cannot cartesian_product(enumerate, enumerate) | transform

[david8dixon]

I am enumerating a vector of vectors and then I take the cartesian product of the enumeration with itself (which works fine), but when I try to transform the result it fails. The following link contains the reproducer:

https://wandbox.org/permlink/AL9HJ8pWbBBntXuN

[pkubik]

Doesn't work for simpler views too:

  auto ints = ranges::views::ints(0, 5);
  auto cp = ranges::views::cartesian_product(ints, ints);
  auto to_one = [](auto&& tup){return 1;};
  auto adjacent_cells_2 = cp | ranges::views::transform(to_one);

[ericniebler]

Doesn't work for simpler views too: ...

Nope, that code works fine.

  auto enumerated = v1 | ranges::views::enumerate;
  auto cp = ranges::views::cartesian_product(enumerated, enumerated);

This is a legit problem. After the above, the range library doesn't think cp is an input range. That's because it has the following associated types:

value type: std::tuple<V, V>, where V is std::pair<std::size_t, std::vector<int>>
reference: ranges::common_tuple<R, R>, where R is ranges::common_pair<std::size_t, std::vector<int>&>

The computed common reference of these types is:
common reference: std::tuple<R, R>, where R is ranges::common_pair<const std::size_t&, std::vector<int>&>

The trouble is, an lvalue reference to the value type is not convertible to the common reference. std::tuple doesn't provide that conversion. The upshot is that the iterators of the cartesian_product view do not satisfy readable.

I think the problem here is that the computed common reference of ranges::common_tuple yields a std::tuple instead of another ranges::common_tuple. I suspect making this change will cause other things to break though. I need to think about this.

[pkubik]

Sorry, it failed for me when I used a regular reference to for the tuple:

 auto to_one = [](auto& tup){return 1;};

I didn't compare the error messages carefully enough. Nevermind.