[libcxx-commits] [libcxx] [libc++] implement `std::flat_multimap` (PR #113835)

Louis Dionne via libcxx-commits libcxx-commits at lists.llvm.org
Fri Nov 8 11:29:17 PST 2024


================
@@ -0,0 +1,1182 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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___FLAT_MAP_FLAT_MULTIMAP_H
+#define _LIBCPP___FLAT_MAP_FLAT_MULTIMAP_H
+
+#include <__algorithm/lexicographical_compare_three_way.h>
+#include <__algorithm/ranges_equal.h>
+#include <__algorithm/ranges_equal_range.h>
+#include <__algorithm/ranges_inplace_merge.h>
+#include <__algorithm/ranges_is_sorted.h>
+#include <__algorithm/ranges_lower_bound.h>
+#include <__algorithm/ranges_partition_point.h>
+#include <__algorithm/ranges_stable_sort.h>
+#include <__algorithm/ranges_unique.h>
+#include <__algorithm/ranges_upper_bound.h>
+#include <__compare/synth_three_way.h>
+#include <__concepts/convertible_to.h>
+#include <__concepts/swappable.h>
+#include <__config>
+#include <__flat_map/sorted_equivalent.h>
+#include <__functional/invoke.h>
+#include <__functional/is_transparent.h>
+#include <__functional/operations.h>
+#include <__iterator/concepts.h>
+#include <__iterator/distance.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/ranges_iterator_traits.h>
+#include <__iterator/reverse_iterator.h>
+#include <__memory/allocator_traits.h>
+#include <__memory/uses_allocator.h>
+#include <__memory/uses_allocator_construction.h>
+#include <__ranges/concepts.h>
+#include <__ranges/container_compatible_range.h>
+#include <__ranges/drop_view.h>
+#include <__ranges/ref_view.h>
+#include <__ranges/subrange.h>
+#include <__ranges/zip_view.h>
+#include <__type_traits/conjunction.h>
+#include <__type_traits/container_traits.h>
+#include <__type_traits/invoke.h>
+#include <__type_traits/is_allocator.h>
+#include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/maybe_const.h>
+#include <__utility/exception_guard.h>
+#include <__utility/pair.h>
+#include <initializer_list>
+#include <stdexcept>
+#include <string>
+#include <vector>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+#if _LIBCPP_STD_VER >= 23
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _Key,
+          class _Tp,
+          class _Compare         = less<_Key>,
+          class _KeyContainer    = vector<_Key>,
+          class _MappedContainer = vector<_Tp>>
+class flat_multimap {
+  template <bool _Const>
+  struct __iterator;
+
+  template <class, class, class, class, class>
+  friend class flat_multimap;
+
+  static_assert(is_same_v<_Key, typename _KeyContainer::value_type>);
+  static_assert(is_same_v<_Tp, typename _MappedContainer::value_type>);
+  static_assert(!is_same_v<_KeyContainer, std::vector<bool>>, "vector<bool> is not a sequence container");
+  static_assert(!is_same_v<_MappedContainer, std::vector<bool>>, "vector<bool> is not a sequence container");
+
+public:
+  // types
+  using key_type               = _Key;
+  using mapped_type            = _Tp;
+  using value_type             = pair<key_type, mapped_type>;
+  using key_compare            = __type_identity_t<_Compare>;
+  using reference              = pair<const key_type&, mapped_type&>;
+  using const_reference        = pair<const key_type&, const mapped_type&>;
+  using size_type              = size_t;
+  using difference_type        = ptrdiff_t;
+  using iterator               = __iterator<false>; // see [container.requirements]
+  using const_iterator         = __iterator<true>;  // see [container.requirements]
+  using reverse_iterator       = std::reverse_iterator<iterator>;
+  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+  using key_container_type     = _KeyContainer;
+  using mapped_container_type  = _MappedContainer;
+
+  class value_compare {
+  private:
+    key_compare __comp_;
+    value_compare(key_compare __c) : __comp_(__c) {}
+    friend flat_multimap;
+
+  public:
+    _LIBCPP_HIDE_FROM_ABI bool operator()(const_reference __x, const_reference __y) const {
+      return __comp_(__x.first, __y.first);
+    }
+  };
+
+  struct containers {
+    key_container_type keys;
+    mapped_container_type values;
+  };
+
+private:
+  template <class _Allocator>
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool __allocator_ctor_constraint =
+      _And<uses_allocator<key_container_type, _Allocator>, uses_allocator<mapped_container_type, _Allocator>>::value;
+
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool __is_compare_transparent = __is_transparent_v<_Compare, _Compare>;
+
+  template <bool _Const>
+  struct __iterator {
+  private:
+    using __key_iterator    = ranges::iterator_t<const key_container_type>;
+    using __mapped_iterator = ranges::iterator_t<__maybe_const<_Const, mapped_container_type>>;
+    using __reference       = pair<iter_reference_t<__key_iterator>, iter_reference_t<__mapped_iterator>>;
+
+    struct __arrow_proxy {
+      __reference __ref_;
+      _LIBCPP_HIDE_FROM_ABI __reference* operator->() { return std::addressof(__ref_); }
+    };
+
+    __key_iterator __key_iter_;
+    __mapped_iterator __mapped_iter_;
+
+    friend flat_multimap;
+
+  public:
+    using iterator_concept = random_access_iterator_tag;
+    // `flat_multimap::iterator` only satisfy "Cpp17InputIterator" named requirements, because
+    // its `reference` is not a reference type.
+    // However, to avoid surprising runtime behaviour when it is used with the
+    // Cpp17 algorithms or operations, iterator_category is set to random_access_iterator_tag.
