUSH
     return __a.allocate(__n, __hint);
     _LIBCPP_SUPPRESS_DEPRECATED_POP
   }
-  template <class _Ap                                                                           = _Alloc,
-            class                                                                               = void,
-            __enable_if_t<!__has_allocate_hint<_Ap, size_type, const_void_pointer>::value, int> = 0>
+  template <class _Ap = _Alloc, __enable_if_t<!__has_allocate_hint_v<_Ap, size_type, const_void_pointer>, int> = 0>
   [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 static pointer
   allocate(allocator_type& __a, size_type __n, const_void_pointer) {
     return __a.allocate(__n);
@@ -304,52 +301,47 @@ struct _LIBCPP_TEMPLATE_VIS allocator_traits {
     __a.deallocate(__p, __n);
   }
 
-  template <class _Tp, class... _Args, __enable_if_t<__has_construct<allocator_type, _Tp*, _Args...>::value, int> = 0>
+  template <class _Tp, class... _Args, __enable_if_t<__has_construct_v<allocator_type, _Tp*, _Args...>, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 static void
   construct(allocator_type& __a, _Tp* __p, _Args&&... __args) {
     _LIBCPP_SUPPRESS_DEPRECATED_PUSH
     __a.construct(__p, std::forward<_Args>(__args)...);
     _LIBCPP_SUPPRESS_DEPRECATED_POP
   }
-  template <class _Tp,
-            class... _Args,
-            class                                                                       = void,
-            __enable_if_t<!__has_construct<allocator_type, _Tp*, _Args...>::value, int> = 0>
+  template <class _Tp, class... _Args, __enable_if_t<!__has_construct_v<allocator_type, _Tp*, _Args...>, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 static void
   construct(allocator_type&, _Tp* __p, _Args&&... __args) {
     std::__construct_at(__p, std::forward<_Args>(__args)...);
   }
 
-  template <class _Tp, __enable_if_t<__has_destroy<allocator_type, _Tp*>::value, int> = 0>
+  template <class _Tp, __enable_if_t<__has_destroy_v<allocator_type, _Tp*>, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 static void destroy(allocator_type& __a, _Tp* __p) {
     _LIBCPP_SUPPRESS_DEPRECATED_PUSH
     __a.destroy(__p);
     _LIBCPP_SUPPRESS_DEPRECATED_POP
   }
-  template <class _Tp, class = void, __enable_if_t<!__has_destroy<allocator_type, _Tp*>::value, int> = 0>
+  template <class _Tp, __enable_if_t<!__has_destroy_v<allocator_type, _Tp*>, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 static void destroy(allocator_type&, _Tp* __p) {
     std::__destroy_at(__p);
   }
 
-  template <class _Ap = _Alloc, __enable_if_t<__has_max_size<const _Ap>::value, int> = 0>
+  template <class _Ap = _Alloc, __enable_if_t<__has_max_size_v<const _Ap>, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 static size_type max_size(const allocator_type& __a) _NOEXCEPT {
     _LIBCPP_SUPPRESS_DEPRECATED_PUSH
     return __a.max_size();
     _LIBCPP_SUPPRESS_DEPRECATED_POP
   }
-  template <class _Ap = _Alloc, class = void, __enable_if_t<!__has_max_size<const _Ap>::value, int> = 0>
+  template <class _Ap = _Alloc, __enable_if_t<!__has_max_size_v<const _Ap>, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 static size_type max_size(const allocator_type&) _NOEXCEPT {
     return numeric_limits<size_type>::max() / sizeof(value_type);
   }
 
-  template <class _Ap = _Alloc, __enable_if_t<__has_select_on_container_copy_construction<const _Ap>::value, int> = 0>
+  template <class _Ap = _Alloc, __enable_if_t<__has_select_on_container_copy_construction_v<const _Ap>, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 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,
-            __enable_if_t<!__has_select_on_container_copy_construction<const _Ap>::value, int> = 0>
+  template <class _Ap = _Alloc, __enable_if_t<!__has_select_on_container_copy_construction_v<const _Ap>, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 static allocator_type
   select_on_container_copy_construction(const allocator_type& __a) {
     return __a;
@@ -372,40 +364,27 @@ struct __check_valid_allocator : true_type {
                 "original allocator");
 };
 
