- 
            ADL used to find and match unused_typestreaming operators for every type inboost::spirit::x3namespace GH#553 GH#552
- 
            Ceased dependence on Boost.TTI
          
- 
            Rule placeholders now a non-trivial type because its copy constructor
            checks for a static initialization fiasco GH#571
          
- 
            Most parsers are now constexpr-constructible GH#570