+    using iterator_category = random_access_iterator_tag;
+    using value_type        = flat_multimap::value_type;
+    using difference_type   = flat_multimap::difference_type;
+
+    _LIBCPP_HIDE_FROM_ABI __iterator() = default;
+
+    _LIBCPP_HIDE_FROM_ABI __iterator(__iterator<!_Const> __i)
+      requires _Const && convertible_to<ranges::iterator_t<key_container_type>, __key_iterator> &&
+                   convertible_to<ranges::iterator_t<mapped_container_type>, __mapped_iterator>
+        : __key_iter_(std::move(__i.__key_iter_)), __mapped_iter_(std::move(__i.__mapped_iter_)) {}
+
+    _LIBCPP_HIDE_FROM_ABI __iterator(__key_iterator __key_iter, __mapped_iterator __mapped_iter)
+        : __key_iter_(std::move(__key_iter)), __mapped_iter_(std::move(__mapped_iter)) {}
+
+    _LIBCPP_HIDE_FROM_ABI __reference operator*() const { return __reference(*__key_iter_, *__mapped_iter_); }
+    _LIBCPP_HIDE_FROM_ABI __arrow_proxy operator->() const { return __arrow_proxy(**this); }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator& operator++() {
+      ++__key_iter_;
+      ++__mapped_iter_;
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator operator++(int) {
+      __iterator __tmp(*this);
+      ++*this;
+      return __tmp;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator& operator--() {
+      --__key_iter_;
+      --__mapped_iter_;
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator operator--(int) {
+      __iterator __tmp(*this);
+      --*this;
+      return __tmp;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator& operator+=(difference_type __x) {
+      __key_iter_ += __x;
+      __mapped_iter_ += __x;
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator& operator-=(difference_type __x) {
+      __key_iter_ -= __x;
+      __mapped_iter_ -= __x;
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __reference operator[](difference_type __n) const { return *(*this + __n); }
+
+    _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const __iterator& __x, const __iterator& __y) {
+      return __x.__key_iter_ == __y.__key_iter_;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend bool operator<(const __iterator& __x, const __iterator& __y) {
+      return __x.__key_iter_ < __y.__key_iter_;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend bool operator>(const __iterator& __x, const __iterator& __y) { return __y < __x; }
+
+    _LIBCPP_HIDE_FROM_ABI friend bool operator<=(const __iterator& __x, const __iterator& __y) { return !(__y < __x); }
+
+    _LIBCPP_HIDE_FROM_ABI friend bool operator>=(const __iterator& __x, const __iterator& __y) { return !(__x < __y); }
+
+    _LIBCPP_HIDE_FROM_ABI friend auto operator<=>(const __iterator& __x, const __iterator& __y)
+      requires three_way_comparable<__key_iterator>
+    {
+      return __x.__key_iter_ <=> __y.__key_iter_;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend __iterator operator+(const __iterator& __i, difference_type __n) {
+      auto __tmp = __i;
+      __tmp += __n;
+      return __tmp;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend __iterator operator+(difference_type __n, const __iterator& __i) { return __i + __n; }
+
+    _LIBCPP_HIDE_FROM_ABI friend __iterator operator-(const __iterator& __i, difference_type __n) {
+      auto __tmp = __i;
+      __tmp -= __n;
+      return __tmp;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend difference_type operator-(const __iterator& __x, const __iterator& __y) {
+      return difference_type(__x.__key_iter_ - __y.__key_iter_);
+    }
+  };
+
+public:
+  // [flat.map.cons], construct/copy/destroy
+  _LIBCPP_HIDE_FROM_ABI flat_multimap() noexcept(
+      is_nothrow_default_constructible_v<_KeyContainer> && is_nothrow_default_constructible_v<_MappedContainer> &&
+      is_nothrow_default_constructible_v<_Compare>)
+      : __containers_(), __compare_() {}
+
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(const flat_multimap&) = default;
+
+  // The copy/move constructors are not specified in the spec, which means they should be defaulted.
+  // However, the move constructor can potentially leave a moved-from object in an inconsistent
+  // state if an exception is thrown.
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(flat_multimap&& __other) noexcept(
+      is_nothrow_move_constructible_v<_KeyContainer> && is_nothrow_move_constructible_v<_MappedContainer> &&
+      is_nothrow_move_constructible_v<_Compare>)
+#  if _LIBCPP_HAS_EXCEPTIONS
+      try
+#  endif // _LIBCPP_HAS_EXCEPTIONS
+      : __containers_(std::move(__other.__containers_)), __compare_(std::move(__other.__compare_)) {
+    __other.clear();
+#  if _LIBCPP_HAS_EXCEPTIONS
+  } catch (...) {
+    __other.clear();
+    // gcc does not like the `throw` keyword in a conditional noexcept function
+    if constexpr (!(is_nothrow_move_constructible_v<_KeyContainer> &&
+                    is_nothrow_move_constructible_v<_MappedContainer> && is_nothrow_move_constructible_v<_Compare>)) {
+      throw;
+    }
+#  endif // _LIBCPP_HAS_EXCEPTIONS
+  }
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(const flat_multimap& __other, const _Allocator& __alloc)
+      : flat_multimap(__ctor_uses_allocator_tag{},
+                      __alloc,
+                      __other.__containers_.keys,
+                      __other.__containers_.values,
+                      __other.__compare_) {}
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(flat_multimap&& __other, const _Allocator& __alloc)
+#  if _LIBCPP_HAS_EXCEPTIONS
+      try
+#  endif // _LIBCPP_HAS_EXCEPTIONS
+      : flat_multimap(__ctor_uses_allocator_tag{},
+                      __alloc,
+                      std::move(__other.__containers_.keys),
+                      std::move(__other.__containers_.values),
+                      std::move(__other.__compare_)) {
+    __other.clear();
+#  if _LIBCPP_HAS_EXCEPTIONS
+  } catch (...) {
+    __other.clear();
+    throw;
+#  endif // _LIBCPP_HAS_EXCEPTIONS
+  }
+
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(
+      key_container_type __key_cont, mapped_container_type __mapped_cont, const key_compare& __comp = key_compare())
+      : __containers_{.keys = std::move(__key_cont), .values = std::move(__mapped_cont)}, __compare_(__comp) {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
+                                     "flat_multimap keys and mapped containers have different size");
+    __sort();
+  }
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(
+      const key_container_type& __key_cont, const mapped_container_type& __mapped_cont, const _Allocator& __alloc)
+      : flat_multimap(__ctor_uses_allocator_tag{}, __alloc, __key_cont, __mapped_cont) {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
+                                     "flat_multimap keys and mapped containers have different size");
+    __sort();
+  }
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI
+  flat_multimap(const key_container_type& __key_cont,
+                const mapped_container_type& __mapped_cont,
+                const key_compare& __comp,
+                const _Allocator& __alloc)
