The concept of ForwardIterator is flawed because it mixed 2 sets of concepts (value access and traversal) into 1 package.
http://www.boost.org/doc/libs/1_65_1/libs/iterator/doc/new-iter-concepts.html
It requires value_type (const)& as return type when dereference is applied, which is not mandatory in spirit parsing. A return type which is convertible to value_type is good enough. ReadableIteratorConcept and ForwardTraversalConcept should be what we need for the iterator check.
For example, the iterator of the range returned by boost::adaptors::transform(std::string, func) is normally not a ForwardIterator. But it fulfills ReadableIteratorConcept and ForwardTraversalConcept and should be able to be parsed by spirit.
