Here’s a small (ok, maybe not minimal) example

#include <range/v3/view/concat.hpp>
#include <range/v3/view/transform.hpp>
#include <unordered_set>
#include <vector>
#include <iostream>

using namespace ranges::views;

int main() {
    constexpr auto removeFrom = [](auto& s){
        return [&s](auto k){
            s.erase(k);
            return 10*k;
        };
    };
    std::vector<int> v{1,2,3,4,5};
    std::unordered_set<int> s{7,2,3,8};
    auto c = concat(v | transform(removeFrom(s)),
                    s | transform([](auto x){ return 100*x; }));
    std::cout << c << std::endl;
}

Notice that iterating on v | transform(removeFrom(s)) will result in removing elements from s.

Now, I know that the evaluation order of the two argumensts to concat is not guaranteed by the standard, but the evaluation of both v | transform(removeFrom(s)) and s | transform([](auto x){ return 100*x; })) is not really doing any iteration yet. But the body of concat will iterate on v | transform(removeFrom(s)) first, and then on
s | transform([](auto x){ return 100*x; })), so is there any problem?

I’m asking the question in general, so both for the 3 valid point in the product range-v3std-ranges × c++17c++20.