Use of 'auto [...] 'before deduction of 'auto' with recursive, concept-based function template

Question

I wanted to create a deep_flatten function template that would produce a range of elements that are deeply joined. For example, if we take into account only nested std::vectors, I can have:

template <typename T>
struct is_vector : public std::false_type { };

template <typename T, typename A>
struct is_vector<std::vector<T, A>> : public std::true_type { };

template <typename T>
auto deepFlatten(const std::vector<std::vector<T>>& vec) {
    using namespace std::ranges;
    if constexpr (is_vector<T>::value) {
        auto range = vec | views::join;
        return deepFlatten(std::vector(range.begin(), range.end()));
    } else {
        auto range = vec | views::join;
        return std::vector(range.begin(), range.end());
    }
}

This enables me to do:

std::vector<std::vector<std::vector<int>>> nested_vectors = {
        {{1, 2, 3}, {4, 5}, {6}},
        {{7},       {8, 9}, {10, 11, 12}},
        {{13}}
};

std::ranges::copy(
        deep_flatten(nested_vectors),
        std::ostream_iterator<int>(std::cout, " ")
);

which prints into the console the following text, as expected:

1 2 3 4 5 6 7 8 9 10 11 12 13

But, I don't like this solution that much. Not only it's inefficient (creating a number of temporary vectors), but it also works only with std::vectors. I figured that I could use some more of c++20 magic and use std::ranges::range concept:

namespace rng {
    template <std::ranges::range Rng>
    auto deep_flatten(Rng&& rng) {
        using namespace std::ranges;

        if constexpr (range<Rng>) {
            return deep_flatten(rng | views::join);
        } else {
            return rng | views::join;
        }
    }
}

This seemed to me pretty straightforward - we have a std::ranges::range and we inspect it's value type. Depending on whether it's a nested range, we recurse or simply return joined elements.

Sadly, it doesn't work. After trying to run:

int main() {
    std::set<std::vector<std::list<int>>> nested_ranges = {
            {{1, 2, 3}, {4, 5}, {6}},
            {{7},       {8, 9}, {10, 11, 12}},
            {{13}}
    };

    std::ranges::copy(
            rng::deep_flatten(nested_ranges),
            std::ostream_iterator<int>(std::cout, " ")
    );
}

I get an error saying that:

In instantiation of 'auto rng::deep_flatten(Rng&&) [with Rng = std::ranges::join_view<std::ranges::ref_view<std::set<std::vector<std::__cxx11::list<int> > > > >]':
     required from 'auto rng::deep_flatten(Rng&&) [with Rng = std::set<std::vector<std::__cxx11::list<int> > >&]'
     required from here
     error: use of 'auto rng::deep_flatten(Rng&&) [with Rng = std::ranges::join_view<std::ranges::ref_view<std::set<std::vector<std::__cxx11::list<int> > > > >]' before deduction of 'auto'
     39 |             return deep_flatten(rng | views::join);
        |                    ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~

Having researched similar problems, I cannot really get why the error appears here.

I am using gcc version 10.1.0 (Rev3, Built by MSYS2 project)

Answer 1

There are two problems here.

The first problem is yours:

namespace rng {
    template <std::ranges::range Rng>
    auto deep_flatten(Rng&& rng) {
        using namespace std::ranges;

        if constexpr (range<Rng>) { // <==
            return deep_flatten(rng | views::join);
        } else {
            return rng | views::join;
        }
    }
}

This function is infinitely recursive. deep_flatten is constrained range<Rng>, so the if constexpr check there is always going to be true, so we're never going to enter the base case. This is just a bug - we're checking the wrong thing, it's not if we're a range, it's if our underlying value is a range. That's:

namespace rng {
    template <typename Rng>
    auto deep_flatten(Rng&& rng) {
        using namespace std::ranges;

        auto joined = rng | views::join;    
        if constexpr (range<range_value_t<decltype(joined)>>) {
            return deep_flatten(joined);
        } else {
            return joined;
        }
    }
}

And here we get into the second problem, which is the standard library's problem. What rng | views::join means is:

The name views​::​join denotes a range adaptor object ([range.adaptor.object]). Given a subexpression E, the expression views​::​join(E) is expression-equivalent to join_­view{E}.

But join_view{E} for an E that's already a specialization of join_view... is a no-op right now because of class template argument deduction (CTAD) - the copy deduction candidate is the best candidate, so our nested join operation actually becomes a single join. Your original implementation gets around this problem because it's not join-ing a join_view, it's always join-ing vectors.

I've submitted LWG 3474.

In the meantime, we can work around the views::join problem by just directly using join_view and specifying the template argument explicitly:

namespace rng {
    template <typename Rng>
    auto deep_flatten(Rng&& rng) {
        using namespace std::ranges;


        auto joined = join_view<views::all_t<Rng>>(rng);

        if constexpr (range<range_value_t<decltype(joined)>>) {
            return deep_flatten(joined);
        } else {
            return joined;
        }
    }
}

This works.