+      : flat_multimap(__ctor_uses_allocator_tag{}, __alloc, __key_cont, __mapped_cont, __comp) {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
+                                     "flat_multimap keys and mapped containers have different size");
+    __sort();
+  }
+
+  _LIBCPP_HIDE_FROM_ABI
+  flat_multimap(sorted_equivalent_t,
+                key_container_type __key_cont,
+                mapped_container_type __mapped_cont,
+                const key_compare& __comp = key_compare())
+      : __containers_{.keys = std::move(__key_cont), .values = std::move(__mapped_cont)}, __compare_(__comp) {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
+                                     "flat_multimap keys and mapped containers have different size");
+    _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(__is_sorted(__containers_.keys), "Key container is not sorted");
+  }
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI
+  flat_multimap(sorted_equivalent_t,
+                const key_container_type& __key_cont,
+                const mapped_container_type& __mapped_cont,
+                const _Allocator& __alloc)
+      : flat_multimap(__ctor_uses_allocator_tag{}, __alloc, __key_cont, __mapped_cont) {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
+                                     "flat_multimap keys and mapped containers have different size");
+    _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(__is_sorted(__containers_.keys), "Key container is not sorted");
+  }
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI
+  flat_multimap(sorted_equivalent_t,
+                const key_container_type& __key_cont,
+                const mapped_container_type& __mapped_cont,
+                const key_compare& __comp,
+                const _Allocator& __alloc)
+      : flat_multimap(__ctor_uses_allocator_tag{}, __alloc, __key_cont, __mapped_cont, __comp) {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
+                                     "flat_multimap keys and mapped containers have different size");
+    _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(__is_sorted(__containers_.keys), "Key container is not sorted");
+  }
+
+  _LIBCPP_HIDE_FROM_ABI explicit flat_multimap(const key_compare& __comp) : __containers_(), __compare_(__comp) {}
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(const key_compare& __comp, const _Allocator& __alloc)
+      : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc, __comp) {}
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI explicit flat_multimap(const _Allocator& __alloc)
+      : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc) {}
+
+  template <class _InputIterator>
+    requires __has_input_iterator_category<_InputIterator>::value
+  _LIBCPP_HIDE_FROM_ABI
+  flat_multimap(_InputIterator __first, _InputIterator __last, const key_compare& __comp = key_compare())
+      : __containers_(), __compare_(__comp) {
+    insert(__first, __last);
+  }
+
+  template <class _InputIterator, class _Allocator>
+    requires(__has_input_iterator_category<_InputIterator>::value && __allocator_ctor_constraint<_Allocator>)
+  _LIBCPP_HIDE_FROM_ABI
+  flat_multimap(_InputIterator __first, _InputIterator __last, const key_compare& __comp, const _Allocator& __alloc)
+      : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc, __comp) {
+    insert(__first, __last);
+  }
+
+  template <class _InputIterator, class _Allocator>
+    requires(__has_input_iterator_category<_InputIterator>::value && __allocator_ctor_constraint<_Allocator>)
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(_InputIterator __first, _InputIterator __last, const _Allocator& __alloc)
+      : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc) {
+    insert(__first, __last);
+  }
+
+  template <_ContainerCompatibleRange<value_type> _Range>
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(from_range_t __fr, _Range&& __rg)
+      : flat_multimap(__fr, std::forward<_Range>(__rg), key_compare()) {}
+
+  template <_ContainerCompatibleRange<value_type> _Range, class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(from_range_t, _Range&& __rg, const _Allocator& __alloc)
+      : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc) {
+    insert_range(std::forward<_Range>(__rg));
+  }
+
+  template <_ContainerCompatibleRange<value_type> _Range>
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(from_range_t, _Range&& __rg, const key_compare& __comp) : flat_multimap(__comp) {
+    insert_range(std::forward<_Range>(__rg));
+  }
+
+  template <_ContainerCompatibleRange<value_type> _Range, class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(from_range_t, _Range&& __rg, const key_compare& __comp, const _Allocator& __alloc)
+      : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc, __comp) {
+    insert_range(std::forward<_Range>(__rg));
+  }
+
+  template <class _InputIterator>
+    requires __has_input_iterator_category<_InputIterator>::value
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(
+      sorted_equivalent_t, _InputIterator __first, _InputIterator __last, const key_compare& __comp = key_compare())
+      : __containers_(), __compare_(__comp) {
+    insert(sorted_equivalent, __first, __last);
+  }
+  template <class _InputIterator, class _Allocator>
+    requires(__has_input_iterator_category<_InputIterator>::value && __allocator_ctor_constraint<_Allocator>)
+  _LIBCPP_HIDE_FROM_ABI
+  flat_multimap(sorted_equivalent_t,
+                _InputIterator __first,
+                _InputIterator __last,
+                const key_compare& __comp,
+                const _Allocator& __alloc)
+      : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc, __comp) {
+    insert(sorted_equivalent, __first, __last);
+  }
+
+  template <class _InputIterator, class _Allocator>
+    requires(__has_input_iterator_category<_InputIterator>::value && __allocator_ctor_constraint<_Allocator>)
+  _LIBCPP_HIDE_FROM_ABI
+  flat_multimap(sorted_equivalent_t, _InputIterator __first, _InputIterator __last, const _Allocator& __alloc)
+      : flat_multimap(__ctor_uses_allocator_empty_tag{}, __alloc) {
+    insert(sorted_equivalent, __first, __last);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(initializer_list<value_type> __il, const key_compare& __comp = key_compare())
+      : flat_multimap(__il.begin(), __il.end(), __comp) {}
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI
+  flat_multimap(initializer_list<value_type> __il, const key_compare& __comp, const _Allocator& __alloc)
+      : flat_multimap(__il.begin(), __il.end(), __comp, __alloc) {}
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(initializer_list<value_type> __il, const _Allocator& __alloc)
+      : flat_multimap(__il.begin(), __il.end(), __alloc) {}
+
+  _LIBCPP_HIDE_FROM_ABI
+  flat_multimap(sorted_equivalent_t, initializer_list<value_type> __il, const key_compare& __comp = key_compare())
+      : flat_multimap(sorted_equivalent, __il.begin(), __il.end(), __comp) {}
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(
+      sorted_equivalent_t, initializer_list<value_type> __il, const key_compare& __comp, const _Allocator& __alloc)
