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From: "hewillk at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/107572] New: cartesian_product_view invokes the begin
 of input_range twice
Date: Tue, 08 Nov 2022 15:25:46 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 13.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107572

            Bug ID: 107572
           Summary: cartesian_product_view invokes the begin of
                    input_range twice
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

ranges#L8069:

      auto __it =3D __detail::__tuple_transform(ranges::begin, _M_bases);
      if (!__empty_tail)
        std::get<0>(__it) =3D
__detail::__cartesian_common_arg_end(std::get<0>(_M_bases));
      return _Iterator<false>{*this, std::move(__it)};

This *always* invokes begin twice for the first range, which is undefined
behavior when it is an input_range.

testcase (https://godbolt.org/z/7hfP8xM6a):

    #include <ranges>
    #include <fmt/ranges.h>
    #include <sstream>

    std::istringstream ints("0 1 2 3 4");
    struct istream_range {
      auto begin() { return std::istream_iterator<int>{ints}; }
      auto end()   { return std::istream_iterator<int>{}; }
    };

    int main() {
      istream_range r;
      fmt::print("{}\n", std::views::cartesian_product(r)); // prints [(0),
(2), (3), (4)]
    }=