Error message dispatch.
This struct and its apply function are used to dispatch a message (e.g., error, waring) from the parser. It is simply an interface to Message. See This struct is practically used as a functor, i.e., a struct or class that defines the function call operator, but instead the function call operator, PEGTL uses the apply() member function for the actions. Thus this struct can be passed to, i.e., specialize a template, such as tk::