+      : flat_multimap(sorted_equivalent, __il.begin(), __il.end(), __comp, __alloc) {}
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(sorted_equivalent_t, initializer_list<value_type> __il, const _Allocator& __alloc)
+      : flat_multimap(sorted_equivalent, __il.begin(), __il.end(), __alloc) {}
+
+  _LIBCPP_HIDE_FROM_ABI flat_multimap& operator=(initializer_list<value_type> __il) {
+    clear();
+    insert(__il);
+    return *this;
+  }
+
+  // copy/move assignment are not specified in the spec (defaulted)
+  // but move assignment can potentially leave moved from object in an inconsistent
+  // state if an exception is thrown
+  _LIBCPP_HIDE_FROM_ABI flat_multimap& operator=(const flat_multimap&) = default;
+
+  _LIBCPP_HIDE_FROM_ABI flat_multimap& operator=(flat_multimap&& __other) noexcept(
+      is_nothrow_move_assignable_v<_KeyContainer> && is_nothrow_move_assignable_v<_MappedContainer> &&
+      is_nothrow_move_assignable_v<_Compare>) {
+    auto __clear_other_guard = std::__make_scope_guard([&]() noexcept { __other.clear() /* noexcept */; });
+    auto __clear_self_guard  = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
+    __containers_            = std::move(__other.__containers_);
+    __compare_               = std::move(__other.__compare_);
+    __clear_self_guard.__complete();
+    return *this;
+  }
+
+  // iterators
+  _LIBCPP_HIDE_FROM_ABI iterator begin() noexcept {
+    return iterator(__containers_.keys.begin(), __containers_.values.begin());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI const_iterator begin() const noexcept {
+    return const_iterator(__containers_.keys.begin(), __containers_.values.begin());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI iterator end() noexcept {
+    return iterator(__containers_.keys.end(), __containers_.values.end());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI const_iterator end() const noexcept {
+    return const_iterator(__containers_.keys.end(), __containers_.values.end());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI reverse_iterator rbegin() noexcept { return reverse_iterator(end()); }
+  _LIBCPP_HIDE_FROM_ABI const_reverse_iterator rbegin() const noexcept { return const_reverse_iterator(end()); }
+  _LIBCPP_HIDE_FROM_ABI reverse_iterator rend() noexcept { return reverse_iterator(begin()); }
+  _LIBCPP_HIDE_FROM_ABI const_reverse_iterator rend() const noexcept { return const_reverse_iterator(begin()); }
+
+  _LIBCPP_HIDE_FROM_ABI const_iterator cbegin() const noexcept { return begin(); }
+  _LIBCPP_HIDE_FROM_ABI const_iterator cend() const noexcept { return end(); }
+  _LIBCPP_HIDE_FROM_ABI const_reverse_iterator crbegin() const noexcept { return const_reverse_iterator(end()); }
+  _LIBCPP_HIDE_FROM_ABI const_reverse_iterator crend() const noexcept { return const_reverse_iterator(begin()); }
+
+  // [flat.map.capacity], capacity
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI bool empty() const noexcept { return __containers_.keys.empty(); }
+
+  _LIBCPP_HIDE_FROM_ABI size_type size() const noexcept { return __containers_.keys.size(); }
+
+  _LIBCPP_HIDE_FROM_ABI size_type max_size() const noexcept {
+    return std::min<size_type>(__containers_.keys.max_size(), __containers_.values.max_size());
+  }
+
+  // [flat.map.modifiers], modifiers
+  template <class... _Args>
+    requires is_constructible_v<pair<key_type, mapped_type>, _Args...> && is_move_constructible_v<key_type> &&
+             is_move_constructible_v<mapped_type>
+  _LIBCPP_HIDE_FROM_ABI iterator emplace(_Args&&... __args) {
+    std::pair<key_type, mapped_type> __pair(std::forward<_Args>(__args)...);
+    return __search_and_emplace_with(ranges::upper_bound, std::move(__pair.first), std::move(__pair.second));
+  }
+
+  template <class... _Args>
+    requires is_constructible_v<pair<key_type, mapped_type>, _Args...>
+  _LIBCPP_HIDE_FROM_ABI iterator emplace_hint(const_iterator __hint, _Args&&... __args) {
+    std::pair<key_type, mapped_type> __pair(std::forward<_Args>(__args)...);
+
+    auto __prev_bigger  = __hint != cbegin() && __compare_(__pair.first, (__hint - 1)->first);
+    auto __next_smaller = __hint != cend() && __compare_(__hint->first, __pair.first);
+
+    if (!__prev_bigger && !__next_smaller) {
+      // hint correct, just emplace
+      return __emplace_exact_pos(
+          __hint.__key_iter_, __hint.__mapped_iter_, std::move(__pair.first), std::move(__pair.second));
+    } else if (__prev_bigger && !__next_smaller) {
+      // the hint position is more to the right then the key should have been.
+      // we want to emplace the element to a position as right as possible
+      // e.g. Insert new element "2" in the following range
+      // 1, 1, 2, 2, 2, 3, 4, 6
+      //                  ^
+      //                  |
+      //                 hint
+      // We want to insert "2" after the last existing "2"
+      return __search_and_emplace_with(ranges::upper_bound, std::move(__pair.first), std::move(__pair.second));
+    } else {
+      // !__prev_bigger && __next_smaller
+      // todo assert this condition.  it is not possible that __prev_bigger && __next_smaller, otherwise the multimap is
+      // not sorted
+
+      // the hint position is more to the left then the key should have been.
+      // we want to emplace the element to a position as left as possible
+      //  1, 1, 2, 2, 2, 3, 4, 6
+      // ^
+      // |
+      // hint
+      // We want to insert "2" before the first existing "2"
+      return __search_and_emplace_with(ranges::lower_bound, std::move(__pair.first), std::move(__pair.second));
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI iterator insert(const value_type& __x) { return emplace(__x); }
+
+  _LIBCPP_HIDE_FROM_ABI iterator insert(value_type&& __x) { return emplace(std::move(__x)); }
+
+  _LIBCPP_HIDE_FROM_ABI iterator insert(const_iterator __hint, const value_type& __x) {
+    return emplace_hint(__hint, __x);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI iterator insert(const_iterator __hint, value_type&& __x) {
+    return emplace_hint(__hint, std::move(__x));
+  }
+
+  template <class _Pp>
+    requires is_constructible_v<pair<key_type, mapped_type>, _Pp>
+  _LIBCPP_HIDE_FROM_ABI iterator insert(_Pp&& __x) {
+    return emplace(std::forward<_Pp>(__x));
+  }
+
+  template <class _Pp>
+    requires is_constructible_v<pair<key_type, mapped_type>, _Pp>
+  _LIBCPP_HIDE_FROM_ABI iterator insert(const_iterator __hint, _Pp&& __x) {
+    return emplace_hint(__hint, std::forward<_Pp>(__x));
+  }
+
+  template <class _InputIterator>
+    requires __has_input_iterator_category<_InputIterator>::value
+  _LIBCPP_HIDE_FROM_ABI void insert(_InputIterator __first, _InputIterator __last) {
+    if constexpr (sized_sentinel_for<_InputIterator, _InputIterator>) {
+      __reserve(__last - __first);
+    }
+    __append_sort_merge</*WasSorted = */ false>(std::move(__first), std::move(__last));
+  }
+
+  template <class _InputIterator>
+    requires __has_input_iterator_category<_InputIterator>::value
+  void insert(sorted_equivalent_t, _InputIterator __first, _InputIterator __last) {
+    if constexpr (sized_sentinel_for<_InputIterator, _InputIterator>) {
+      __reserve(__last - __first);
