Add transparent subscript

include/boost/unordered/unordered_flat_map.hpp

@@ -483,6 +483,15 @@ namespace boost {
         return table_.try_emplace(std::move(key)).first->second;
       }
 
+      template <class K>
+      typename std::enable_if<
+        boost::unordered::detail::are_transparent<K, hasher, key_equal>::value,
+        mapped_type&>::type
+      operator[](K&& key)
+      {
+        return table_.try_emplace(std::forward<K>(key)).first->second;
+      }
+
       BOOST_FORCEINLINE size_type count(key_type const& key) const
       {
         auto pos = table_.find(key);

include/boost/unordered/unordered_map.hpp

@@ -973,6 +973,10 @@ namespace boost {
 
       mapped_type& operator[](const key_type&);
       mapped_type& operator[](BOOST_RV_REF(key_type));
+      template <class Key>
+      typename boost::enable_if_c<detail::are_transparent<Key, H, P>::value,
+        mapped_type&>::type
+      operator[](BOOST_FWD_REF(Key) k);
       mapped_type& at(const key_type&);
       mapped_type const& at(const key_type&) const;
 
@@ -2215,6 +2219,15 @@ namespace boost {
       return table_.try_emplace_unique(boost::move(k)).first->second;
     }
 
+    template <class K, class T, class H, class P, class A>
+    template <class Key>
+    typename boost::enable_if_c<detail::are_transparent<Key, H, P>::value,
+      typename unordered_map<K, T, H, P, A>::mapped_type&>::type
+    unordered_map<K, T, H, P, A>::operator[](BOOST_FWD_REF(Key) k)
+    {
+      return table_.try_emplace_unique(boost::forward<Key>(k)).first->second;
+    }
+
     template <class K, class T, class H, class P, class A>
     typename unordered_map<K, T, H, P, A>::mapped_type&
     unordered_map<K, T, H, P, A>::at(const key_type& k)