Compile error involving zip_view and iter_move

[duanqn]

Esteemed maintainers and C++ experts,
I'm implementing flat_map in #3896 and I encountered a compile error specifically in the test configuration /MTd /D_ITERATOR_DEBUG_LEVEL=2 /permissive. All other configurations have the flag /permissive- and the tests are passing. The compiler error message was posted here. I don't have much experience working with ranges and I couldn't figure out the reason behind this error, but it looks like a bug with zip_view rather than my flat_map implementation.

Many thanks to achabense who composed a minimal repro source code:

#include <ranges>
#include <vector>
using namespace std;

void test() {
    vector<int> a(10, 1);
    vector<double> b(10, 2.0);
    auto zipv = views::zip(a, b);

    *zipv.begin() = _RANGES iter_move(zipv.begin());
}

int main() {
    test();
}

I'd love some help with this, thanks!

[frederick-vs-ja]

This issue was originally discovered during implementing C++20 ranges utilities (#1083 (comment)) and well discussed during implementing zip_view (#3035 (comment)).

In short, MSVC doesn't properly support the combination of hidden friend function iter_move (inhabiting in the std::ranges namespace) and the ranges::iter_move customization point object (inhabiting in an inline namespace inside std::ranges) in permissive modes. As a result, many C++ ranges components are not properly supported in permissive modes, and many tests in MSVC STL have to use "strict" matrices.

I think we can support flat_map in permissive modes by not using zip_view though...

[duanqn]

Got it. Thanks for the answer!
I will try to remove the usage of zip_view as an improvement after I finish the core functionalities.

[CaseyCarter]

Hidden friends aren't hidden in permissive mode; any use of what should be a hidden friend iter_swap or iter_move in std::ranges will cause compile errors. This is well-known and unavoidable.

If you can easily avoid using zip_view then by all means do, but don't go out of your way to do so.