+    }
+
+    __append_sort_merge</*WasSorted = */ true>(std::move(__first), std::move(__last));
+  }
+
+  template <_ContainerCompatibleRange<value_type> _Range>
+  _LIBCPP_HIDE_FROM_ABI void insert_range(_Range&& __range) {
+    if constexpr (ranges::sized_range<_Range>) {
+      __reserve(ranges::size(__range));
+    }
+
+    __append_sort_merge</*WasSorted = */ false>(ranges::begin(__range), ranges::end(__range));
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void insert(initializer_list<value_type> __il) { insert(__il.begin(), __il.end()); }
+
+  _LIBCPP_HIDE_FROM_ABI void insert(sorted_equivalent_t, initializer_list<value_type> __il) {
+    insert(sorted_equivalent, __il.begin(), __il.end());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI containers extract() && {
+    auto __guard = std::__make_scope_guard([&]() noexcept { clear() /* noexcept */; });
+    auto __ret   = std::move(__containers_);
+    return __ret;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void replace(key_container_type&& __key_cont, mapped_container_type&& __mapped_cont) {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(
+        __key_cont.size() == __mapped_cont.size(), "flat_multimap keys and mapped containers have different size");
+
+    _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(__is_sorted(__key_cont), "Key container is not sorted");
+    auto __guard         = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
+    __containers_.keys   = std::move(__key_cont);
+    __containers_.values = std::move(__mapped_cont);
+    __guard.__complete();
+  }
+
+  _LIBCPP_HIDE_FROM_ABI iterator erase(iterator __position) {
+    return __erase(__position.__key_iter_, __position.__mapped_iter_);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI iterator erase(const_iterator __position) {
+    return __erase(__position.__key_iter_, __position.__mapped_iter_);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI size_type erase(const key_type& __x) {
+    auto [__first, __last] = equal_range(__x);
+    auto __res             = __last - __first;
+    erase(__first, __last);
+    return __res;
+  }
+
+  template <class _Kp>
+    requires(__is_compare_transparent && !is_convertible_v<_Kp &&, iterator> &&
+             !is_convertible_v<_Kp &&, const_iterator>)
+  _LIBCPP_HIDE_FROM_ABI size_type erase(_Kp&& __x) {
+    auto [__first, __last] = equal_range(__x);
+    auto __res             = __last - __first;
+    erase(__first, __last);
+    return __res;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI iterator erase(const_iterator __first, const_iterator __last) {
+    auto __on_failure = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
+    auto __key_it     = __containers_.keys.erase(__first.__key_iter_, __last.__key_iter_);
+    auto __mapped_it  = __containers_.values.erase(__first.__mapped_iter_, __last.__mapped_iter_);
+    __on_failure.__complete();
+    return iterator(std::move(__key_it), std::move(__mapped_it));
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void swap(flat_multimap& __y) noexcept {
+    // warning: The spec has unconditional noexcept, which means that
+    // if any of the following functions throw an exception,
+    // std::terminate will be called
+    ranges::swap(__compare_, __y.__compare_);
+    ranges::swap(__containers_.keys, __y.__containers_.keys);
+    ranges::swap(__containers_.values, __y.__containers_.values);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void clear() noexcept {
+    __containers_.keys.clear();
+    __containers_.values.clear();
+  }
+
+  // observers
+  _LIBCPP_HIDE_FROM_ABI key_compare key_comp() const { return __compare_; }
+  _LIBCPP_HIDE_FROM_ABI value_compare value_comp() const { return value_compare(__compare_); }
+
+  _LIBCPP_HIDE_FROM_ABI const key_container_type& keys() const noexcept { return __containers_.keys; }
+  _LIBCPP_HIDE_FROM_ABI const mapped_container_type& values() const noexcept { return __containers_.values; }
+
+  // map operations
+  _LIBCPP_HIDE_FROM_ABI iterator find(const key_type& __x) { return __find_impl(*this, __x); }
+
+  _LIBCPP_HIDE_FROM_ABI const_iterator find(const key_type& __x) const { return __find_impl(*this, __x); }
+
+  template <class _Kp>
+    requires __is_compare_transparent
+  _LIBCPP_HIDE_FROM_ABI iterator find(const _Kp& __x) {
+    return __find_impl(*this, __x);
+  }
+
+  template <class _Kp>
+    requires __is_compare_transparent
+  _LIBCPP_HIDE_FROM_ABI const_iterator find(const _Kp& __x) const {
+    return __find_impl(*this, __x);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI size_type count(const key_type& __x) const {
+    auto [__first, __last] = equal_range(__x);
+    return __last - __first;
+  }
+
+  template <class _Kp>
+    requires __is_compare_transparent
+  _LIBCPP_HIDE_FROM_ABI size_type count(const _Kp& __x) const {
+    auto [__first, __last] = equal_range(__x);
+    return __last - __first;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI bool contains(const key_type& __x) const { return find(__x) != end(); }
+
+  template <class _Kp>
+    requires __is_compare_transparent
+  _LIBCPP_HIDE_FROM_ABI bool contains(const _Kp& __x) const {
+    return find(__x) != end();
+  }
+
+  _LIBCPP_HIDE_FROM_ABI iterator lower_bound(const key_type& __x) { return __lower_bound<iterator>(*this, __x); }
+
+  _LIBCPP_HIDE_FROM_ABI const_iterator lower_bound(const key_type& __x) const {
+    return __lower_bound<const_iterator>(*this, __x);
+  }
+
+  template <class _Kp>
+    requires __is_compare_transparent
+  _LIBCPP_HIDE_FROM_ABI iterator lower_bound(const _Kp& __x) {
+    return __lower_bound<iterator>(*this, __x);
+  }
+
+  template <class _Kp>
+    requires __is_compare_transparent
+  _LIBCPP_HIDE_FROM_ABI const_iterator lower_bound(const _Kp& __x) const {
+    return __lower_bound<const_iterator>(*this, __x);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI iterator upper_bound(const key_type& __x) { return __upper_bound<iterator>(*this, __x); }
+
+  _LIBCPP_HIDE_FROM_ABI const_iterator upper_bound(const key_type& __x) const {
+    return __upper_bound<const_iterator>(*this, __x);
+  }
+
+  template <class _Kp>
+    requires __is_compare_transparent
+  _LIBCPP_HIDE_FROM_ABI iterator upper_bound(const _Kp& __x) {
+    return __upper_bound<iterator>(*this, __x);
+  }
+
+  template <class _Kp>
+    requires __is_compare_transparent
+  _LIBCPP_HIDE_FROM_ABI const_iterator upper_bound(const _Kp& __x) const {
+    return __upper_bound<const_iterator>(*this, __x);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI pair<iterator, iterator> equal_range(const key_type& __x) {
+    return __equal_range_impl(*this, __x);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI pair<const_iterator, const_iterator> equal_range(const key_type& __x) const {
+    return __equal_range_impl(*this, __x);
+  }
+
+  template <class _Kp>
+    requires __is_compare_transparent
+  _LIBCPP_HIDE_FROM_ABI pair<iterator, iterator> equal_range(const _Kp& __x) {
+    return __equal_range_impl(*this, __x);
+  }
+  template <class _Kp>
+    requires __is_compare_transparent
+  _LIBCPP_HIDE_FROM_ABI pair<const_iterator, const_iterator> equal_range(const _Kp& __x) const {
