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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/102181] std::advance and
 std::views::iota<std::int64_t> don't work
Date: Thu, 02 Sep 2021 21:59:19 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102181

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
https://en.cppreference.com/w/cpp/iterator/advance says "InputIt must meet =
the
requirements of LegacyInputIterator."

https://en.cppreference.com/w/cpp/named_req/InputIterator gives a sample
concept you can use to test whether something meets those requirements. The
iterator_t<iota_view<int64_t, int64_t>> type does not satisfy that concept.

I think GCC is behaving according to the standard.

You can use std::ranges::advance because that requires
std::input_or_output_iterator, which is satisfied by your iota_view's itera=
tor.


#include <ranges>

int main() {
  using type =3D std::int64_t; // using type =3D int works!
  auto v =3D std::views::iota(static_cast<type>(0), static_cast<type>(100));
  auto b =3D v.begin();

  static_assert( ! __LegacyInputIterator<decltype(b)> );
  // So this can't work:
  // std::advance(b, 1);

  static_assert( std::input_iterator<decltype(b)> );
  // So this can work:
  std::ranges::advance(b, static_cast<type>(1));
}=