[libc++] Add the __is_replaceable type trait

libcxx/include/CMakeLists.txt

@@ -840,6 +840,7 @@ set(files
   __type_traits/is_reference.h
   __type_traits/is_reference_wrapper.h
   __type_traits/is_referenceable.h
+  __type_traits/is_replaceable.h
   __type_traits/is_same.h
   __type_traits/is_scalar.h
   __type_traits/is_signed.h

libcxx/include/__exception/exception_ptr.h

@@ -65,8 +65,11 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
   friend _LIBCPP_HIDE_FROM_ABI exception_ptr make_exception_ptr(_Ep) _NOEXCEPT;
 
 public:
-  // exception_ptr is basically a COW string.
+  // exception_ptr is basically a COW string so it is trivially relocatable.
+  // However, it's not replaceable because destroying and move-constructing could cause
+  // the underlying refcount to hit 0 if we're self-assigning.
   using __trivially_relocatable _LIBCPP_NODEBUG = exception_ptr;
+  using __replaceable _LIBCPP_NODEBUG           = void;
 
   _LIBCPP_HIDE_FROM_ABI exception_ptr() _NOEXCEPT : __ptr_() {}
   _LIBCPP_HIDE_FROM_ABI exception_ptr(nullptr_t) _NOEXCEPT : __ptr_() {}

libcxx/include/__expected/expected.h

@@ -29,6 +29,7 @@
 #include <__type_traits/is_nothrow_assignable.h>
 #include <__type_traits/is_nothrow_constructible.h>
 #include <__type_traits/is_reference.h>
+#include <__type_traits/is_replaceable.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_swappable.h>
 #include <__type_traits/is_trivially_constructible.h>
@@ -470,6 +471,8 @@ class expected : private __expected_base<_Tp, _Err> {
       __conditional_t<__libcpp_is_trivially_relocatable<_Tp>::value && __libcpp_is_trivially_relocatable<_Err>::value,
                       expected,
                       void>;
+  using __replaceable _LIBCPP_NODEBUG =
+      __conditional_t<__is_replaceable_v<_Tp> && __is_replaceable_v<_Err>, expected, void>;
 
   template <class _Up>
   using rebind = expected<_Up, error_type>;

libcxx/include/__locale

@@ -50,8 +50,11 @@ _LIBCPP_HIDE_FROM_ABI const _Facet& use_facet(const locale&);
 
 class _LIBCPP_EXPORTED_FROM_ABI locale {
 public:
-  // locale is essentially a shared_ptr that doesn't support weak_ptrs and never got a move constructor.
+  // locale is essentially a shared_ptr that doesn't support weak_ptrs and never got a move constructor,
+  // so it is trivially relocatable. However, it is not replaceable because self-assignment must prevent
+  // the refcount from hitting 0.
   using __trivially_relocatable _LIBCPP_NODEBUG = locale;
+  using __replaceable _LIBCPP_NODEBUG           = void;
 
   // types:
   class _LIBCPP_EXPORTED_FROM_ABI facet;

libcxx/include/__memory/shared_ptr.h

@@ -317,7 +317,11 @@ class _LIBCPP_SHARED_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS shared_ptr {
 
   // A shared_ptr contains only two raw pointers which point to the heap and move constructing already doesn't require
   // any bookkeeping, so it's always trivially relocatable.
+  //
+  // However, it's not replaceable because of self-assignment, which must prevent the refcount from
+  // hitting 0.
   using __trivially_relocatable _LIBCPP_NODEBUG = shared_ptr;
+  using __replaceable _LIBCPP_NODEBUG           = void;
 
 private:
   element_type* __ptr_;
@@ -1212,7 +1216,10 @@ class _LIBCPP_SHARED_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS weak_ptr {
 
   // A weak_ptr contains only two raw pointers which point to the heap and move constructing already doesn't require
   // any bookkeeping, so it's always trivially relocatable.
+  //
+  // However, it's not replaceable because we must preserve a non-zero refcount through self-assignment.
   using __trivially_relocatable _LIBCPP_NODEBUG = weak_ptr;
+  using __replaceable _LIBCPP_NODEBUG           = void;
 
 private:
   element_type* __ptr_;

