On 12/09/2010 09:18, Thomas Heller wrote:
Phoenix3 uses result_of protocol, and result_type wherever the return type is monomorphic.
It uses result_of on the grammar, which in turns calls apply_transform on the default transform, the latter not happening in a template deduction context and thus preventing SFINAE to happen.
My take would be to use boost::result_of<Expr(T1, T2, ...)>::type before calling the phoenix expression and use SFINAE here.
I don't understand what you mean, and how that would help at all. What needs to be in an SFINAE context, here, is something of the kind decltype(make_mutable<T0>() << make_mutable<T1>()), with T0 and T1 template parameters. If you try the above code, you'll get an error in the default_shift_left transform, which already uses decltype to deduce the return type, but not in a template context. If you change default_shift_left's operator() to be a template and change its result type deduction to be in a template context as well, then you get an error that says no overload matches within default_shift_left. That's because the SFINAE doesn't propagate up to the grammar through apply_transform etc., and making that work involves doing the changes that I said. It's really a limitation in Proto at this point, not in Phoenix.