[libc++] LWG3870: Remove voidify

Instead of changing the cast sequence to implicit conversion in
_`voidify`_, I think it is better to totally remove `__voidify` and use
`static_cast` to `void*`, which has equivalent effects.

Test coverage for const iterators are removed.

Now most affected algorithms are underconstrained, for which I submitted
https://cplusplus.github.io/LWG/issue3888. I'm not sure whether we
should speculatively implement it at this moment, and thus haven't
added any `*.verify.cpp`.

In some control block types and `optional`, the stored objects are
changed to have cv-unqualified type.

Author: frederick-vs-ja

libcxx/docs/Status/Cxx23Issues.csv

@@ -296,7 +296,7 @@
 "`LWG3862 <https://wg21.link/LWG3862>`__","``basic_const_iterator``'s ``common_type`` specialization is underconstrained","2023-02 (Issaquah)","","",""
 "`LWG3865 <https://wg21.link/LWG3865>`__","Sorting a range of ``pairs``","2023-02 (Issaquah)","|Complete|","17.0",""
 "`LWG3869 <https://wg21.link/LWG3869>`__","Deprecate ``std::errc`` constants related to UNIX STREAMS","2023-02 (Issaquah)","|Complete|","19.0",""
-"`LWG3870 <https://wg21.link/LWG3870>`__","Remove ``voidify``","2023-02 (Issaquah)","","",""
+"`LWG3870 <https://wg21.link/LWG3870>`__","Remove ``voidify``","2023-02 (Issaquah)","|Complete|","20.0",""
 "`LWG3871 <https://wg21.link/LWG3871>`__","Adjust note about ``terminate``","2023-02 (Issaquah)","","",""
 "`LWG3872 <https://wg21.link/LWG3872>`__","``basic_const_iterator`` should have custom ``iter_move``","2023-02 (Issaquah)","","",""
 "`LWG3875 <https://wg21.link/LWG3875>`__","``std::ranges::repeat_view<T, IntegerClass>::iterator`` may be ill-formed","2023-02 (Issaquah)","|Complete|","17.0",""

libcxx/include/CMakeLists.txt

@@ -560,7 +560,6 @@ set(files
   __memory/unique_temporary_buffer.h
   __memory/uses_allocator.h
   __memory/uses_allocator_construction.h
-  __memory/voidify.h
   __memory_resource/memory_resource.h
   __memory_resource/monotonic_buffer_resource.h
   __memory_resource/polymorphic_allocator.h

libcxx/include/__memory/construct_at.h

@@ -14,7 +14,6 @@
 #include <__config>
 #include <__iterator/access.h>
 #include <__memory/addressof.h>
-#include <__memory/voidify.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_array.h>
 #include <__utility/declval.h>
@@ -38,7 +37,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Tp, class... _Args, class = decltype(::new(std::declval<void*>()) _Tp(std::declval<_Args>()...))>
 _LIBCPP_HIDE_FROM_ABI constexpr _Tp* construct_at(_Tp* __location, _Args&&... __args) {
   _LIBCPP_ASSERT_NON_NULL(__location != nullptr, "null pointer given to construct_at");
-  return ::new (std::__voidify(*__location)) _Tp(std::forward<_Args>(__args)...);
+  return ::new (static_cast<void*>(__location)) _Tp(std::forward<_Args>(__args)...);
 }
 
 #endif
@@ -49,7 +48,7 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Tp* __construct_at(_Tp* __l
   return std::construct_at(__location, std::forward<_Args>(__args)...);
 #else
   return _LIBCPP_ASSERT_NON_NULL(__location != nullptr, "null pointer given to construct_at"),
-         ::new (std::__voidify(*__location)) _Tp(std::forward<_Args>(__args)...);
+         ::new (static_cast<void*>(__location)) _Tp(std::forward<_Args>(__args)...);
 #endif
 }
 

libcxx/include/__memory/shared_ptr.h

@@ -248,33 +248,35 @@ struct __for_overwrite_tag {};
 
 template <class _Tp, class _Alloc>
 struct __shared_ptr_emplace : __shared_weak_count {
+  using __value_type = __remove_cv_t<_Tp>;
+
   template <class... _Args,
             class _Allocator                                                                         = _Alloc,
             __enable_if_t<is_same<typename _Allocator::value_type, __for_overwrite_tag>::value, int> = 0>
   _LIBCPP_HIDE_FROM_ABI explicit __shared_ptr_emplace(_Alloc __a, _Args&&...) : __storage_(std::move(__a)) {
     static_assert(
         sizeof...(_Args) == 0, "No argument should be provided to the control block when using _for_overwrite");
-    ::new ((void*)__get_elem()) _Tp;
+    ::new (static_cast<void*>(__get_elem())) __value_type;
   }
 
   template <class... _Args,
             class _Allocator                                                                          = _Alloc,
             __enable_if_t<!is_same<typename _Allocator::value_type, __for_overwrite_tag>::value, int> = 0>
   _LIBCPP_HIDE_FROM_ABI explicit __shared_ptr_emplace(_Alloc __a, _Args&&... __args) : __storage_(std::move(__a)) {
-    using _TpAlloc = typename __allocator_traits_rebind<_Alloc, __remove_cv_t<_Tp> >::type;
+    using _TpAlloc = typename __allocator_traits_rebind<_Alloc, __value_type>::type;
     _TpAlloc __tmp(*__get_alloc());
     allocator_traits<_TpAlloc>::construct(__tmp, __get_elem(), std::forward<_Args>(__args)...);
   }
 