+    return __equal_range_impl(*this, __x);
+  }
+
+  friend _LIBCPP_HIDE_FROM_ABI bool operator==(const flat_multimap& __x, const flat_multimap& __y) {
+    return ranges::equal(__x, __y);
+  }
+
+  friend _LIBCPP_HIDE_FROM_ABI auto operator<=>(const flat_multimap& __x, const flat_multimap& __y) {
+    return std::lexicographical_compare_three_way(
+        __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way);
+  }
+
+  friend _LIBCPP_HIDE_FROM_ABI void swap(flat_multimap& __x, flat_multimap& __y) noexcept { __x.swap(__y); }
+
+private:
+  struct __ctor_uses_allocator_tag {
+    explicit _LIBCPP_HIDE_FROM_ABI __ctor_uses_allocator_tag() = default;
+  };
+  struct __ctor_uses_allocator_empty_tag {
+    explicit _LIBCPP_HIDE_FROM_ABI __ctor_uses_allocator_empty_tag() = default;
+  };
+
+  template <class _Allocator, class _KeyCont, class _MappedCont, class... _CompArg>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI
+  flat_multimap(__ctor_uses_allocator_tag,
+                const _Allocator& __alloc,
+                _KeyCont&& __key_cont,
+                _MappedCont&& __mapped_cont,
+                _CompArg&&... __comp)
+      : __containers_{.keys = std::make_obj_using_allocator<key_container_type>(
+                          __alloc, std::forward<_KeyCont>(__key_cont)),
+                      .values = std::make_obj_using_allocator<mapped_container_type>(
+                          __alloc, std::forward<_MappedCont>(__mapped_cont))},
+        __compare_(std::forward<_CompArg>(__comp)...) {}
+
+  template <class _Allocator, class... _CompArg>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_multimap(__ctor_uses_allocator_empty_tag, const _Allocator& __alloc, _CompArg&&... __comp)
+      : __containers_{.keys   = std::make_obj_using_allocator<key_container_type>(__alloc),
+                      .values = std::make_obj_using_allocator<mapped_container_type>(__alloc)},
+        __compare_(std::forward<_CompArg>(__comp)...) {}
+
+  _LIBCPP_HIDE_FROM_ABI bool __is_sorted(auto&& __key_container) const {
+    return ranges::is_sorted(__key_container, __compare_);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void __sort() {
+    auto __zv = ranges::views::zip(__containers_.keys, __containers_.values);
+    ranges::stable_sort(__zv, __compare_, [](const auto& __p) -> decltype(auto) { return std::get<0>(__p); });
+  }
+
+  template <class _InputIterator, class _Sentinel>
+  _LIBCPP_HIDE_FROM_ABI size_type __append(_InputIterator __first, _Sentinel __last) {
+    size_type __num_of_appended = 0;
+    for (; __first != __last; ++__first) {
+      value_type __kv = *__first;
+      __containers_.keys.insert(__containers_.keys.end(), std::move(__kv.first));
+      __containers_.values.insert(__containers_.values.end(), std::move(__kv.second));
+      ++__num_of_appended;
+    }
+    return __num_of_appended;
+  }
+
+  template <bool _WasSorted, class _InputIterator, class _Sentinel>
+  _LIBCPP_HIDE_FROM_ABI void __append_sort_merge(_InputIterator __first, _Sentinel __last) {
+    auto __on_failure        = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
+    size_t __num_of_appended = __append(std::move(__first), std::move(__last));
+    if (__num_of_appended != 0) {
+      auto __zv                  = ranges::views::zip(__containers_.keys, __containers_.values);
+      auto __append_start_offset = __containers_.keys.size() - __num_of_appended;
+      auto __end                 = __zv.end();
+      auto __compare_key         = [this](const auto& __p1, const auto& __p2) {
+        return __compare_(std::get<0>(__p1), std::get<0>(__p2));
+      };
+      if constexpr (!_WasSorted) {
+        ranges::stable_sort(__zv.begin() + __append_start_offset, __end, __compare_key);
+      } else {
+        _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(
+            __is_sorted(__containers_.keys | ranges::views::drop(__append_start_offset)),
+            "Key container is not sorted");
+      }
+      ranges::inplace_merge(__zv.begin(), __zv.begin() + __append_start_offset, __end, __compare_key);
+    }
+    __on_failure.__complete();
+  }
+
+  template <class _Self, class _Kp>
+  _LIBCPP_HIDE_FROM_ABI static auto __find_impl(_Self&& __self, const _Kp& __key) {
+    auto __it   = __self.lower_bound(__key);
+    auto __last = __self.end();
+    if (__it == __last || __self.__compare_(__key, __it->first)) {
+      return __last;
+    }
+    return __it;
+  }
+
+  template <class _Self, class _Kp>
+  _LIBCPP_HIDE_FROM_ABI static auto __equal_range_impl(_Self&& __self, const _Kp& __key) {
+    auto [__key_first, __key_last] = ranges::equal_range(__self.__containers_.keys, __key, __self.__compare_);
+
+    const auto __make_mapped_iter = [&](const auto& __key_iter) {
+      return __self.__containers_.values.begin() +
+             static_cast<ranges::range_difference_t<mapped_container_type>>(
+                 ranges::distance(__self.__containers_.keys.begin(), __key_iter));
+    };
+
+    using __iterator_type = ranges::iterator_t<decltype(__self)>;
+    return std::make_pair(__iterator_type(__key_first, __make_mapped_iter(__key_first)),
+                          __iterator_type(__key_last, __make_mapped_iter(__key_last)));
+  }
+
+  template <class _Res, class _Self, class _Kp>
+  _LIBCPP_HIDE_FROM_ABI static _Res __lower_bound(_Self&& __self, _Kp& __x) {
+    return __binary_search<_Res>(__self, ranges::lower_bound, __x);
+  }
+
+  template <class _Res, class _Self, class _Kp>
+  _LIBCPP_HIDE_FROM_ABI static _Res __upper_bound(_Self&& __self, _Kp& __x) {
+    return __binary_search<_Res>(__self, ranges::upper_bound, __x);
+  }
+
+  template <class _Res, class _Self, class _Fn, class _Kp>
+  _LIBCPP_HIDE_FROM_ABI static _Res __binary_search(_Self&& __self, _Fn __search_fn, _Kp& __x) {
+    auto __key_iter = __search_fn(__self.__containers_.keys, __x, __self.__compare_);
+    auto __mapped_iter =
+        __self.__containers_.values.begin() +
+        static_cast<ranges::range_difference_t<mapped_container_type>>(
+            ranges::distance(__self.__containers_.keys.begin(), __key_iter));
+
+    return _Res(std::move(__key_iter), std::move(__mapped_iter));
+  }
+
+  template <class _Fn, class _KeyArg, class... _MArgs>
+  _LIBCPP_HIDE_FROM_ABI iterator
+  __search_and_emplace_with(_Fn&& __search_fn, _KeyArg&& __key, _MArgs&&... __mapped_args) {
+    auto __key_it    = __search_fn(__containers_.keys, __key, __compare_);
+    auto __mapped_it = __containers_.values.begin() + ranges::distance(__containers_.keys.begin(), __key_it);
+
+    return __emplace_exact_pos(
+        std::move(__key_it),
+        std::move(__mapped_it),
+        std::forward<_KeyArg>(__key),
+        std::forward<_MArgs>(__mapped_args)...);
+  }
+
+  template <class _IterK, class _IterM, class _KeyArg, class... _MArgs>
+  _LIBCPP_HIDE_FROM_ABI iterator
+  __emplace_exact_pos(_IterK&& __it_key, _IterM&& __it_mapped, _KeyArg&& __key, _MArgs&&... __mapped_args) {
+    auto __on_key_failed = std::__make_exception_guard([&]() noexcept {
+      if constexpr (__container_traits<_KeyContainer>::__emplacement_has_strong_exception_safety_guarantee) {
+        // Nothing to roll back!
