https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64814
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> --- I think the behaviour you're seeing is correct (and Clang gives the same result). The problem is that increments to the input iterator happen inside the copy_n call, to a copy of the iterator not to readIter itself. This means it is not equivalent to your for (int j = 0; j < 4; ++j) loop which operates directly on readIter. Each time through the outer loop the readIter variable has not been incremented, so has not cached the next element from the input stream.