   _LIBCPP_HIDE_FROM_ABI _Alloc* __get_alloc() _NOEXCEPT { return __storage_.__get_alloc(); }
 
-  _LIBCPP_HIDE_FROM_ABI _Tp* __get_elem() _NOEXCEPT { return __storage_.__get_elem(); }
+  _LIBCPP_HIDE_FROM_ABI __value_type* __get_elem() _NOEXCEPT { return __storage_.__get_elem(); }
 
 private:
   template <class _Allocator                                                                         = _Alloc,
             __enable_if_t<is_same<typename _Allocator::value_type, __for_overwrite_tag>::value, int> = 0>
   _LIBCPP_HIDE_FROM_ABI void __on_zero_shared_impl() _NOEXCEPT {
-    __get_elem()->~_Tp();
+    __get_elem()->~__value_type();
   }
 
   template <class _Allocator                                                                          = _Alloc,
@@ -300,7 +302,7 @@ struct __shared_ptr_emplace : __shared_weak_count {
   // through `std::allocate_shared` and `std::make_shared`.
   struct _Storage {
     struct _Data {
-      _LIBCPP_COMPRESSED_PAIR(_Alloc, __alloc_, _Tp, __elem_);
+      _LIBCPP_COMPRESSED_PAIR(_Alloc, __alloc_, __value_type, __elem_);
     };
 
     _ALIGNAS_TYPE(_Data) char __buffer_[sizeof(_Data)];
@@ -312,7 +314,7 @@ struct __shared_ptr_emplace : __shared_weak_count {
       return std::addressof(reinterpret_cast<_Data*>(__buffer_)->__alloc_);
     }
 
-    _LIBCPP_HIDE_FROM_ABI _LIBCPP_NO_CFI _Tp* __get_elem() _NOEXCEPT {
+    _LIBCPP_HIDE_FROM_ABI _LIBCPP_NO_CFI __value_type* __get_elem() _NOEXCEPT {
       return std::addressof(reinterpret_cast<_Data*>(__buffer_)->__elem_);
     }
   };