-// __is_default_allocator
+// __is_default_allocator_v
 template <class _Tp>
-struct __is_default_allocator : false_type {};
-
-template <class>
-class allocator;
+inline const bool __is_std_allocator_v = false;
 
 template <class _Tp>
-struct __is_default_allocator<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> {};
+inline const bool __is_std_allocator_v<allocator<_Tp> > = true;
 
+// __is_cpp17_move_insertable_v
 template <class _Alloc>
-struct __is_cpp17_move_insertable<
-    _Alloc,
-    __enable_if_t< !__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 > {};
+inline const bool __is_cpp17_move_insertable_v =
+    is_move_constructible<typename _Alloc::value_type>::value ||
+    (!__is_std_allocator_v<_Alloc> &&
+     __has_construct_v<_Alloc, typename _Alloc::value_type*, typename _Alloc::value_type&&>);
 
+// __is_cpp17_copy_insertable_v
 template <class _Alloc>
-struct __is_cpp17_copy_insertable<
-    _Alloc,
-    __enable_if_t< !__is_default_allocator<_Alloc>::value &&
-                   __has_construct<_Alloc, typename _Alloc::value_type*, const typename _Alloc::value_type&>::value > >
-    : __is_cpp17_move_insertable<_Alloc> {};
+inline const bool __is_cpp17_copy_insertable_v =
+    __is_cpp17_move_insertable_v<_Alloc> &&
+    (is_copy_constructible<typename _Alloc::value_type>::value ||
+     (!__is_std_allocator_v<_Alloc> &&
+      __has_construct_v<_Alloc, typename _Alloc::value_type*, const typename _Alloc::value_type&>));
 
 #undef _LIBCPP_ALLOCATOR_TRAITS_HAS_XXX
 
diff --git a/libcxx/include/__memory/uninitialized_algorithms.h b/libcxx/include/__memory/uninitialized_algorithms.h
index d7f210d12a21..e80236640052 100644
--- a/libcxx/include/__memory/uninitialized_algorithms.h
+++ b/libcxx/include/__memory/uninitialized_algorithms.h
@@ -16,6 +16,7 @@
 #include <__algorithm/unwrap_range.h>
 #include <__config>
 #include <__cstddef/size_t.h>
+#include <__fwd/memory.h>
 #include <__iterator/iterator_traits.h>
 #include <__iterator/reverse_iterator.h>
 #include <__memory/addressof.h>
@@ -32,7 +33,6 @@
 #include <__type_traits/is_trivially_constructible.h>
 #include <__type_traits/is_trivially_relocatable.h>
 #include <__type_traits/is_unbounded_array.h>
-#include <__type_traits/negation.h>
 #include <__type_traits/remove_const.h>
 #include <__type_traits/remove_extent.h>
 #include <__utility/exception_guard.h>
@@ -549,17 +549,17 @@ __uninitialized_allocator_copy_impl(_Alloc& __alloc, _Iter1 __first1, _Sent1 __l
 }
 
 template <class _Alloc, class _Type>
-struct __allocator_has_trivial_copy_construct : _Not<__has_construct<_Alloc, _Type*, const _Type&> > {};
+inline const bool __allocator_has_trivial_copy_construct_v = !__has_construct_v<_Alloc, _Type*, const _Type&>;
 
 template <class _Type>
-struct __allocator_has_trivial_copy_construct<allocator<_Type>, _Type> : true_type {};
+inline const bool __allocator_has_trivial_copy_construct_v<allocator<_Type>, _Type> = true;
 
 template <class _Alloc,
           class _In,
           class _Out,
           __enable_if_t<is_trivially_copy_constructible<_In>::value && is_trivially_copy_assignable<_In>::value &&
                             is_same<__remove_const_t<_In>, __remove_const_t<_Out> >::value &&
-                            __allocator_has_trivial_copy_construct<_Alloc, _In>::value,
+                            __allocator_has_trivial_copy_construct_v<_Alloc, _In>,
                         int> = 0>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Out*
 __uninitialized_allocator_copy_impl(_Alloc&, _In* __first1, _In* __last1, _Out* __first2) {
@@ -585,16 +585,16 @@ __uninitialized_allocator_copy(_Alloc& __alloc, _Iter1 __first1, _Sent1 __last1,
 }
 
