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From: "hewillk at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/99752] New: ranges::find_end should return empty
 subrange when search range is empty
Date: Wed, 24 Mar 2021 14:19:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99752

            Bug ID: 99752
           Summary: ranges::find_end should return empty subrange when
                    search range is empty
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

In ranges::find_end implementation:

674     else
675       {
676         auto __i =3D ranges::next(__first1, __last1);
677         if (__first2 =3D=3D __last2)
678           return {__i, __i};

when the examined range is not bidirectional_range and the search range is
empty, we still do the ranges::next call, this makes the following code
infinite loop:

#include <algorithm>
#include <ranges>

int main() {
  auto r =3D std::views::iota(0);
  std::ranges::empty_view<int> e;
  std::ranges::find_end(r, e);
}

But according to the [alg.find.end]: "i be last1 if [first2, last2) is empt=
y",=20
so I think this is a library bug since in such case ranges::find_end never
return.

Same situations in ranges::rotate with performance loss in more rare
conditions:

1573      {
1574    auto __lasti =3D ranges::next(__first, __last);
1575    if (__first =3D=3D __middle)
1576      return {__lasti, __lasti};
1577    if (__last =3D=3D __middle)
1578      return {std::move(__first), std::move(__lasti)};=