Index: libcxx/include/__hash_table
===================================================================
--- libcxx/include/__hash_table
+++ libcxx/include/__hash_table
@@ -1058,8 +1058,26 @@
         );
     }
 
+private:
+    _LIBCPP_INLINE_VISIBILITY
+    __next_pointer __node_insert_multi_prepare(size_t __cp_hash,
+                                               value_type& __cp_val);
+    _LIBCPP_INLINE_VISIBILITY
+    void __node_insert_multi_perform(__node_pointer __cp,
+                                     __next_pointer __pn) _NOEXCEPT;
+
+    _LIBCPP_INLINE_VISIBILITY
+    __next_pointer __node_insert_unique_prepare(size_t __nd_hash,
+                                                value_type& __nd_val);
+    _LIBCPP_INLINE_VISIBILITY
+    void __node_insert_unique_perform(__node_pointer __ptr) _NOEXCEPT;
+
+public:
+    _LIBCPP_INLINE_VISIBILITY
     pair<iterator, bool> __node_insert_unique(__node_pointer __nd);
+    _LIBCPP_INLINE_VISIBILITY
     iterator             __node_insert_multi(__node_pointer __nd);
+    _LIBCPP_INLINE_VISIBILITY
     iterator             __node_insert_multi(const_iterator __p,
                                              __node_pointer __nd);
 
@@ -1170,6 +1188,9 @@
     _LIBCPP_INLINE_VISIBILITY
     iterator __node_handle_insert_unique(const_iterator __hint,
                                          _NodeHandle&& __nh);
+    template <class _Table>
+    _LIBCPP_INLINE_VISIBILITY
+    void __node_handle_merge_unique(_Table& __source);
 
     template <class _NodeHandle>
     _LIBCPP_INLINE_VISIBILITY
@@ -1177,6 +1198,9 @@
     template <class _NodeHandle>
     _LIBCPP_INLINE_VISIBILITY
     iterator __node_handle_insert_multi(const_iterator __hint, _NodeHandle&& __nh);
+    template <class _Table>
+    _LIBCPP_INLINE_VISIBILITY
+    void __node_handle_merge_multi(_Table& __source);
 
     template <class _NodeHandle>
     _LIBCPP_INLINE_VISIBILITY
@@ -1850,60 +1874,78 @@
 }
 
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
-pair<typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator, bool>
-__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_unique(__node_pointer __nd)
+_LIBCPP_INLINE_VISIBILITY
+typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::__next_pointer
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_unique_prepare(
+    size_t __hash, value_type& __value)
 {
-    __nd->__hash_ = hash_function()(__nd->__value_);
     size_type __bc = bucket_count();
-    bool __inserted = false;
-    __next_pointer __ndptr;
-    size_t __chash;
+
     if (__bc != 0)
     {
-        __chash = __constrain_hash(__nd->__hash_, __bc);
-        __ndptr = __bucket_list_[__chash];
+        size_t __chash = __constrain_hash(__hash, __bc);
+        __next_pointer __ndptr = __bucket_list_[__chash];
         if (__ndptr != nullptr)
         {
             for (__ndptr = __ndptr->__next_; __ndptr != nullptr &&
                                              __constrain_hash(__ndptr->__hash(), __bc) == __chash;
                                                      __ndptr = __ndptr->__next_)
             {
-                if (key_eq()(__ndptr->__upcast()->__value_, __nd->__value_))
-                    goto __done;
+                if (key_eq()(__ndptr->__upcast()->__value_, __value))
+                    return __ndptr;
             }
         }
     }
+    if (size()+1 > __bc * max_load_factor() || __bc == 0)
     {
-        if (size()+1 > __bc * max_load_factor() || __bc == 0)
-        {
-            rehash(_VSTD::max<size_type>(2 * __bc + !__is_hash_power2(__bc),
-                           size_type(ceil(float(size() + 1) / max_load_factor()))));
-            __bc = bucket_count();
-            __chash = __constrain_hash(__nd->__hash_, __bc);
-        }
-        // insert_after __bucket_list_[__chash], or __first_node if bucket is null
-        __next_pointer __pn = __bucket_list_[__chash];
-        if (__pn == nullptr)
-        {
-            __pn =__p1_.first().__ptr();
-            __nd->__next_ = __pn->__next_;
-            __pn->__next_ = __nd->__ptr();
-            // fix up __bucket_list_
-            __bucket_list_[__chash] = __pn;
-            if (__nd->__next_ != nullptr)
-                __bucket_list_[__constrain_hash(__nd->__next_->__hash(), __bc)] = __nd->__ptr();
-        }
-        else
-        {
-            __nd->__next_ = __pn->__next_;
-            __pn->__next_ = __nd->__ptr();
-        }
+        rehash(_VSTD::max<size_type>(2 * __bc + !__is_hash_power2(__bc),
+                                     size_type(ceil(float(size() + 1) / max_load_factor()))));
+    }
+    return nullptr;
+}
+
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
+_LIBCPP_INLINE_VISIBILITY
+void
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_unique_perform(
+    __node_pointer __nd) _NOEXCEPT
+{
+    size_type __bc = bucket_count();
+    size_t __chash = __constrain_hash(__nd->__hash(), __bc);
+    // insert_after __bucket_list_[__chash], or __first_node if bucket is null
+    __next_pointer __pn = __bucket_list_[__chash];
+    if (__pn == nullptr)
+    {
+        __pn =__p1_.first().__ptr();
+        __nd->__next_ = __pn->__next_;
+        __pn->__next_ = __nd->__ptr();
+        // fix up __bucket_list_
+        __bucket_list_[__chash] = __pn;
+        if (__nd->__next_ != nullptr)
+            __bucket_list_[__constrain_hash(__nd->__next_->__hash(), __bc)] = __nd->__ptr();
+    }
+    else
+    {
+        __nd->__next_ = __pn->__next_;
+        __pn->__next_ = __nd->__ptr();
+    }
+    ++size();
+}
+
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
+pair<typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator, bool>
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_unique(__node_pointer __nd)
+{
+    __nd->__hash_ = hash_function()(__nd->__value_);
+    __next_pointer __ndptr =
+        __node_insert_unique_prepare(__nd->__hash(), __nd->__value_);
+    bool __inserted = false;
+    if (__ndptr == nullptr)
+    {
+        __node_insert_unique_perform(__nd);
         __ndptr = __nd->__ptr();
-        // increment size
-        ++size();
         __inserted = true;
     }
-__done:
 #if _LIBCPP_DEBUG_LEVEL >= 2
     return pair<iterator, bool>(iterator(__ndptr, this), __inserted);
 #else
@@ -1912,10 +1954,10 @@
 }
 
