Zach Laine wrote:
Anyway, I can't get the compiler to treat the result of get_arity as a constant expression using std::max, even in C++17 mode with a recent-ish Clang, so I'm leaving the example as-is for now.
Yes, unfortunately, the straightforward way doesn't work. This does: template <boost::yap::expr_kind Kind, typename... Arg> auto operator() (boost::yap::expr_tag<Kind>, Arg &&... arg) { auto constexpr r = std::max( { decltype( boost::yap::transform( boost::yap::as_expr(std::forward<Arg>(arg)), get_arity{} ) )::value... } ); return boost::hana::llong<r>{}; } And you're right, it works even with -std=c++14. Whether this is really an improvement over what we started with is questionable. :-)