错误:形成指针指向引用类型'const std::pair&'…我无法理解这个错误。

I'm getting some errors when trying to use -> in an iterator type. When I dig in the library defining the iterator, it seems to me that everyhing is allright and that there is no reason for the error. Here is the code, part of boost::multi_array:I'm getting some errors when trying to use -> i