libcxx/include/__memory/unique_ptr.h

@@ -39,6 +39,7 @@
 #include <__type_traits/is_function.h>
 #include <__type_traits/is_pointer.h>
 #include <__type_traits/is_reference.h>
+#include <__type_traits/is_replaceable.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_swappable.h>
 #include <__type_traits/is_trivially_relocatable.h>
@@ -144,6 +145,8 @@ class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS unique_ptr {
       __libcpp_is_trivially_relocatable<pointer>::value && __libcpp_is_trivially_relocatable<deleter_type>::value,
       unique_ptr,
       void>;
+  using __replaceable _LIBCPP_NODEBUG =
+      __conditional_t<__is_replaceable_v<pointer> && __is_replaceable_v<deleter_type>, unique_ptr, void>;
 
 private:
   _LIBCPP_COMPRESSED_PAIR(pointer, __ptr_, deleter_type, __deleter_);
@@ -410,6 +413,8 @@ class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS unique_ptr<_Tp[], _Dp>
       __libcpp_is_trivially_relocatable<pointer>::value && __libcpp_is_trivially_relocatable<deleter_type>::value,
       unique_ptr,
       void>;
+  using __replaceable _LIBCPP_NODEBUG =
+      __conditional_t<__is_replaceable_v<pointer> && __is_replaceable_v<deleter_type>, unique_ptr, void>;
 
 private:
   template <class _Up, class _OtherDeleter>

libcxx/include/__split_buffer

@@ -28,6 +28,7 @@
 #include <__type_traits/integral_constant.h>
 #include <__type_traits/is_nothrow_assignable.h>
 #include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/is_replaceable.h>
 #include <__type_traits/is_swappable.h>
 #include <__type_traits/is_trivially_destructible.h>
 #include <__type_traits/is_trivially_relocatable.h>
@@ -72,6 +73,10 @@ public:
       __libcpp_is_trivially_relocatable<pointer>::value && __libcpp_is_trivially_relocatable<allocator_type>::value,
       __split_buffer,
       void>;
+  using __replaceable _LIBCPP_NODEBUG =
+      __conditional_t<__is_replaceable_v<pointer> && __container_allocator_is_replaceable<__alloc_traits>::value,
+                      __split_buffer,
+                      void>;
 
   pointer __first_;
   pointer __begin_;

libcxx/include/__type_traits/is_replaceable.h

@@ -0,0 +1,62 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H
+#define _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H
+
+#include <__config>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/is_trivially_copyable.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// A type is replaceable if `x = std::move(y)` is equivalent to:
+//
+//  std::destroy_at(&x)
+//  std::construct_at(&x, std::move(y))
+//
+// This allows turning a move-assignment into a sequence of destroy + move-construct, which
+// is often more efficient. This is especially relevant when the move-construct is in fact
+// part of a trivial relocation from somewhere else, in which case there is a huge win.
+//
+// Note that this requires language support in order to be really effective, but we
+// currently emulate the base template with something very conservative.
+template <class _Tp, class = void>
+struct __is_replaceable : is_trivially_copyable<_Tp> {};
+
+template <class _Tp>
+struct __is_replaceable<_Tp, __enable_if_t<is_same<_Tp, typename _Tp::__replaceable>::value> > : true_type {};
+
+template <class _Tp>
+inline const bool __is_replaceable_v = __is_replaceable<_Tp>::value;
+
+// Determines whether an allocator member of a container is replaceable.
+//
+// We take into account whether the allocator is propagated on assignments. If the allocator
+// always compares equal, then it doesn't matter whether we propagate it or not on assignments,
+// the result will be the same and we can just as much move-construct it instead.
+//
+// If the allocator does not always compare equal, we check whether it propagates on assignment
+// and it is replaceable.
+template <class _AllocatorTraits>
+struct __container_allocator_is_replaceable
+    : integral_constant<bool,
+                        _AllocatorTraits::is_always_equal::value ||
+                            (_AllocatorTraits::propagate_on_container_move_assignment::value &&
+                             _AllocatorTraits::propagate_on_container_copy_assignment::value &&
+                             __is_replaceable_v<typename _AllocatorTraits::allocator_type>)> {};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___TYPE_TRAITS_IS_REPLACEABLE_H

libcxx/include/__utility/pair.h

@@ -31,6 +31,8 @@
 #include <__type_traits/is_implicitly_default_constructible.h>
 #include <__type_traits/is_nothrow_assignable.h>
 #include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/is_replaceable.h>
+#include <__type_traits/is_same.h>
 #include <__type_traits/is_swappable.h>
 #include <__type_traits/is_trivially_relocatable.h>
 #include <__type_traits/nat.h>
@@ -72,6 +74,7 @@ struct _LIBCPP_TEMPLATE_VIS pair
       __conditional_t<__libcpp_is_trivially_relocatable<_T1>::value && __libcpp_is_trivially_relocatable<_T2>::value,
                       pair,
                       void>;
+  using __replaceable _LIBCPP_NODEBUG = __conditional_t<__is_replaceable_v<_T1> && __is_replaceable_v<_T2>, pair, void>;
 
