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From: "hewillk at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/100631] ranges::elements_view:: _Sentinel is missing
 __distance_from() that can access _M_current of _Iterator
Date: Mon, 17 May 2021 04:50:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100631

--- Comment #2 from =E5=BA=B7=E6=A1=93=E7=91=8B <hewillk at gmail dot com> =
---
In addition, I think elements_view::_Sentinel::_M_equal should be a template
function, and elements_view::_Iterator also needs to declare _Sentinel<!_Co=
nst>
as a friend, otherwise the following valid codes will be rejected:

https://godbolt.org/z/7aa75vqsP


#include <ranges>

int main() {
  auto r =3D std::views::iota(0)
    | std::views::transform([](int) { return std::make_pair(42, "hello"); })
    | std::views::keys;

  auto b =3D std::ranges::cbegin(r);
  auto e =3D std::end(r);
  b.base() =3D=3D e.base();

  b =3D=3D e; // error
}


<source>:12:9:   required from here
/opt/compiler-explorer/gcc-trunk-20210513/include/c++/12.0.0/ranges:3675:34:
error: cannot convert 'const
std::ranges::elements_view<std::ranges::transform_view<std::ranges::iota_vi=
ew<int,
std::unreachable_sentinel_t>, main()::<lambda(int)> >, 0>::_Iterator<true>'=
 to
'const
std::ranges::elements_view<std::ranges::transform_view<std::ranges::iota_vi=
ew<int,
std::unreachable_sentinel_t>, main()::<lambda(int)> >, 0>::_Iterator<false>=
&'
 3675 |             { return __y._M_equal(__x); }
      |                      ~~~~~~~~~~~~^~~~~
/opt/compiler-explorer/gcc-trunk-20210513/include/c++/12.0.0/ranges:3645:45:
note:   initializing argument 1 of 'constexpr bool
std::ranges::elements_view<_Vp, _Nm>::_Sentinel<_Const>::_M_equal(const
std::ranges::elements_view<_Vp, _Nm>::_Iterator<_Const>&) const [with bool
_Const =3D false; _Vp =3D std::ranges::transform_view<std::ranges::iota_vie=
w<int,
std::unreachable_sentinel_t>, main()::<lambda(int)> >; long unsigned int _N=
m =3D
0]'
 3645 |           _M_equal(const _Iterator<_Const>& __x) const
      |                    ~~~~~~~~~~~~~~~~~~~~~~~~~^~~=