Header file visitor.hpp

namespace cpp
{
    namespace {}
    
    template <typename R>
    struct VariantVisitorTraits;
    
    template <typename R, typename F, typename = void>
    struct VariantVisitor;
    
    template <typename R, typename F>
    struct VariantVisitor<R, F, void_t<typename F::ResultType>>;
    
    template <typename R, typename ... Fs>
    VariantVisitor<R, Function<std::decay_t<Fs>...>> visitor(Fs&&... fs);
}