   _LIBCPP_HIDE_FROM_ABI pair(pair const&) = default;
   _LIBCPP_HIDE_FROM_ABI pair(pair&&)      = default;

libcxx/include/__vector/vector.h

@@ -55,6 +55,7 @@
 #include <__type_traits/is_nothrow_assignable.h>
 #include <__type_traits/is_nothrow_constructible.h>
 #include <__type_traits/is_pointer.h>
+#include <__type_traits/is_replaceable.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_trivially_relocatable.h>
 #include <__type_traits/type_identity.h>
@@ -120,6 +121,10 @@ class _LIBCPP_TEMPLATE_VIS vector {
       __libcpp_is_trivially_relocatable<pointer>::value && __libcpp_is_trivially_relocatable<allocator_type>::value,
       vector,
       void>;
+  using __replaceable _LIBCPP_NODEBUG =
+      __conditional_t<__is_replaceable_v<pointer> && __container_allocator_is_replaceable<__alloc_traits>::value,
+                      vector,
+                      void>;
 
   static_assert(__check_valid_allocator<allocator_type>::value, "");
   static_assert(is_same<typename allocator_type::value_type, value_type>::value,

libcxx/include/array

@@ -134,6 +134,7 @@ template <size_t I, class T, size_t N> const T&& get(const array<T, N>&&) noexce
 #  include <__type_traits/is_const.h>
 #  include <__type_traits/is_constructible.h>
 #  include <__type_traits/is_nothrow_constructible.h>
+#  include <__type_traits/is_replaceable.h>
 #  include <__type_traits/is_same.h>
 #  include <__type_traits/is_swappable.h>
 #  include <__type_traits/is_trivially_relocatable.h>
@@ -175,6 +176,7 @@ template <class _Tp, size_t _Size>
 struct _LIBCPP_TEMPLATE_VIS array {
   using __trivially_relocatable _LIBCPP_NODEBUG =
       __conditional_t<__libcpp_is_trivially_relocatable<_Tp>::value, array, void>;
+  using __replaceable _LIBCPP_NODEBUG = __conditional_t<__is_replaceable_v<_Tp>, array, void>;
 
   // types:
   using __self _LIBCPP_NODEBUG = array;

libcxx/include/deque

@@ -230,6 +230,7 @@ template <class T, class Allocator, class Predicate>
 #  include <__type_traits/is_convertible.h>
 #  include <__type_traits/is_nothrow_assignable.h>
 #  include <__type_traits/is_nothrow_constructible.h>
+#  include <__type_traits/is_replaceable.h>
 #  include <__type_traits/is_same.h>
 #  include <__type_traits/is_swappable.h>
 #  include <__type_traits/is_trivially_relocatable.h>
@@ -530,6 +531,10 @@ public:
       __libcpp_is_trivially_relocatable<__map>::value && __libcpp_is_trivially_relocatable<allocator_type>::value,
       deque,
       void>;
+  using __replaceable _LIBCPP_NODEBUG =
+      __conditional_t<__is_replaceable_v<__map> && __container_allocator_is_replaceable<__alloc_traits>::value,
+                      deque,
+                      void>;
 
   static_assert(is_nothrow_default_constructible<allocator_type>::value ==
                     is_nothrow_default_constructible<__pointer_allocator>::value,

libcxx/include/module.modulemap

@@ -275,6 +275,10 @@ module std_core [system] {
       header "__type_traits/is_referenceable.h"
       export std_core.type_traits.integral_constant
     }
+    module is_replaceable {
+      header "__type_traits/is_replaceable.h"
+      export std_core.type_traits.integral_constant
+    }
     module is_same {
       header "__type_traits/is_same.h"
       export std_core.type_traits.integral_constant

libcxx/include/optional

@@ -210,6 +210,7 @@ namespace std {
 #  include <__type_traits/is_nothrow_constructible.h>
 #  include <__type_traits/is_object.h>
 #  include <__type_traits/is_reference.h>
+#  include <__type_traits/is_replaceable.h>
 #  include <__type_traits/is_same.h>
 #  include <__type_traits/is_scalar.h>
 #  include <__type_traits/is_swappable.h>
@@ -590,6 +591,7 @@ public:
 
   using __trivially_relocatable _LIBCPP_NODEBUG =
       conditional_t<__libcpp_is_trivially_relocatable<_Tp>::value, optional, void>;
+  using __replaceable _LIBCPP_NODEBUG = conditional_t<__is_replaceable_v<_Tp>, optional, void>;
 
 private:
   // Disable the reference extension using this static assert.