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
-typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator
-__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_multi(__node_pointer __cp)
+typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::__next_pointer
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_multi_prepare(
+    size_t __cp_hash, value_type& __cp_val)
 {
-    __cp->__hash_ = hash_function()(__cp->__value_);
     size_type __bc = bucket_count();
     if (size()+1 > __bc * max_load_factor() || __bc == 0)
     {
@@ -1923,20 +1965,9 @@
                        size_type(ceil(float(size() + 1) / max_load_factor()))));
         __bc = bucket_count();
     }
-    size_t __chash = __constrain_hash(__cp->__hash_, __bc);
+    size_t __chash = __constrain_hash(__cp_hash, __bc);
     __next_pointer __pn = __bucket_list_[__chash];
-    if (__pn == nullptr)
-    {
-        __pn =__p1_.first().__ptr();
-        __cp->__next_ = __pn->__next_;
-        __pn->__next_ = __cp->__ptr();
-        // fix up __bucket_list_
-        __bucket_list_[__chash] = __pn;
-        if (__cp->__next_ != nullptr)
-            __bucket_list_[__constrain_hash(__cp->__next_->__hash(), __bc)]
-                = __cp->__ptr();
-    }
-    else
+    if (__pn != nullptr)
     {
         for (bool __found = false; __pn->__next_ != nullptr &&
                                    __constrain_hash(__pn->__next_->__hash(), __bc) == __chash;
@@ -1947,8 +1978,8 @@
             //      true        true        loop
             //      false       true        set __found to true
             //      true        false       break
-            if (__found != (__pn->__next_->__hash() == __cp->__hash_ &&
-                            key_eq()(__pn->__next_->__upcast()->__value_, __cp->__value_)))
+            if (__found != (__pn->__next_->__hash() == __cp_hash &&
+                            key_eq()(__pn->__next_->__upcast()->__value_, __cp_val)))
             {
                 if (!__found)
                     __found = true;
@@ -1956,6 +1987,30 @@
                     break;
             }
         }
+    }
+    return __pn;
+}
+
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
+void
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_multi_perform(
+    __node_pointer __cp, __next_pointer __pn) _NOEXCEPT
+{
+    size_type __bc = bucket_count();
+    size_t __chash = __constrain_hash(__cp->__hash_, __bc);
+    if (__pn == nullptr)
+    {
+        __pn =__p1_.first().__ptr();
+        __cp->__next_ = __pn->__next_;
+        __pn->__next_ = __cp->__ptr();
+        // fix up __bucket_list_
+        __bucket_list_[__chash] = __pn;
+        if (__cp->__next_ != nullptr)
+            __bucket_list_[__constrain_hash(__cp->__next_->__hash(), __bc)]
+                = __cp->__ptr();
+    }
+    else
+    {
         __cp->__next_ = __pn->__next_;
         __pn->__next_ = __cp->__ptr();
         if (__cp->__next_ != nullptr)
@@ -1966,6 +2021,17 @@
         }
     }
     ++size();
+}
+
+
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
+typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_insert_multi(__node_pointer __cp)
+{
+    __cp->__hash_ = hash_function()(__cp->__value_);
+    __next_pointer __pn = __node_insert_multi_prepare(__cp->__hash(), __cp->__value_);
+    __node_insert_multi_perform(__cp, __pn);
+
 #if _LIBCPP_DEBUG_LEVEL >= 2
     return iterator(__cp->__ptr(), this);
 #else
@@ -2216,6 +2282,33 @@
     return _NodeHandle(remove(__p).release(), __alloc);
 }
 
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
+template <class _Table>
+_LIBCPP_INLINE_VISIBILITY
+void
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_handle_merge_unique(
+    _Table& __source)
+{
+    static_assert(is_same<__node, typename _Table::__node>::value, "");
+
+    for (typename _Table::iterator __it = __source.begin();
+         __it != __source.end();)
+    {
+        __node_pointer __src_ptr = __it.__node_->__upcast();
+        size_t __hash = hash_function()(__src_ptr->__value_);
+        __next_pointer __ndptr =
+            __node_insert_unique_prepare(__hash, __src_ptr->__value_);
+        if (__ndptr == nullptr)
+        {
+            (void)__source.remove(__it++).release();
+            __src_ptr->__hash_ = __hash;
+            __node_insert_unique_perform(__src_ptr);
+        }
+        else
+            ++__it;
+    }
+}
+
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
 template <class _NodeHandle>
 _LIBCPP_INLINE_VISIBILITY
@@ -2244,6 +2337,27 @@
     return __result;
 }
 
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
+template <class _Table>
+_LIBCPP_INLINE_VISIBILITY
+void
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__node_handle_merge_multi(
+    _Table& __source)
+{
+    static_assert(is_same<typename _Table::__node, __node>::value, "");
+
+    for (typename _Table::iterator __it = __source.begin();
+         __it != __source.end();)
+    {
+        __node_pointer __src_ptr = __it.__node_->__upcast();
+        size_t __src_hash = hash_function()(__src_ptr->__value_);
+        __next_pointer __pn =
+            __node_insert_multi_prepare(__src_hash, __src_ptr->__value_);
+        (void)__source.remove(__it++).release();
+        __src_ptr->__hash_ = __src_hash;
+        __node_insert_multi_perform(__src_ptr, __pn);
+    }
+}
 #endif  // _LIBCPP_STD_VER > 14
 
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
Index: libcxx/include/__node_handle
===================================================================
--- libcxx/include/__node_handle
+++ libcxx/include/__node_handle
@@ -26,8 +26,7 @@
 
 #if _LIBCPP_STD_VER > 14
 
-// FIXME: Uncomment this when we support the 'merge' functionality.
-// #define __cpp_lib_node_extract 201606L
+#define __cpp_lib_node_extract 201606L
 
 // Specialized in __tree & __hash_table for their _NodeType.
 template <class _NodeType, class _Alloc>
Index: libcxx/include/__tree
===================================================================
--- libcxx/include/__tree
+++ libcxx/include/__tree
@@ -1341,11 +1341,15 @@
 
 #endif // !_LIBCPP_CXX03_LANG
 
+    _LIBCPP_INLINE_VISIBILITY
     pair<iterator, bool> __node_insert_unique(__node_pointer __nd);
+    _LIBCPP_INLINE_VISIBILITY
     iterator             __node_insert_unique(const_iterator __p,
                                               __node_pointer __nd);
 
+    _LIBCPP_INLINE_VISIBILITY
     iterator __node_insert_multi(__node_pointer __nd);
+    _LIBCPP_INLINE_VISIBILITY
     iterator __node_insert_multi(const_iterator __p, __node_pointer __nd);
 
 
@@ -1358,6 +1362,9 @@
     template <class _NodeHandle>
     _LIBCPP_INLINE_VISIBILITY
     iterator __node_handle_insert_unique(const_iterator, _NodeHandle&&);
+    template <class _Tree>
+    _LIBCPP_INLINE_VISIBILITY
+    void __node_handle_merge_unique(_Tree& __source);
 
     template <class _NodeHandle>
     _LIBCPP_INLINE_VISIBILITY
@@ -1365,6 +1372,9 @@
     template <class _NodeHandle>
     _LIBCPP_INLINE_VISIBILITY
     iterator __node_handle_insert_multi(const_iterator, _NodeHandle&&);
+    template <class _Tree>
+    _LIBCPP_INLINE_VISIBILITY
+    void __node_handle_merge_multi(_Tree& __source);
 
 
     template <class _NodeHandle>
@@ -2471,6 +2481,30 @@
     return _NodeHandle(__np, __alloc());
 }
 
