[libc++] Fix __gnu_cxx::hash_multiset copy construction (llvm#160466)

Author: philnik777

libcxx/include/ext/hash_set

@@ -534,10 +534,7 @@ hash_multiset<_Value, _Hash, _Pred, _Alloc>::hash_multiset(
 }
 
 template <class _Value, class _Hash, class _Pred, class _Alloc>
-hash_multiset<_Value, _Hash, _Pred, _Alloc>::hash_multiset(const hash_multiset& __u) : __table_(__u.__table_) {
-  __table_.__rehash_multi(__u.bucket_count());
-  insert(__u.begin(), __u.end());
-}
+hash_multiset<_Value, _Hash, _Pred, _Alloc>::hash_multiset(const hash_multiset& __u) : __table_(__u.__table_) {}
 
 template <class _Value, class _Hash, class _Pred, class _Alloc>
 template <class _InputIterator>

libcxx/test/extensions/gnu/hash_multiset/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_multiset::hash_multiset(const hash_multiset&)
+
+#include <cassert>
+#include <ext/hash_set>
+
+int main(int, char**) {
+  __gnu_cxx::hash_multiset<int> set;
+
+  set.insert(1);
+  set.insert(1);
+
+  auto set2 = set;
+
+  assert(set2.size() == 2);
+
+  return 0;
+}