 template <class _Alloc, class _Type>
-struct __allocator_has_trivial_move_construct : _Not<__has_construct<_Alloc, _Type*, _Type&&> > {};
+inline const bool __allocator_has_trivial_move_construct_v = !__has_construct_v<_Alloc, _Type*, _Type&&>;
 
 template <class _Type>
-struct __allocator_has_trivial_move_construct<allocator<_Type>, _Type> : true_type {};
+inline const bool __allocator_has_trivial_move_construct_v<allocator<_Type>, _Type> = true;
 
 template <class _Alloc, class _Tp>
-struct __allocator_has_trivial_destroy : _Not<__has_destroy<_Alloc, _Tp*> > {};
+inline const bool __allocator_has_trivial_destroy_v = !__has_destroy_v<_Alloc, _Tp*>;
 
 template <class _Tp, class _Up>
-struct __allocator_has_trivial_destroy<allocator<_Tp>, _Up> : true_type {};
+inline const bool __allocator_has_trivial_destroy_v<allocator<_Tp>, _Up> = true;
 
 // __uninitialized_allocator_relocate relocates the objects in [__first, __last) into __result.
 // Relocation means that the objects in [__first, __last) are placed into __result as-if by move-construct and destroy,
@@ -613,11 +613,11 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void __uninitialized_allocat
     _Alloc& __alloc, _ContiguousIterator __first, _ContiguousIterator __last, _ContiguousIterator __result) {
   static_assert(__libcpp_is_contiguous_iterator<_ContiguousIterator>::value, "");
   using _ValueType = typename iterator_traits<_ContiguousIterator>::value_type;
-  static_assert(__is_cpp17_move_insertable<_Alloc>::value,
-                "The specified type does not meet the requirements of Cpp17MoveInsertable");
+  static_assert(
+      __is_cpp17_move_insertable_v<_Alloc>, "The specified type does not meet the requirements of Cpp17MoveInsertable");
   if (__libcpp_is_constant_evaluated() || !__libcpp_is_trivially_relocatable<_ValueType>::value ||
-      !__allocator_has_trivial_move_construct<_Alloc, _ValueType>::value ||
-      !__allocator_has_trivial_destroy<_Alloc, _ValueType>::value) {
+      !__allocator_has_trivial_move_construct_v<_Alloc, _ValueType> ||
+      !__allocator_has_trivial_destroy_v<_Alloc, _ValueType>) {
     auto __destruct_first = __result;
     auto __guard          = std::__make_exception_guard(
         _AllocatorDestroyRangeReverse<_Alloc, _ContiguousIterator>(__alloc, __destruct_first, __result));
diff --git a/libcxx/include/__vector/container_traits.h b/libcxx/include/__vector/container_traits.h
index 7b1497a404f2..337b9c5c8368 100644
--- a/libcxx/include/__vector/container_traits.h
+++ b/libcxx/include/__vector/container_traits.h
@@ -31,7 +31,7 @@ struct __container_traits<vector<_Tp, _Allocator> > {
   //  there are no effects. Otherwise, if an exception is thrown by the move constructor of a non-Cpp17CopyInsertable T,
   //  the effects are unspecified.
   static _LIBCPP_CONSTEXPR const bool __emplacement_has_strong_exception_safety_guarantee =
-      _Or<is_nothrow_move_constructible<_Tp>, __is_cpp17_copy_insertable<_Allocator> >::value;
+      is_nothrow_move_constructible<_Tp>::value || __is_cpp17_copy_insertable_v<_Allocator>;
 };
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/deque b/libcxx/include/deque
index 04788c277e42..bab76d442a21 100644
--- a/libcxx/include/deque
+++ b/libcxx/include/deque
@@ -2625,7 +2625,7 @@ struct __container_traits<deque<_Tp, _Allocator> > {
   //  either end, there are no effects. Otherwise, if an exception is thrown by the move constructor of a
   //  non-Cpp17CopyInsertable T, the effects are unspecified.
   static _LIBCPP_CONSTEXPR const bool __emplacement_has_strong_exception_safety_guarantee =
-      _Or<is_nothrow_move_constructible<_Tp>, __is_cpp17_copy_insertable<_Allocator> >::value;
+      is_nothrow_move_constructible<_Tp>::value || __is_cpp17_copy_insertable_v<_Allocator>;
 };
 
 _LIBCPP_END_NAMESPACE_STD