+template <class _Tp, class _Compare, class _Allocator>
+template <class _Tree>
+_LIBCPP_INLINE_VISIBILITY
+void
+__tree<_Tp, _Compare, _Allocator>::__node_handle_merge_unique(_Tree& __source)
+{
+    static_assert(is_same<typename _Tree::__node_pointer, __node_pointer>::value, "");
+
+    for (typename _Tree::iterator __i = __source.begin();
+         __i != __source.end();)
+    {
+        __node_pointer __src_ptr = __i.__get_np();
+        __parent_pointer __parent;
+        __node_base_pointer& __child = __find_equal(
+            __parent, _NodeTypes::__get_key(__src_ptr->__value_));
+        ++__i;
+        if (__child != nullptr)
+            continue;
+        __source.__remove_node_pointer(__src_ptr);
+        __insert_node_at(__parent, __child,
+                         static_cast<__node_base_pointer>(__src_ptr));
+    }
+}
+
 template <class _Tp, class _Compare, class _Allocator>
 template <class _NodeHandle>
 _LIBCPP_INLINE_VISIBILITY
@@ -2507,6 +2541,28 @@
     return iterator(__ptr);
 }
 
+template <class _Tp, class _Compare, class _Allocator>
+template <class _Tree>
+_LIBCPP_INLINE_VISIBILITY
+void
+__tree<_Tp, _Compare, _Allocator>::__node_handle_merge_multi(_Tree& __source)
+{
+    static_assert(is_same<typename _Tree::__node_pointer, __node_pointer>::value, "");
+
+    for (typename _Tree::iterator __i = __source.begin();
+         __i != __source.end();)
+    {
+        __node_pointer __src_ptr = __i.__get_np();
+        __parent_pointer __parent;
+        __node_base_pointer& __child = __find_leaf_high(
+            __parent, _NodeTypes::__get_key(__src_ptr->__value_));
+        ++__i;
+        __source.__remove_node_pointer(__src_ptr);
+        __insert_node_at(__parent, __child,
+                         static_cast<__node_base_pointer>(__src_ptr));
+    }
+}
+
 #endif  // _LIBCPP_STD_VER > 14
 
 template <class _Tp, class _Compare, class _Allocator>
Index: libcxx/include/map
===================================================================
--- libcxx/include/map
+++ libcxx/include/map
@@ -167,6 +167,15 @@
     iterator  erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class C2>
+      void merge(map<Key, T, C2, Allocator>& source);         // C++17
+    template<class C2>
+      void merge(map<Key, T, C2, Allocator>&& source);        // C++17
+    template<class C2>
+      void merge(multimap<Key, T, C2, Allocator>& source);    // C++17
+    template<class C2>
+      void merge(multimap<Key, T, C2, Allocator>&& source);   // C++17
+
     void swap(map& m)
         noexcept(allocator_traits<allocator_type>::is_always_equal::value &&
             is_nothrow_swappable<key_compare>::value); // C++17
@@ -368,6 +377,15 @@
     iterator  erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class C2>
+      void merge(multimap<Key, T, C2, Allocator>& source);    // C++17
+    template<class C2>
+      void merge(multimap<Key, T, C2, Allocator>&& source);   // C++17
+    template<class C2>
+      void merge(map<Key, T, C2, Allocator>& source);         // C++17
+    template<class C2>
+      void merge(map<Key, T, C2, Allocator>&& source);        // C++17
+
     void swap(multimap& m)
         noexcept(allocator_traits<allocator_type>::is_always_equal::value &&
             is_nothrow_swappable<key_compare>::value); // C++17
@@ -925,6 +943,8 @@
     typedef __insert_return_type<iterator, node_type> insert_return_type;
 #endif
 
+    _LIBCPP_INLINE_VISIBILITY __base& __get_tree() { return __tree_; }
+
     _LIBCPP_INLINE_VISIBILITY
     map()
         _NOEXCEPT_(
@@ -1299,6 +1319,38 @@
     {
         return __tree_.template __node_handle_extract<node_type>(__it.__i_);
     }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(map<key_type, mapped_type, _C2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__get_tree());
+    }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(map<key_type, mapped_type, _C2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__get_tree());
+    }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multimap<key_type, mapped_type, _C2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__get_tree());
+    }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multimap<key_type, mapped_type, _C2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__get_tree());
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
@@ -1614,6 +1666,8 @@
     typedef __map_node_handle<typename __base::__node, allocator_type> node_type;
 #endif
 
+    _LIBCPP_INLINE_VISIBILITY __base& __get_tree() { return __tree_; }
+
     _LIBCPP_INLINE_VISIBILITY
     multimap()
         _NOEXCEPT_(
@@ -1881,6 +1935,38 @@
         return __tree_.template __node_handle_extract<node_type>(
             __it.__i_);
     }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multimap<key_type, mapped_type, _C2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __tree_.__node_handle_merge_multi(__source.__get_tree());
+    }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multimap<key_type, mapped_type, _C2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __tree_.__node_handle_merge_multi(__source.__get_tree());
+    }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(map<key_type, mapped_type, _C2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __tree_.__node_handle_merge_multi(__source.__get_tree());
+    }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(map<key_type, mapped_type, _C2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __tree_.__node_handle_merge_multi(__source.__get_tree());
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
Index: libcxx/include/set
===================================================================
--- libcxx/include/set
+++ libcxx/include/set
@@ -128,6 +128,15 @@
     iterator  erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class C2>
+      void merge(set<Key, C2, Allocator>& source);         // C++17
+    template<class C2>
+      void merge(set<Key, C2, Allocator>&& source);        // C++17
+    template<class C2>
+      void merge(multiset<Key, C2, Allocator>& source);    // C++17
+    template<class C2>
+      void merge(multiset<Key, C2, Allocator>&& source);   // C++17
+
     void swap(set& s)
         noexcept(
             __is_nothrow_swappable<key_compare>::value &&
@@ -316,6 +325,15 @@
     iterator  erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class C2>
+      void merge(multiset<Key, C2, Allocator>& source);    // C++17
+    template<class C2>
+      void merge(multiset<Key, C2, Allocator>&& source);   // C++17
+    template<class C2>
+      void merge(set<Key, C2, Allocator>& source);         // C++17
+    template<class C2>
+      void merge(set<Key, C2, Allocator>&& source);        // C++17
+
     void swap(multiset& s)
         noexcept(
             __is_nothrow_swappable<key_compare>::value &&
@@ -409,6 +427,9 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+template <class _Key, class _Compare, class _Allocator>
+class multiset;
+
 template <class _Key, class _Compare = less<_Key>,
           class _Allocator = allocator<_Key> >
 class _LIBCPP_TEMPLATE_VIS set
@@ -448,6 +469,8 @@
     typedef __insert_return_type<iterator, node_type> insert_return_type;
 #endif
 
+    _LIBCPP_INLINE_VISIBILITY __base& __get_tree() { return __tree_; }
+
     _LIBCPP_INLINE_VISIBILITY
     set()
         _NOEXCEPT_(
@@ -680,6 +703,38 @@
     {
         return __tree_.template __node_handle_extract<node_type>(__it);
     }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(set<key_type, _C2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__get_tree());
+    }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(set<key_type, _C2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__get_tree());
+    }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multiset<key_type, _C2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__get_tree());
+    }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multiset<key_type, _C2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_unique(__source.__get_tree());
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
@@ -890,6 +945,8 @@
     typedef __set_node_handle<typename __base::__node, allocator_type> node_type;
 #endif
 