+      } else {
+        // we need to clear both because we don't know the state of our keys anymore
+        clear() /* noexcept */;
+      }
+    });
+    auto __key_it        = __containers_.keys.emplace(__it_key, std::forward<_KeyArg>(__key));
+    __on_key_failed.__complete();
+
+    auto __on_value_failed = std::__make_exception_guard([&]() noexcept {
+      if constexpr (!__container_traits<_MappedContainer>::__emplacement_has_strong_exception_safety_guarantee) {
+        // we need to clear both because we don't know the state of our values anymore
+        clear() /* noexcept */;
+      } else {
+        // In this case, we know the values are just like before we attempted emplacement,
+        // and we also know that the keys have been emplaced successfully. Just roll back the keys.
+#  if _LIBCPP_HAS_EXCEPTIONS
+        try {
+#  endif // _LIBCPP_HAS_EXCEPTIONS
+          __containers_.keys.erase(__key_it);
+#  if _LIBCPP_HAS_EXCEPTIONS
+        } catch (...) {
+          // Now things are funky for real. We're failing to rollback the keys.
+          // Just give up and clear the whole thing.
+          //
+          // Also, swallow the exception that happened during the rollback and let the
+          // original value-emplacement exception propagate normally.
+          clear() /* noexcept */;
+        }
+#  endif // _LIBCPP_HAS_EXCEPTIONS
+      }
+    });
+    auto __mapped_it = __containers_.values.emplace(__it_mapped, std::forward<_MArgs>(__mapped_args)...);
+    __on_value_failed.__complete();
+
+    return iterator(std::move(__key_it), std::move(__mapped_it));
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void __reserve(size_t __size) {
+    if constexpr (requires { __containers_.keys.reserve(__size); }) {
+      __containers_.keys.reserve(__size);
+    }
+
+    if constexpr (requires { __containers_.values.reserve(__size); }) {
+      __containers_.values.reserve(__size);
+    }
+  }
+
+  template <class _KIter, class _MIter>
+  _LIBCPP_HIDE_FROM_ABI iterator __erase(_KIter __key_iter_to_remove, _MIter __mapped_iter_to_remove) {
+    auto __on_failure  = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
+    auto __key_iter    = __containers_.keys.erase(__key_iter_to_remove);
+    auto __mapped_iter = __containers_.values.erase(__mapped_iter_to_remove);
+    __on_failure.__complete();
+    return iterator(std::move(__key_iter), std::move(__mapped_iter));
+  }
+
+  template <class _Key2, class _Tp2, class _Compare2, class _KeyContainer2, class _MappedContainer2, class _Predicate>
+  friend typename flat_multimap<_Key2, _Tp2, _Compare2, _KeyContainer2, _MappedContainer2>::size_type
+  erase_if(flat_multimap<_Key2, _Tp2, _Compare2, _KeyContainer2, _MappedContainer2>&, _Predicate);
+
+  containers __containers_;
+  [[no_unique_address]] key_compare __compare_;
+
+  struct __key_equiv {
+    _LIBCPP_HIDE_FROM_ABI __key_equiv(key_compare __c) : __comp_(__c) {}
+    _LIBCPP_HIDE_FROM_ABI bool operator()(const_reference __x, const_reference __y) const {
+      return !__comp_(std::get<0>(__x), std::get<0>(__y)) && !__comp_(std::get<0>(__y), std::get<0>(__x));
+    }
+    key_compare __comp_;
+  };
+};
+
+template <class _KeyContainer, class _MappedContainer, class _Compare = less<typename _KeyContainer::value_type>>
+  requires(!__is_allocator<_Compare>::value && !__is_allocator<_KeyContainer>::value &&
+           !__is_allocator<_MappedContainer>::value &&
+           is_invocable_v<const _Compare&,
+                          const typename _KeyContainer::value_type&,
+                          const typename _KeyContainer::value_type&>)
+flat_multimap(_KeyContainer, _MappedContainer, _Compare = _Compare())
+    -> flat_multimap<typename _KeyContainer::value_type,
+                     typename _MappedContainer::value_type,
+                     _Compare,
+                     _KeyContainer,
+                     _MappedContainer>;
+
+template <class _KeyContainer, class _MappedContainer, class _Allocator>
+  requires(uses_allocator_v<_KeyContainer, _Allocator> && uses_allocator_v<_MappedContainer, _Allocator> &&
+           !__is_allocator<_KeyContainer>::value && !__is_allocator<_MappedContainer>::value)
+flat_multimap(_KeyContainer, _MappedContainer, _Allocator)
+    -> flat_multimap<typename _KeyContainer::value_type,
+                     typename _MappedContainer::value_type,
+                     less<typename _KeyContainer::value_type>,
+                     _KeyContainer,
+                     _MappedContainer>;
+
+template <class _KeyContainer, class _MappedContainer, class _Compare, class _Allocator>
+  requires(!__is_allocator<_Compare>::value && !__is_allocator<_KeyContainer>::value &&
+           !__is_allocator<_MappedContainer>::value && uses_allocator_v<_KeyContainer, _Allocator> &&
+           uses_allocator_v<_MappedContainer, _Allocator> &&
+           is_invocable_v<const _Compare&,
+                          const typename _KeyContainer::value_type&,
+                          const typename _KeyContainer::value_type&>)
+flat_multimap(_KeyContainer, _MappedContainer, _Compare, _Allocator)
+    -> flat_multimap<typename _KeyContainer::value_type,
+                     typename _MappedContainer::value_type,
+                     _Compare,
+                     _KeyContainer,
+                     _MappedContainer>;
+
+template <class _KeyContainer, class _MappedContainer, class _Compare = less<typename _KeyContainer::value_type>>
