c++23 std::ranges::copy_n advances InputIterator one more time than necessary

[Be3y4uu-K0T]

Repeated error, but for copy_n from ranges. (old resolved bug)

Test program (godbolt):

#include <iostream>
#include <sstream>
#include <vector>
#include <iterator>

int main()
{
    std::istringstream s("1 2 3 4 5");
    std::vector<int> v;
    std::ranges::copy_n(std::istream_iterator<int>(s), 2, std::back_inserter(v));
    std::ranges::copy_n(std::istream_iterator<int>(s), 2, back_inserter(v));

    std::ranges::copy(v, std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
}

Run:
clang++ -std=c++23 index.cc -o index && ./index

Actual output:
1 2 4 5

Expected output:
1 2 3 4

Environment:

% clang++ -v
Homebrew clang version 18.1.8
Target: arm64-apple-darwin24.3.0
Thread model: posix
InstalledDir: /opt/homebrew/bin

[frederick-vs-ja]

Curiously, std::ranges::copy_n performs one more increment than std::copy_n for input iterators in both libstdc++ and libc++.

See also 99847d2, perhaps we also want apply the change to ranges::copy_n. CC @philnik777.

[philnik777]

This behaviour is mandated by the standard, since we have to return the first + n iterator, which in this case points to the 3 and removes it from the stream. This seems more like LWG issue territory than libc++ bug.

[frederick-vs-ja]

Also, a modified version can achieve the intended behavior (Godbolt link). Note that constructing a non-end-of-stream istream_iterator performs one read.

#include <algorithm>
#include <iostream>
#include <sstream>
#include <vector>
#include <iterator>

int main()
{
    std::istringstream s("1 2 3 4 5");
    std::vector<int> v;
    auto [init, outit] = std::ranges::copy_n(std::istream_iterator<int>(s), 2, std::back_inserter(v));
    std::ranges::copy_n(init, 2, outit);

    std::ranges::copy(v, std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
}

[Be3y4uu-K0T]

This behaviour is mandated by the standard, since we have to return the first + n iterator, which in this case points to the 3 and removes it from the stream. This seems more like LWG issue territory than libc++ bug.

Since it's not about the implementation itself, I'm thinking of closing this issue here!

[philnik777]

Let's keep it open until we have an LWG issue.