+    _LIBCPP_INLINE_VISIBILITY __base& __get_tree() { return __tree_; }
+
     // construct/copy/destroy:
     _LIBCPP_INLINE_VISIBILITY
     multiset()
@@ -1121,6 +1178,38 @@
     {
         return __tree_.template __node_handle_extract<node_type>(__it);
     }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multiset<key_type, _C2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_multi(__source.__get_tree());
+    }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(multiset<key_type, _C2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_multi(__source.__get_tree());
+    }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(set<key_type, _C2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_multi(__source.__get_tree());
+    }
+    template <class _C2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(set<key_type, _C2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __tree_.__node_handle_merge_multi(__source.__get_tree());
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
Index: libcxx/include/unordered_map
===================================================================
--- libcxx/include/unordered_map
+++ libcxx/include/unordered_map
@@ -153,6 +153,15 @@
     iterator erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class H2, class P2>
+      void merge(unordered_map<Key, T, H2, P2, Allocator>& source);         // C++17
+    template<class H2, class P2>
+      void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);        // C++17
+    template<class H2, class P2>
+      void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);    // C++17
+    template<class H2, class P2>
+      void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);   // C++17
+
     void swap(unordered_map&)
         noexcept(
             (!allocator_type::propagate_on_container_swap::value ||
@@ -325,6 +334,15 @@
     iterator erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class H2, class P2>
+      void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);    // C++17
+    template<class H2, class P2>
+      void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);   // C++17
+    template<class H2, class P2>
+      void merge(unordered_map<Key, T, H2, P2, Allocator>& source);         // C++17
+    template<class H2, class P2>
+      void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);        // C++17
+
     void swap(unordered_multimap&)
         noexcept(
             (!allocator_type::propagate_on_container_swap::value ||
@@ -807,6 +825,9 @@
     template <class> friend class _LIBCPP_TEMPLATE_VIS __hash_const_local_iterator;
 };
 
+template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
+class unordered_multimap;
+
 template <class _Key, class _Tp, class _Hash = hash<_Key>, class _Pred = equal_to<_Key>,
           class _Alloc = allocator<pair<const _Key, _Tp> > >
 class _LIBCPP_TEMPLATE_VIS unordered_map
@@ -864,6 +885,8 @@
     typedef __insert_return_type<iterator, node_type> insert_return_type;
 #endif
 
+    _LIBCPP_INLINE_VISIBILITY __table& __get_table() { return __table_; }
+
     _LIBCPP_INLINE_VISIBILITY
     unordered_map()
         _NOEXCEPT_(is_nothrow_default_constructible<__table>::value)
@@ -1187,6 +1210,39 @@
         return __table_.template __node_handle_extract<node_type>(
             __it.__i_);
     }
+
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_map<key_type, mapped_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_unique(__source.__get_table());
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_map<key_type, mapped_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_unique(__source.__get_table());
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multimap<key_type, mapped_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_unique(__source.__get_table());
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multimap<key_type, mapped_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_unique(__source.__get_table());
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
@@ -1645,6 +1701,8 @@
     typedef __map_node_handle<__node, allocator_type> node_type;
 #endif
 
+    _LIBCPP_INLINE_VISIBILITY __table& __get_table() { return __table_; }
+
     _LIBCPP_INLINE_VISIBILITY
     unordered_multimap()
         _NOEXCEPT_(is_nothrow_default_constructible<__table>::value)
@@ -1846,6 +1904,39 @@
         return __table_.template __node_handle_extract<node_type>(
             __it.__i_);
     }
+
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multimap<key_type, mapped_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__get_table());
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multimap<key_type, mapped_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__get_table());
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_map<key_type, mapped_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__get_table());
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_map<key_type, mapped_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__get_table());
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
Index: libcxx/include/unordered_set
===================================================================
--- libcxx/include/unordered_set
+++ libcxx/include/unordered_set
@@ -127,6 +127,15 @@
     iterator erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class H2, class P2>
+      void merge(unordered_set<Key, H2, P2, Allocator>& source);         // C++17
+    template<class H2, class P2>
+      void merge(unordered_set<Key, H2, P2, Allocator>&& source);        // C++17
+    template<class H2, class P2>
+      void merge(unordered_multiset<Key, H2, P2, Allocator>& source);    // C++17
+    template<class H2, class P2>
+      void merge(unordered_multiset<Key, H2, P2, Allocator>&& source);   // C++17
+
     void swap(unordered_set&)
        noexcept(allocator_traits<Allocator>::is_always_equal::value &&
                  noexcept(swap(declval<hasher&>(), declval<hasher&>())) &&
@@ -282,6 +291,15 @@
     iterator erase(const_iterator first, const_iterator last);
     void clear() noexcept;
 
+    template<class H2, class P2>
+      void merge(unordered_multiset<Key, H2, P2, Allocator>& source);    // C++17
+    template<class H2, class P2>
+      void merge(unordered_multiset<Key, H2, P2, Allocator>&& source);   // C++17
+    template<class H2, class P2>
+      void merge(unordered_set<Key, H2, P2, Allocator>& source);         // C++17
+    template<class H2, class P2>
+      void merge(unordered_set<Key, H2, P2, Allocator>&& source);        // C++17
+
     void swap(unordered_multiset&)
        noexcept(allocator_traits<Allocator>::is_always_equal::value &&
                  noexcept(swap(declval<hasher&>(), declval<hasher&>())) &&
@@ -347,6 +365,9 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+template <class _Value, class _Hash, class _Pred, class _Alloc>
+class unordered_multiset;
+
 template <class _Value, class _Hash = hash<_Value>, class _Pred = equal_to<_Value>,
           class _Alloc = allocator<_Value> >
 class _LIBCPP_TEMPLATE_VIS unordered_set
@@ -384,6 +405,8 @@
     typedef __insert_return_type<iterator, node_type> insert_return_type;
 #endif
 
+    _LIBCPP_INLINE_VISIBILITY __table& __get_table() { return __table_; }
+
     _LIBCPP_INLINE_VISIBILITY
     unordered_set()
         _NOEXCEPT_(is_nothrow_default_constructible<__table>::value)
@@ -589,6 +612,39 @@
     {
         return __table_.template __node_handle_extract<node_type>(__it);
     }
+
+    template<class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_set<key_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __table_.__node_handle_merge_unique(__source.__get_table());
+    }
+    template<class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_set<key_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __table_.__node_handle_merge_unique(__source.__get_table());
+    }
+    template<class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multiset<key_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __table_.__node_handle_merge_unique(__source.__get_table());
+    }
+    template<class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multiset<key_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        __table_.__node_handle_merge_unique(__source.__get_table());
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
@@ -937,6 +993,8 @@
     typedef __set_node_handle<typename __table::__node, allocator_type> node_type;
 #endif
 
