error with cartesian_product_view using 2 views

[dixlorenz]

There is a bug with just 2 ranges:

for (auto &&[a, b, c] :
tl::views::cartesian_product(std::views::iota(0, 3), std::views::iota(0, 3), std::views::iota(0, 3)) |
std::views::transform([](auto val) { return val; }))
std::cout << a << ' ' << b << ' ' << c << '\n';

works fine, but:

for (auto &&[a, b] : tl::views::cartesian_product(std::views::iota(0, 3), std::views::iota(0, 3)) |
std::views::transform([](auto val) { return val; }))
std::cout << a << ' ' << b << '\n';

fails to compile because when cartesian_product only uses 2 ranges, tuple_transform returns a pair instead of a tuple and cartesian_product_view is not using that consistently. The iterator fails to be an input_iterator because it is not indirectly_readable, "common_reference_with<iter_reference_t<_In>&&, iter_value_t<_In>&> &&" fails because iter_reference_t uses a pair and iter_value_t uses a tuple. At least that's what I think is happening; once I eliminated the specialization for pair in tuple_or_pair_impl it works. That's definitely not a fix though, but I don't know where the actual bug lies.