Fixed std::initializer_list assignment issues for open-addressing containers (#277)

* fixed #276

* used range insert, stylistic this->

* assigned non-empty std::initializer_lists

test/helpers/replace_allocator.hpp

@@ -0,0 +1,44 @@
+
+// Copyright 2024 Joaquin M Lopez Munz.
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or move at http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_UNORDERED_TEST_REPLACE_ALLOCATOR
+#define BOOST_UNORDERED_TEST_REPLACE_ALLOCATOR
+
+#include <boost/core/allocator_access.hpp> 
+#include <utility>
+
+namespace test {
+template <typename Container, typename Allocator>
+  struct replace_allocator_impl;
+
+  template <typename Container, typename Allocator>
+  using replace_allocator = 
+    typename replace_allocator_impl<Container, Allocator>::type;
+
+  template <
+    typename K, typename H, typename P, typename A,
+    template <typename, typename, typename, typename> class Set,
+    typename Allocator
+  >
+  struct replace_allocator_impl<Set<K, H, P, A>, Allocator>
+  {
+    using type = Set<
+      K, H, P, boost::allocator_rebind_t<Allocator, K> >;
+  };
+
+  template <
+    typename K, typename H, typename T, typename P, typename A,
+    template <typename, typename, typename, typename, typename> class Map,
+    typename Allocator
+  >
+  struct replace_allocator_impl<Map<K, T, H, P, A>, Allocator>
+  {
+    using type = Map<
+      K, T, H, P,
+      boost::allocator_rebind_t<Allocator, std::pair<K const, T> > >;
+  };
+} // namespace test
+
+#endif // !defined(BOOST_UNORDERED_TEST_REPLACE_ALLOCATOR)