+    _LIBCPP_INLINE_VISIBILITY __table& __get_table() { return __table_; }
+
     _LIBCPP_INLINE_VISIBILITY
     unordered_multiset()
         _NOEXCEPT_(is_nothrow_default_constructible<__table>::value)
@@ -1101,6 +1159,39 @@
     {
         return __table_.template __node_handle_extract<node_type>(__key);
     }
+
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multiset<key_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__get_table());
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_multiset<key_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__get_table());
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_set<key_type, _H2, _P2, allocator_type>& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__get_table());
+    }
+    template <class _H2, class _P2>
+    _LIBCPP_INLINE_VISIBILITY
+    void merge(unordered_set<key_type, _H2, _P2, allocator_type>&& __source)
+    {
+        _LIBCPP_ASSERT(__source.get_allocator() == get_allocator(),
+                       "merging container with incompatible allocator");
+        return __table_.__node_handle_merge_multi(__source.__get_table());
+    }
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
Index: libcxx/test/std/containers/associative/map/map.modifiers/merge.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/containers/associative/map/map.modifiers/merge.pass.cpp
@@ -0,0 +1,125 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// <map>
+
+// class map
+
+// template <class C2>
+//   void merge(map<key_type, C2, allocator_type>& source);
+
+#include <map>
+#include "test_macros.h"
+#include "Counter.h"
+
+template <class Map>
+bool map_equal(const Map& map, Map other)
+{
+    return map == other;
+}
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+struct throw_comparator
+{
+    bool& should_throw_;
+
+    throw_comparator(bool& should_throw) : should_throw_(should_throw) {}
+
+    template <class T>
+    bool operator()(const T& lhs, const T& rhs) const
+    {
+        if (should_throw_)
+            throw 0;
+        return lhs < rhs;
+    }
+};
+#endif
+
+int main()
+{
+    {
+        std::map<int, int> src{{1, 0}, {3, 0}, {5, 0}};
+        std::map<int, int> dst{{2, 0}, {4, 0}, {5, 0}};
+        dst.merge(src);
+        assert(map_equal(src, {{5,0}}));
+        assert(map_equal(dst, {{1, 0}, {2, 0}, {3, 0}, {4, 0}, {5, 0}}));
+    }
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        bool do_throw = false;
+        typedef std::map<Counter<int>, int, throw_comparator> map_type;
+        map_type src({{1, 0}, {3, 0}, {5, 0}}, throw_comparator(do_throw));
+        map_type dst({{2, 0}, {4, 0}, {5, 0}}, throw_comparator(do_throw));
+
+        assert(Counter_base::gConstructed == 6);
+
+        do_throw = true;
+        try
+        {
+            dst.merge(src);
+        }
+        catch (int)
+        {
+            do_throw = false;
+        }
+        assert(!do_throw);
+        assert(map_equal(src, map_type({{1, 0}, {3, 0}, {5, 0}}, throw_comparator(do_throw))));
+        assert(map_equal(dst, map_type({{2, 0}, {4, 0}, {5, 0}}, throw_comparator(do_throw))));
+    }
+#endif
+    assert(Counter_base::gConstructed == 0);
+    struct comparator
+    {
+        comparator() = default;
+
+        bool operator()(const Counter<int>& lhs, const Counter<int>& rhs) const
+        {
+            return lhs < rhs;
+        }
+    };
+    {
+        typedef std::map<Counter<int>, int, std::less<Counter<int>>> first_map_type;
+        typedef std::map<Counter<int>, int, comparator> second_map_type;
+        typedef std::multimap<Counter<int>, int, comparator> third_map_type;
+
+        first_map_type first{{1, 0}, {2, 0}, {3, 0}};
+        second_map_type second{{2, 0}, {3, 0}, {4, 0}};
+        third_map_type third{{1, 0}, {3, 0}};
+
+        assert(Counter_base::gConstructed == 8);
+
+        first.merge(second);
+        first.merge(std::move(second));
+        first.merge(third);
+        first.merge(std::move(third));
+
+        assert(map_equal(first, {{1, 0}, {2, 0}, {3, 0}, {4, 0}}));
+        assert(map_equal(second, {{2, 0}, {3, 0}}));
+        assert(map_equal(third, {{1, 0}, {3, 0}}));
+
+        assert(Counter_base::gConstructed == 8);
+    }
+    assert(Counter_base::gConstructed == 0);
+    {
+        std::map<int, int> first;
+        {
+            std::map<int, int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+        {
+            std::multimap<int, int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+    }
+}
Index: libcxx/test/std/containers/associative/multimap/multimap.modifiers/merge.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/containers/associative/multimap/multimap.modifiers/merge.pass.cpp
@@ -0,0 +1,125 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// <map>
+
+// class multimap
+
+// template <class C2>
+//   void merge(multimap<key_type, C2, allocator_type>& source);
+
+#include <map>
+#include "test_macros.h"
+#include "Counter.h"
+
+template <class Map>
+bool map_equal(const Map& map, Map other)
+{
+    return map == other;
+}
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+struct throw_comparator
+{
+    bool& should_throw_;
+
+    throw_comparator(bool& should_throw) : should_throw_(should_throw) {}
+
+    template <class T>
+    bool operator()(const T& lhs, const T& rhs) const
+    {
+        if (should_throw_)
+            throw 0;
+        return lhs < rhs;
+    }
+};
+#endif
+
+int main()
+{
+    {
+        std::multimap<int, int> src{{1, 0}, {3, 0}, {5, 0}};
+        std::multimap<int, int> dst{{2, 0}, {4, 0}, {5, 0}};
+        dst.merge(src);
+        assert(map_equal(src, {}));
+        assert(map_equal(dst, {{1, 0}, {2, 0}, {3, 0}, {4, 0}, {5, 0}, {5, 0}}));
+    }
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        bool do_throw = false;
+        typedef std::multimap<Counter<int>, int, throw_comparator> map_type;
+        map_type src({{1, 0}, {3, 0}, {5, 0}}, throw_comparator(do_throw));
+        map_type dst({{2, 0}, {4, 0}, {5, 0}}, throw_comparator(do_throw));
+
+        assert(Counter_base::gConstructed == 6);
+
+        do_throw = true;
+        try
+        {
+            dst.merge(src);
+        }
+        catch (int)
+        {
+            do_throw = false;
+        }
+        assert(!do_throw);
+        assert(map_equal(src, map_type({{1, 0}, {3, 0}, {5, 0}}, throw_comparator(do_throw))));
+        assert(map_equal(dst, map_type({{2, 0}, {4, 0}, {5, 0}}, throw_comparator(do_throw))));
+    }
+#endif
+    assert(Counter_base::gConstructed == 0);
+    struct comparator
+    {
+        comparator() = default;
+
+        bool operator()(const Counter<int>& lhs, const Counter<int>& rhs) const
+        {
+            return lhs < rhs;
+        }
+    };
+    {
+        typedef std::multimap<Counter<int>, int, std::less<Counter<int>>> first_map_type;
+        typedef std::multimap<Counter<int>, int, comparator> second_map_type;
+        typedef std::map<Counter<int>, int, comparator> third_map_type;
+
+        first_map_type first{{1, 0}, {2, 0}, {3, 0}};
+        second_map_type second{{2, 0}, {3, 0}, {4, 0}};
+        third_map_type third{{1, 0}, {3, 0}};
+
+        assert(Counter_base::gConstructed == 8);
+
+        first.merge(second);
+        first.merge(std::move(second));
+        first.merge(third);