libcxx/include/string

@@ -630,6 +630,7 @@ basic_string<char32_t> operator""s( const char32_t *str, size_t len );
 #  include <__type_traits/is_convertible.h>
 #  include <__type_traits/is_nothrow_assignable.h>
 #  include <__type_traits/is_nothrow_constructible.h>
+#  include <__type_traits/is_replaceable.h>
 #  include <__type_traits/is_same.h>
 #  include <__type_traits/is_standard_layout.h>
 #  include <__type_traits/is_trivial.h>
@@ -754,13 +755,20 @@ public:
   // external memory. In such cases, the destructor is responsible for unpoisoning
   // the memory to avoid triggering false positives.
   // Therefore it's crucial to ensure the destructor is called.
+  //
+  // However, it is replaceable since implementing move-assignment as a destroy + move-construct
+  // will maintain the right ASAN state.
   using __trivially_relocatable = void;
 #  else
   using __trivially_relocatable _LIBCPP_NODEBUG = __conditional_t<
       __libcpp_is_trivially_relocatable<allocator_type>::value && __libcpp_is_trivially_relocatable<pointer>::value,
       basic_string,
       void>;
 #  endif
+  using __replaceable _LIBCPP_NODEBUG =
+      __conditional_t<__is_replaceable_v<pointer> && __container_allocator_is_replaceable<__alloc_traits>::value,
+                      basic_string,
+                      void>;
 
 #  if _LIBCPP_HAS_ASAN && _LIBCPP_INSTRUMENTED_WITH_ASAN
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pointer __asan_volatile_wrapper(pointer const& __ptr) const {

libcxx/include/tuple

@@ -250,6 +250,7 @@ template <class... Types>
 #  include <__type_traits/is_nothrow_assignable.h>
 #  include <__type_traits/is_nothrow_constructible.h>
 #  include <__type_traits/is_reference.h>
+#  include <__type_traits/is_replaceable.h>
 #  include <__type_traits/is_same.h>
 #  include <__type_traits/is_swappable.h>
 #  include <__type_traits/is_trivially_relocatable.h>
@@ -555,6 +556,7 @@ class _LIBCPP_TEMPLATE_VIS _LIBCPP_NO_SPECIALIZATIONS tuple {
 public:
   using __trivially_relocatable _LIBCPP_NODEBUG =
       __conditional_t<_And<__libcpp_is_trivially_relocatable<_Tp>...>::value, tuple, void>;
+  using __replaceable _LIBCPP_NODEBUG = __conditional_t<_And<__is_replaceable<_Tp>...>::value, tuple, void>;
 
   // [tuple.cnstr]
 

libcxx/include/variant

@@ -246,6 +246,7 @@ namespace std {
 #  include <__type_traits/is_nothrow_assignable.h>
 #  include <__type_traits/is_nothrow_constructible.h>
 #  include <__type_traits/is_reference.h>
+#  include <__type_traits/is_replaceable.h>
 #  include <__type_traits/is_same.h>
 #  include <__type_traits/is_swappable.h>
 #  include <__type_traits/is_trivially_assignable.h>
@@ -1176,6 +1177,7 @@ class _LIBCPP_TEMPLATE_VIS _LIBCPP_DECLSPEC_EMPTY_BASES _LIBCPP_NO_SPECIALIZATIO
 public:
   using __trivially_relocatable _LIBCPP_NODEBUG =
       conditional_t<_And<__libcpp_is_trivially_relocatable<_Types>...>::value, variant, void>;
+  using __replaceable _LIBCPP_NODEBUG = conditional_t<_And<__is_replaceable<_Types>...>::value, variant, void>;
 
   template <bool _Dummy                                                                               = true,
             enable_if_t<__dependent_type<is_default_constructible<__first_type>, _Dummy>::value, int> = 0>