Inferring return type of templated member functions in CRTP

Question

Is it possible to infer the return type of a templated member function in a CRTP base class?

While inferring argument types works well, it fails wit

Answer 1

An extra indirection is your friend:

template <typename D, typename T>
struct f_impl_result
{
    typedef decltype(static_cast<D*>(0)->f_impl(std::declval<T>())) type;
};

template <typename Derived>
struct base
{
    template <typename T>
    auto f(T x) -> typename f_impl_result<Derived, T>::type
    {
        return static_cast<Derived&>(*this).f_impl(x);
    }
};