+        first.merge(std::move(third));
+
+        assert(map_equal(first, {{1, 0}, {1, 0}, {2, 0}, {2, 0}, {3, 0}, {3, 0}, {3, 0}, {4, 0}}));
+        assert(map_equal(second, {}));
+        assert(map_equal(third, {}));
+
+        assert(Counter_base::gConstructed == 8);
+    }
+    assert(Counter_base::gConstructed == 0);
+    {
+        std::multimap<int, int> first;
+        {
+            std::multimap<int, int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+        {
+            std::multimap<int, int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+    }
+}
Index: libcxx/test/std/containers/associative/multiset/merge.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/containers/associative/multiset/merge.pass.cpp
@@ -0,0 +1,134 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// <set>
+
+// class multiset
+
+// template <class C2>
+//   void merge(multiset<Key, C2, Allocator>& source);
+//
+// template <class C2>
+//   void merge(multiset<Key, C2, Allocator>&& source);
+//
+// template <class C2>
+//   void merge(set<Key, C2, Allocator>& source);
+//
+// template <class C2>
+//   void merge(set<Key, C2, Allocator>&& source);
+
+#include <set>
+#include "test_macros.h"
+#include "Counter.h"
+
+template <class Set>
+bool set_equal(const Set& set, Set other)
+{
+    return set == other;
+}
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+struct throw_comparator
+{
+    bool& should_throw_;
+
+    throw_comparator(bool& should_throw) : should_throw_(should_throw) {}
+
+    template <class T>
+    bool operator()(const T& lhs, const T& rhs) const
+    {
+        if (should_throw_)
+            throw 0;
+        return lhs < rhs;
+    }
+};
+#endif
+
+int main()
+{
+    {
+        std::multiset<int> src{1, 3, 5};
+        std::multiset<int> dst{2, 4, 5};
+        dst.merge(src);
+        assert(set_equal(src, {}));
+        assert(set_equal(dst, {1, 2, 3, 4, 5, 5}));
+    }
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        bool do_throw = false;
+        typedef std::multiset<Counter<int>, throw_comparator> set_type;
+        set_type src({1, 3, 5}, throw_comparator(do_throw));
+        set_type dst({2, 4, 5}, throw_comparator(do_throw));
+
+        assert(Counter_base::gConstructed == 6);
+
+        do_throw = true;
+        try
+        {
+            dst.merge(src);
+        }
+        catch (int)
+        {
+            do_throw = false;
+        }
+        assert(!do_throw);
+        assert(set_equal(src, set_type({1, 3, 5}, throw_comparator(do_throw))));
+        assert(set_equal(dst, set_type({2, 4, 5}, throw_comparator(do_throw))));
+    }
+#endif
+    assert(Counter_base::gConstructed == 0);
+    struct comparator
+    {
+        comparator() = default;
+
+        bool operator()(const Counter<int>& lhs, const Counter<int>& rhs) const
+        {
+            return lhs < rhs;
+        }
+    };
+    {
+        typedef std::multiset<Counter<int>, std::less<Counter<int>>> first_set_type;
+        typedef std::multiset<Counter<int>, comparator> second_set_type;
+        typedef std::set<Counter<int>, comparator> third_set_type;
+
+        first_set_type first{1, 2, 3};
+        second_set_type second{2, 3, 4};
+        third_set_type third{1, 3};
+
+        assert(Counter_base::gConstructed == 8);
+
+        first.merge(second);
+        first.merge(std::move(second));
+        first.merge(third);
+        first.merge(std::move(third));
+
+        assert(set_equal(first, {1, 1, 2, 2, 3, 3, 3, 4}));
+        assert(set_equal(second, {}));
+        assert(set_equal(third, {}));
+
+        assert(Counter_base::gConstructed == 8);
+    }
+    assert(Counter_base::gConstructed == 0);
+    {
+        std::multiset<int> first;
+        {
+            std::multiset<int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+        {
+            std::set<int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+    }
+}
Index: libcxx/test/std/containers/associative/set/merge.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/containers/associative/set/merge.pass.cpp
@@ -0,0 +1,125 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// <set>
+
+// class set
+
+// template <class C2>
+//   void merge(set<key_type, C2, allocator_type>& source);
+
+#include <set>
+#include "test_macros.h"
+#include "Counter.h"
+
+template <class Set>
+bool set_equal(const Set& set, Set other)
+{
+    return set == other;
+}
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+struct throw_comparator
+{
+    bool& should_throw_;
+
+    throw_comparator(bool& should_throw) : should_throw_(should_throw) {}
+
+    template <class T>
+    bool operator()(const T& lhs, const T& rhs) const
+    {
+        if (should_throw_)
+            throw 0;
+        return lhs < rhs;
+    }
+};
+#endif
+
+int main()
+{
+    {
+        std::set<int> src{1, 3, 5};
+        std::set<int> dst{2, 4, 5};
+        dst.merge(src);
+        assert(set_equal(src, {5}));
+        assert(set_equal(dst, {1, 2, 3, 4, 5}));
+    }
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        bool do_throw = false;
+        typedef std::set<Counter<int>, throw_comparator> set_type;
+        set_type src({1, 3, 5}, throw_comparator(do_throw));
+        set_type dst({2, 4, 5}, throw_comparator(do_throw));
+
+        assert(Counter_base::gConstructed == 6);
+
+        do_throw = true;
+        try
+        {
+            dst.merge(src);
+        }
+        catch (int)
+        {
+            do_throw = false;
+        }
+        assert(!do_throw);
+        assert(set_equal(src, set_type({1, 3, 5}, throw_comparator(do_throw))));
+        assert(set_equal(dst, set_type({2, 4, 5}, throw_comparator(do_throw))));
+    }
+#endif
+    assert(Counter_base::gConstructed == 0);
+    struct comparator
+    {
+        comparator() = default;
+
+        bool operator()(const Counter<int>& lhs, const Counter<int>& rhs) const
+        {
+            return lhs < rhs;
+        }
+    };
+    {
+        typedef std::set<Counter<int>, std::less<Counter<int>>> first_set_type;
+        typedef std::set<Counter<int>, comparator> second_set_type;
+        typedef std::multiset<Counter<int>, comparator> third_set_type;
+
+        first_set_type first{1, 2, 3};
+        second_set_type second{2, 3, 4};
+        third_set_type third{1, 3};
+
+        assert(Counter_base::gConstructed == 8);
+
+        first.merge(second);
+        first.merge(std::move(second));
+        first.merge(third);
+        first.merge(std::move(third));
+
+        assert(set_equal(first, {1, 2, 3, 4}));
+        assert(set_equal(second, {2, 3}));
+        assert(set_equal(third, {1, 3}));
+
+        assert(Counter_base::gConstructed == 8);
+    }
+    assert(Counter_base::gConstructed == 0);
+    {
+        std::set<int> first;
+        {
+            std::set<int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+        {
+            std::multiset<int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+    }
+}
Index: libcxx/test/std/containers/unord/unord.map/unord.map.modifiers/merge.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/containers/unord/unord.map/unord.map.modifiers/merge.pass.cpp
@@ -0,0 +1,138 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// <unordered_map>
+
+// class unordered_map
+
+// template <class C2>
