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From: "hewillk at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/100770] New: Incorrect if constexpr statement in
 ranges::unique_copy
Date: Wed, 26 May 2021 14:49:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100770

            Bug ID: 100770
           Summary: Incorrect if constexpr statement in
                    ranges::unique_copy
           Product: gcc
           Version: 12.0
            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::unique_copy in ranges_algo.h#L1435:


  else if constexpr (input_iterator<_Out>
                     && same_as<iter_value_t<_Iter>, iter_value_t<_Out>>)


Since there is no short circuit in if constexpr statement, this will
incorrectly abort compilation when iter_value_t<_Out> is ill-formed.=20
(thanks T.C. for pointing that out).



#include <algorithm>
#include <iostream>
#include <sstream>

int main() {
  std::istringstream str("42 42 42");
  std::ranges::unique_copy(
    std::istream_iterator<int>(str),=20
    std::istream_iterator<int>(),
    std::ostream_iterator<int>(std::cout, " "));
}

https://godbolt.org/z/PM58b3KrE=