libcxx/include/__memory/uninitialized_algorithms.h

@@ -21,7 +21,6 @@
 #include <__memory/allocator_traits.h>
 #include <__memory/construct_at.h>
 #include <__memory/pointer_traits.h>
-#include <__memory/voidify.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/extent.h>
 #include <__type_traits/is_array.h>
@@ -64,7 +63,7 @@ inline _LIBCPP_HIDE_FROM_ABI pair<_InputIterator, _ForwardIterator> __uninitiali
   try {
 #endif
     for (; __ifirst != __ilast && !__stop_copying(__idx); ++__ifirst, (void)++__idx)
-      ::new (std::__voidify(*__idx)) _ValueType(*__ifirst);
+      ::new (static_cast<void*>(std::addressof(*__idx))) _ValueType(*__ifirst);
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
     std::__destroy(__ofirst, __idx);
@@ -94,7 +93,7 @@ __uninitialized_copy_n(_InputIterator __ifirst, _Size __n, _ForwardIterator __of
   try {
 #endif
     for (; __n > 0 && !__stop_copying(__idx); ++__ifirst, (void)++__idx, (void)--__n)
-      ::new (std::__voidify(*__idx)) _ValueType(*__ifirst);
+      ::new (static_cast<void*>(std::addressof(*__idx))) _ValueType(*__ifirst);
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
     std::__destroy(__ofirst, __idx);
@@ -124,7 +123,7 @@ __uninitialized_fill(_ForwardIterator __first, _Sentinel __last, const _Tp& __x)
   try {
 #endif
     for (; __idx != __last; ++__idx)
-      ::new (std::__voidify(*__idx)) _ValueType(__x);
+      ::new (static_cast<void*>(std::addressof(*__idx))) _ValueType(__x);
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
     std::__destroy(__first, __idx);
@@ -152,7 +151,7 @@ __uninitialized_fill_n(_ForwardIterator __first, _Size __n, const _Tp& __x) {
   try {
 #endif
     for (; __n > 0; ++__idx, (void)--__n)
-      ::new (std::__voidify(*__idx)) _ValueType(__x);
+      ::new (static_cast<void*>(std::addressof(*__idx))) _ValueType(__x);
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
     std::__destroy(__first, __idx);
@@ -182,7 +181,7 @@ __uninitialized_default_construct(_ForwardIterator __first, _Sentinel __last) {
   try {
 #  endif
     for (; __idx != __last; ++__idx)
-      ::new (std::__voidify(*__idx)) _ValueType;
+      ::new (static_cast<void*>(std::addressof(*__idx))) _ValueType;
 #  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
     std::__destroy(__first, __idx);
@@ -208,7 +207,7 @@ inline _LIBCPP_HIDE_FROM_ABI _ForwardIterator __uninitialized_default_construct_
   try {
 #  endif
     for (; __n > 0; ++__idx, (void)--__n)
-      ::new (std::__voidify(*__idx)) _ValueType;
+      ::new (static_cast<void*>(std::addressof(*__idx))) _ValueType;
 #  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
     std::__destroy(__first, __idx);
@@ -235,7 +234,7 @@ __uninitialized_value_construct(_ForwardIterator __first, _Sentinel __last) {
   try {
 #  endif
     for (; __idx != __last; ++__idx)
-      ::new (std::__voidify(*__idx)) _ValueType();
+      ::new (static_cast<void*>(std::addressof(*__idx))) _ValueType();
 #  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
     std::__destroy(__first, __idx);
@@ -261,7 +260,7 @@ inline _LIBCPP_HIDE_FROM_ABI _ForwardIterator __uninitialized_value_construct_n(
   try {
 #  endif
     for (; __n > 0; ++__idx, (void)--__n)
-      ::new (std::__voidify(*__idx)) _ValueType();
+      ::new (static_cast<void*>(std::addressof(*__idx))) _ValueType();
 #  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
     std::__destroy(__first, __idx);
@@ -297,7 +296,7 @@ inline _LIBCPP_HIDE_FROM_ABI pair<_InputIterator, _ForwardIterator> __uninitiali
   try {
 #  endif
     for (; __ifirst != __ilast && !__stop_moving(__idx); ++__idx, (void)++__ifirst) {
-      ::new (std::__voidify(*__idx)) _ValueType(__iter_move(__ifirst));
+      ::new (static_cast<void*>(std::addressof(*__idx))) _ValueType(__iter_move(__ifirst));
     }
 #  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
@@ -335,7 +334,7 @@ inline _LIBCPP_HIDE_FROM_ABI pair<_InputIterator, _ForwardIterator> __uninitiali
   try {
 #  endif
     for (; __n > 0 && !__stop_moving(__idx); ++__idx, (void)++__ifirst, --__n)
-      ::new (std::__voidify(*__idx)) _ValueType(__iter_move(__ifirst));
+      ::new (static_cast<void*>(std::addressof(*__idx))) _ValueType(__iter_move(__ifirst));
 #  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   } catch (...) {
     std::__destroy(__ofirst, __idx);

libcxx/include/__memory/voidify.h

@@ -1523,7 +1523,6 @@ module std [system] {
     }
     module uses_allocator                     { header "__memory/uses_allocator.h" }
     module uses_allocator_construction        { header "__memory/uses_allocator_construction.h" }
-    module voidify                            { header "__memory/voidify.h" }
 
     header "memory"
     export *

libcxx/include/module.modulemap

@@ -287,7 +287,7 @@ struct __optional_destruct_base<_Tp, false> {
   static_assert(is_object_v<value_type>, "instantiation of optional with a non-object type is undefined behavior");
   union {
     char __null_state_;
-    value_type __val_;
+    remove_cv_t<value_type> __val_;
   };
   bool __engaged_;
 
@@ -323,7 +323,7 @@ struct __optional_destruct_base<_Tp, true> {
   static_assert(is_object_v<value_type>, "instantiation of optional with a non-object type is undefined behavior");
   union {
     char __null_state_;
-    value_type __val_;
+    remove_cv_t<value_type> __val_;
   };
   bool __engaged_;
 
@@ -377,7 +377,7 @@ struct __optional_storage_base : __optional_destruct_base<_Tp> {
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __assign_from(_That&& __opt) {
     if (this->__engaged_ == __opt.has_value()) {
       if (this->__engaged_)
-        this->__val_ = std::forward<_That>(__opt).__get();
+        static_cast<_Tp&>(this->__val_) = std::forward<_That>(__opt).__get();
     } else {
       if (this->__engaged_)
         this->reset();

libcxx/include/optional

@@ -80,21 +80,6 @@ constexpr bool test()
         a.deallocate(p, 2);
     }
 
-    {
-        std::allocator<Counted> a;
-        Counted const* p = a.allocate(2);
-        int count = 0;
-        std::construct_at(p, count);
-        assert(count == 1);
-        std::construct_at(p+1, count);
-        assert(count == 2);
-        (p+1)->~Counted();
-        assert(count == 1);
-        p->~Counted();
-        assert(count == 0);
-        a.deallocate(const_cast<Counted*>(p), 2);
-    }
-
     return true;
 }
 

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

@@ -99,16 +99,6 @@ constexpr bool test() {
     alloc.deallocate(out, 2);
   }
 
-  // Works with const pointers.
-  {
-    int x = 1;
-    const int* ptr = &x;
-
-    const int* result = std::ranges::construct_at(ptr, 42);
-    assert(result == ptr);
-    assert(x == 42);
-  }
-
   return true;
 }