+//   void merge(unordered_map<key_type, C2, allocator_type>& source);
+
+#include <unordered_map>
+#include "test_macros.h"
+#include "Counter.h"
+
+template <class Map>
+bool map_equal(const Map& map, Map other)
+{
+    return map == other;
+}
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+template <class T>
+struct throw_hasher
+{
+    bool& should_throw_;
+
+    throw_hasher(bool& should_throw) : should_throw_(should_throw) {}
+
+    typedef size_t result_type;
+    typedef T argument_type;
+
+    size_t operator()(const T& p) const
+    {
+        if (should_throw_)
+            throw 0;
+        return std::hash<T>()(p);
+    }
+};
+#endif
+
+int main()
+{
+    {
+        std::unordered_map<int, int> src{{1, 0}, {3, 0}, {5, 0}};
+        std::unordered_map<int, int> dst{{2, 0}, {4, 0}, {5, 0}};
+        dst.merge(src);
+        assert(map_equal(src, {{5,0}}));
+        assert(map_equal(dst, {{1, 0}, {2, 0}, {3, 0}, {4, 0}, {5, 0}}));
+    }
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        bool do_throw = false;
+        typedef std::unordered_map<Counter<int>, int, throw_hasher<Counter<int>>> map_type;
+        map_type src({{1, 0}, {3, 0}, {5, 0}}, 0, throw_hasher<Counter<int>>(do_throw));
+        map_type dst({{2, 0}, {4, 0}, {5, 0}}, 0, throw_hasher<Counter<int>>(do_throw));
+
+        assert(Counter_base::gConstructed == 6);
+
+        do_throw = true;
+        try
+        {
+            dst.merge(src);
+        }
+        catch (int)
+        {
+            do_throw = false;
+        }
+        assert(!do_throw);
+        assert(map_equal(src, map_type({{1, 0}, {3, 0}, {5, 0}}, 0, throw_hasher<Counter<int>>(do_throw))));
+        assert(map_equal(dst, map_type({{2, 0}, {4, 0}, {5, 0}}, 0, throw_hasher<Counter<int>>(do_throw))));
+    }
+#endif
+    assert(Counter_base::gConstructed == 0);
+    struct equal
+    {
+        equal() = default;
+
+        bool operator()(const Counter<int>& lhs, const Counter<int>& rhs) const
+        {
+            return lhs == rhs;
+        }
+    };
+    struct hasher
+    {
+        hasher() = default;
+        typedef Counter<int> argument_type;
+        typedef size_t result_type;
+        size_t operator()(const Counter<int>& p) const
+        {
+            return std::hash<Counter<int>>()(p);
+        }
+    };
+    {
+        typedef std::unordered_map<Counter<int>, int, std::hash<Counter<int>>, std::equal_to<Counter<int>>> first_map_type;
+        typedef std::unordered_map<Counter<int>, int, hasher, equal> second_map_type;
+        typedef std::unordered_multimap<Counter<int>, int, hasher, equal> third_map_type;
+
+        first_map_type first{{1, 0}, {2, 0}, {3, 0}};
+        second_map_type second{{2, 0}, {3, 0}, {4, 0}};
+        third_map_type third{{1, 0}, {3, 0}};
+
+        assert(Counter_base::gConstructed == 8);
+
+        first.merge(second);
+        first.merge(std::move(second));
+        first.merge(third);
+        first.merge(std::move(third));
+
+        assert(map_equal(first, {{1, 0}, {2, 0}, {3, 0}, {4, 0}}));
+        assert(map_equal(second, {{2, 0}, {3, 0}}));
+        assert(map_equal(third, {{1, 0}, {3, 0}}));
+
+        assert(Counter_base::gConstructed == 8);
+    }
+    assert(Counter_base::gConstructed == 0);
+    {
+        std::unordered_map<int, int> first;
+        {
+            std::unordered_map<int, int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+        {
+            std::unordered_multimap<int, int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+    }
+}
Index: libcxx/test/std/containers/unord/unord.multimap/unord.multimap.modifiers/merge.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/containers/unord/unord.multimap/unord.multimap.modifiers/merge.pass.cpp
@@ -0,0 +1,138 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// <unordered_map>
+
+// class unordered_multimap
+
+// template <class C2>
+//   void merge(unordered_multimap<key_type, C2, allocator_type>& source);
+
+#include <unordered_map>
+#include "test_macros.h"
+#include "Counter.h"
+
+template <class Map>
+bool map_equal(const Map& map, Map other)
+{
+    return map == other;
+}
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+template <class T>
+struct throw_hasher
+{
+    bool& should_throw_;
+
+    throw_hasher(bool& should_throw) : should_throw_(should_throw) {}
+
+    typedef size_t result_type;
+    typedef T argument_type;
+
+    size_t operator()(const T& p) const
+    {
+        if (should_throw_)
+            throw 0;
+        return std::hash<T>()(p);
+    }
+};
+#endif
+
+int main()
+{
+    {
+        std::unordered_multimap<int, int> src{{1, 0}, {3, 0}, {5, 0}};
+        std::unordered_multimap<int, int> dst{{2, 0}, {4, 0}, {5, 0}};
+        dst.merge(src);
+        assert(map_equal(src, {}));
+        assert(map_equal(dst, {{1, 0}, {2, 0}, {3, 0}, {4, 0}, {5, 0}, {5, 0}}));
+    }
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        bool do_throw = false;
+        typedef std::unordered_multimap<Counter<int>, int, throw_hasher<Counter<int>>> map_type;
+        map_type src({{1, 0}, {3, 0}, {5, 0}}, 0, throw_hasher<Counter<int>>(do_throw));
+        map_type dst({{2, 0}, {4, 0}, {5, 0}}, 0, throw_hasher<Counter<int>>(do_throw));
+
+        assert(Counter_base::gConstructed == 6);
+
+        do_throw = true;
+        try
+        {
+            dst.merge(src);
+        }
+        catch (int)
+        {
+            do_throw = false;
+        }
+        assert(!do_throw);
+        assert(map_equal(src, map_type({{1, 0}, {3, 0}, {5, 0}}, 0, throw_hasher<Counter<int>>(do_throw))));
+        assert(map_equal(dst, map_type({{2, 0}, {4, 0}, {5, 0}}, 0, throw_hasher<Counter<int>>(do_throw))));
+    }
+#endif
+    assert(Counter_base::gConstructed == 0);
+    struct equal
+    {
+        equal() = default;
+
+        bool operator()(const Counter<int>& lhs, const Counter<int>& rhs) const
+        {
+            return lhs == rhs;
+        }
+    };
+    struct hasher
+    {
+        hasher() = default;
+        typedef Counter<int> argument_type;
+        typedef size_t result_type;
+        size_t operator()(const Counter<int>& p) const
+        {
+            return std::hash<Counter<int>>()(p);
+        }
+    };
+    {
+        typedef std::unordered_multimap<Counter<int>, int, std::hash<Counter<int>>, std::equal_to<Counter<int>>> first_map_type;
+        typedef std::unordered_multimap<Counter<int>, int, hasher, equal> second_map_type;
+        typedef std::unordered_map<Counter<int>, int, hasher, equal> third_map_type;
+
+        first_map_type first{{1, 0}, {2, 0}, {3, 0}};
+        second_map_type second{{2, 0}, {3, 0}, {4, 0}};
+        third_map_type third{{1, 0}, {3, 0}};
+
+        assert(Counter_base::gConstructed == 8);
+
+        first.merge(second);
+        first.merge(std::move(second));
+        first.merge(third);
+        first.merge(std::move(third));
+
+        assert(map_equal(first, {{1, 0}, {2, 0}, {3, 0}, {4, 0}, {2, 0}, {3, 0}, {1, 0}, {3, 0}}));
+        assert(map_equal(second, {}));
+        assert(map_equal(third, {}));
+
+        assert(Counter_base::gConstructed == 8);
+    }
+    assert(Counter_base::gConstructed == 0);
+    {
+        std::unordered_multimap<int, int> first;
+        {
+            std::unordered_multimap<int, int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+        {
