[libc++] Fix __gnu_cxx::hash_multimap copy construction

libcxx/include/ext/hash_map

@@ -787,10 +787,7 @@ hash_multimap<_Key, _Tp, _Hash, _Pred, _Alloc>::hash_multimap(
 }
 
 template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
-hash_multimap<_Key, _Tp, _Hash, _Pred, _Alloc>::hash_multimap(const hash_multimap& __u) : __table_(__u.__table_) {
-  __table_.__rehash_multi(__u.bucket_count());
-  insert(__u.begin(), __u.end());
-}
+hash_multimap<_Key, _Tp, _Hash, _Pred, _Alloc>::hash_multimap(const hash_multimap& __u) : __table_(__u.__table_) {}
 
 template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
 template <class _InputIterator>

libcxx/test/extensions/gnu/hash_multimap/copy.pass.cpp

@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// ADDITIONAL_COMPILE_FLAGS: -Wno-deprecated
+
+// hash_multimap::hash_multimap(const hash_multimap&)
+
+#include <cassert>
+#include <ext/hash_map>
+
+int main(int, char**) {
+  __gnu_cxx::hash_multimap<int, int> map;
+
+  map.insert(std::make_pair(1, 1));
+  map.insert(std::make_pair(1, 1));
+
+  auto map2 = map;
+
+  assert(map2.size() == 2);
+
+  return 0;
+}