forward_as_tuple not working as expected

[Mogball]

The current code for map insert is

template<typename K, typename V>
pair<iterator, bool> insert(K &&key, V &&val) 
{ return m_table.insert_unique(make_tuple(forward<K>(key), forward<V>(val))); }

but if you replace it with

return m_table.insert_unique(forward_as_tuple(key, val));

insertion will create duplicates for dynamic types like dynamic_string