<iterator>: move_sentinel could be unwrappable

[JMazurkiewicz]

As the title says, std::move_sentinel could be unwrappable.

Outdated (fixed by #3024)

Since std::move_sentinel<X> is not unwrappable, it cannot be used with ranges algorithms when corresponding std::move_iterator<X> is unwrappable.

#include <algorithm>
#include <cassert>
#include <iterator>
#include <vector>

using namespace std;

int main() {
    vector<int> ints = {1, 2, 3};
    auto is_positive = [](auto x) { return x > 0; };
    assert(ranges::all_of(move_iterator{ints.begin()}, move_sentinel{ints.end()}, is_positive));
}

Expected behavior: This code should compile.

Additional context:

• Compiler explorer,
• Noticed while implementing test coverage for P2446R2: views::as_rvalue #3008.

[JMazurkiewicz]

This is no longer a bug (thanks to #3024), so it should be labelled with enhancement or performance.

[StephanTLavavej]

Relabeled - thanks @JMazurkiewicz!