+  requires(!__is_allocator<_Compare>::value && !__is_allocator<_KeyContainer>::value &&
+           !__is_allocator<_MappedContainer>::value &&
+           is_invocable_v<const _Compare&,
+                          const typename _KeyContainer::value_type&,
+                          const typename _KeyContainer::value_type&>)
+flat_multimap(sorted_equivalent_t, _KeyContainer, _MappedContainer, _Compare = _Compare())
+    -> flat_multimap<typename _KeyContainer::value_type,
+                     typename _MappedContainer::value_type,
+                     _Compare,
+                     _KeyContainer,
+                     _MappedContainer>;
+
+template <class _KeyContainer, class _MappedContainer, class _Allocator>
+  requires(uses_allocator_v<_KeyContainer, _Allocator> && uses_allocator_v<_MappedContainer, _Allocator> &&
+           !__is_allocator<_KeyContainer>::value && !__is_allocator<_MappedContainer>::value)
+flat_multimap(sorted_equivalent_t, _KeyContainer, _MappedContainer, _Allocator)
+    -> flat_multimap<typename _KeyContainer::value_type,
+                     typename _MappedContainer::value_type,
+                     less<typename _KeyContainer::value_type>,
+                     _KeyContainer,
+                     _MappedContainer>;
+
+template <class _KeyContainer, class _MappedContainer, class _Compare, class _Allocator>
+  requires(!__is_allocator<_Compare>::value && !__is_allocator<_KeyContainer>::value &&
+           !__is_allocator<_MappedContainer>::value && uses_allocator_v<_KeyContainer, _Allocator> &&
+           uses_allocator_v<_MappedContainer, _Allocator> &&
+           is_invocable_v<const _Compare&,
+                          const typename _KeyContainer::value_type&,
+                          const typename _KeyContainer::value_type&>)
+flat_multimap(sorted_equivalent_t, _KeyContainer, _MappedContainer, _Compare, _Allocator)
+    -> flat_multimap<typename _KeyContainer::value_type,
+                     typename _MappedContainer::value_type,
+                     _Compare,
+                     _KeyContainer,
+                     _MappedContainer>;
+
+template <class _InputIterator, class _Compare = less<__iter_key_type<_InputIterator>>>
+  requires(__has_input_iterator_category<_InputIterator>::value && !__is_allocator<_Compare>::value)
+flat_multimap(_InputIterator, _InputIterator, _Compare = _Compare())
+    -> flat_multimap<__iter_key_type<_InputIterator>, __iter_mapped_type<_InputIterator>, _Compare>;
+
+template <class _InputIterator, class _Compare = less<__iter_key_type<_InputIterator>>>
+  requires(__has_input_iterator_category<_InputIterator>::value && !__is_allocator<_Compare>::value)
+flat_multimap(sorted_equivalent_t, _InputIterator, _InputIterator, _Compare = _Compare())
+    -> flat_multimap<__iter_key_type<_InputIterator>, __iter_mapped_type<_InputIterator>, _Compare>;
+
+template <ranges::input_range _Range,
+          class _Compare   = less<__range_key_type<_Range>>,
+          class _Allocator = allocator<byte>,
+          class            = __enable_if_t<!__is_allocator<_Compare>::value && __is_allocator<_Allocator>::value>>
+flat_multimap(from_range_t, _Range&&, _Compare = _Compare(), _Allocator = _Allocator())
+    -> flat_multimap<
+        __range_key_type<_Range>,
+        __range_mapped_type<_Range>,
+        _Compare,
+        vector<__range_key_type<_Range>, __allocator_traits_rebind_t<_Allocator, __range_key_type<_Range>>>,
+        vector<__range_mapped_type<_Range>, __allocator_traits_rebind_t<_Allocator, __range_mapped_type<_Range>>>>;
+
+template <ranges::input_range _Range, class _Allocator, class = __enable_if_t<__is_allocator<_Allocator>::value>>
+flat_multimap(from_range_t, _Range&&, _Allocator)
+    -> flat_multimap<
+        __range_key_type<_Range>,
+        __range_mapped_type<_Range>,
+        less<__range_key_type<_Range>>,
+        vector<__range_key_type<_Range>, __allocator_traits_rebind_t<_Allocator, __range_key_type<_Range>>>,
+        vector<__range_mapped_type<_Range>, __allocator_traits_rebind_t<_Allocator, __range_mapped_type<_Range>>>>;
+
+template <class _Key, class _Tp, class _Compare = less<_Key>>
+  requires(!__is_allocator<_Compare>::value)
+flat_multimap(initializer_list<pair<_Key, _Tp>>, _Compare = _Compare()) -> flat_multimap<_Key, _Tp, _Compare>;
+
+template <class _Key, class _Tp, class _Compare = less<_Key>>
+  requires(!__is_allocator<_Compare>::value)
+flat_multimap(sorted_equivalent_t, initializer_list<pair<_Key, _Tp>>, _Compare = _Compare())
+    -> flat_multimap<_Key, _Tp, _Compare>;
+
+template <class _Key, class _Tp, class _Compare, class _KeyContainer, class _MappedContainer, class _Allocator>
+struct uses_allocator<flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>, _Allocator>
+    : bool_constant<uses_allocator_v<_KeyContainer, _Allocator> && uses_allocator_v<_MappedContainer, _Allocator>> {};
+
+template <class _Key, class _Tp, class _Compare, class _KeyContainer, class _MappedContainer, class _Predicate>
+_LIBCPP_HIDE_FROM_ABI typename flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>::size_type
+erase_if(flat_multimap<_Key, _Tp, _Compare, _KeyContainer, _MappedContainer>& __flat_multimap, _Predicate __pred) {
+  // todo
----------------
ldionne wrote:

```suggestion

```

https://github.com/llvm/llvm-project/pull/113835


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