+            std::unordered_map<int, int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+    }
+}
Index: libcxx/test/std/containers/unord/unord.multiset/merge.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/containers/unord/unord.multiset/merge.pass.cpp
@@ -0,0 +1,135 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// <unordered_set>
+
+// class unordered_multiset
+
+// template <class C2>
+//   void merge(unordered_multiset<key_type, C2, allocator_type>& source);
+
+#include <unordered_set>
+#include "test_macros.h"
+#include "Counter.h"
+
+template <class Set>
+bool set_equal(const Set& set, Set other)
+{
+    return set == other;
+}
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+template <class T>
+struct throw_hasher
+{
+    bool& should_throw_;
+
+    throw_hasher(bool& should_throw) : should_throw_(should_throw) {}
+
+    typedef size_t result_type;
+    typedef T argument_type;
+
+    size_t operator()(const T& p) const
+    {
+        if (should_throw_)
+            throw 0;
+        return std::hash<T>()(p);
+    }
+};
+#endif
+
+int main()
+{
+    {
+        std::unordered_multiset<int> src{1, 3, 5};
+        std::unordered_multiset<int> dst{2, 4, 5};
+        dst.merge(src);
+        assert(set_equal(src, {}));
+        assert(set_equal(dst, {1, 2, 3, 4, 5, 5}));
+    }
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        bool do_throw = false;
+        typedef std::unordered_multiset<Counter<int>, throw_hasher<Counter<int>>> set_type;
+        set_type src({1, 3, 5}, 0, throw_hasher<Counter<int>>(do_throw));
+        set_type dst({2, 4, 5}, 0, throw_hasher<Counter<int>>(do_throw));
+
+        assert(Counter_base::gConstructed == 6);
+
+        do_throw = true;
+        try
+        {
+            dst.merge(src);
+        }
+        catch (int)
+        {
+            do_throw = false;
+        }
+        assert(!do_throw);
+        assert(set_equal(src, set_type({1, 3, 5}, 0, throw_hasher<Counter<int>>(do_throw))));
+        assert(set_equal(dst, set_type({2, 4, 5}, 0, throw_hasher<Counter<int>>(do_throw))));
+    }
+#endif
+    assert(Counter_base::gConstructed == 0);
+    struct equal
+    {
+        equal() = default;
+
+        bool operator()(const Counter<int>& lhs, const Counter<int>& rhs) const
+        {
+            return lhs == rhs;
+        }
+    };
+    struct hasher
+    {
+        hasher() = default;
+        typedef Counter<int> argument_type;
+        typedef size_t result_type;
+        size_t operator()(const Counter<int>& p) const { return std::hash<Counter<int>>()(p); }
+    };
+    {
+        typedef std::unordered_multiset<Counter<int>, std::hash<Counter<int>>, std::equal_to<Counter<int>>> first_set_type;
+        typedef std::unordered_multiset<Counter<int>, hasher, equal> second_set_type;
+        typedef std::unordered_set<Counter<int>, hasher, equal> third_set_type;
+
+        first_set_type first{1, 2, 3};
+        second_set_type second{2, 3, 4};
+        third_set_type third{1, 3};
+
+        assert(Counter_base::gConstructed == 8);
+
+        first.merge(second);
+        first.merge(std::move(second));
+        first.merge(third);
+        first.merge(std::move(third));
+
+        assert(set_equal(first, {1, 2, 3, 4, 2, 3, 1, 3}));
+        assert(set_equal(second, {}));
+        assert(set_equal(third, {}));
+
+        assert(Counter_base::gConstructed == 8);
+    }
+    assert(Counter_base::gConstructed == 0);
+    {
+        std::unordered_multiset<int> first;
+        {
+            std::unordered_multiset<int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+        {
+            std::unordered_set<int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+    }
+}
Index: libcxx/test/std/containers/unord/unord.set/merge.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/containers/unord/unord.set/merge.pass.cpp
@@ -0,0 +1,135 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// <unordered_set>
+
+// class unordered_set
+
+// template <class C2>
+//   void merge(unordered_set<key_type, C2, allocator_type>& source);
+
+#include <unordered_set>
+#include "test_macros.h"
+#include "Counter.h"
+
+template <class Set>
+bool set_equal(const Set& set, Set other)
+{
+    return set == other;
+}
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+template <class T>
+struct throw_hasher
+{
+    bool& should_throw_;
+
+    throw_hasher(bool& should_throw) : should_throw_(should_throw) {}
+
+    typedef size_t result_type;
+    typedef T argument_type;
+
+    size_t operator()(const T& p) const
+    {
+        if (should_throw_)
+            throw 0;
+        return std::hash<T>()(p);
+    }
+};
+#endif
+
+int main()
+{
+    {
+        std::unordered_set<int> src{1, 3, 5};
+        std::unordered_set<int> dst{2, 4, 5};
+        dst.merge(src);
+        assert(set_equal(src, {5}));
+        assert(set_equal(dst, {1, 2, 3, 4, 5}));
+    }
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        bool do_throw = false;
+        typedef std::unordered_set<Counter<int>, throw_hasher<Counter<int>>> set_type;
+        set_type src({1, 3, 5}, 0, throw_hasher<Counter<int>>(do_throw));
+        set_type dst({2, 4, 5}, 0, throw_hasher<Counter<int>>(do_throw));
+
+        assert(Counter_base::gConstructed == 6);
+
+        do_throw = true;
+        try
+        {
+            dst.merge(src);
+        }
+        catch (int)
+        {
+            do_throw = false;
+        }
+        assert(!do_throw);
+        assert(set_equal(src, set_type({1, 3, 5}, 0, throw_hasher<Counter<int>>(do_throw))));
+        assert(set_equal(dst, set_type({2, 4, 5}, 0, throw_hasher<Counter<int>>(do_throw))));
+    }
+#endif
+    assert(Counter_base::gConstructed == 0);
+    struct equal
+    {
+        equal() = default;
+
+        bool operator()(const Counter<int>& lhs, const Counter<int>& rhs) const
+        {
+            return lhs == rhs;
+        }
+    };
+    struct hasher
+    {
+        hasher() = default;
+        typedef Counter<int> argument_type;
+        typedef size_t result_type;
+        size_t operator()(const Counter<int>& p) const { return std::hash<Counter<int>>()(p); }
+    };
+    {
+        typedef std::unordered_set<Counter<int>, std::hash<Counter<int>>, std::equal_to<Counter<int>>> first_set_type;
+        typedef std::unordered_set<Counter<int>, hasher, equal> second_set_type;
+        typedef std::unordered_multiset<Counter<int>, hasher, equal> third_set_type;
+
+        first_set_type first{1, 2, 3};
+        second_set_type second{2, 3, 4};
+        third_set_type third{1, 3};
+
+        assert(Counter_base::gConstructed == 8);
+
+        first.merge(second);
+        first.merge(std::move(second));
+        first.merge(third);
+        first.merge(std::move(third));
+
+        assert(set_equal(first, {1, 2, 3, 4}));
+        assert(set_equal(second, {2, 3}));
+        assert(set_equal(third, {1, 3}));
+
+        assert(Counter_base::gConstructed == 8);
+    }
+    assert(Counter_base::gConstructed == 0);
+    {
+        std::unordered_set<int> first;
+        {
+            std::unordered_set<int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+        {
+            std::unordered_multiset<int> second;
+            first.merge(second);
+            first.merge(std::move(second));
+        }
+    }
+}