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From: "richard.corden at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/104098] New: bits/stl_iterator.h fails to compile for
 __cplusplus > 201703L but with  __cpp_concepts undefined
Date: Tue, 18 Jan 2022 15:10:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104098

            Bug ID: 104098
           Summary: bits/stl_iterator.h fails to compile for __cplusplus >
                    201703L but with  __cpp_concepts undefined
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: richard.corden at gmail dot com
  Target Milestone: ---

Hi,

The following commit changed a preprocessor condition, removing a check for
__cpp_lib_concepts:

=20
https://github.com/gcc-mirror/gcc/commit/d5cbe0f0d4b7bc11f80b2236521f90ec94=
e95767#diff-d1fe6b9164baa7e8e1119a4d194b6b2481439be0cdf5da5794e8001537db4e84


The result being that this header cannot be used with __cplusplus after 201=
703L
unless "__cpp_concepts" is defined.

Compiling the following with "g++ -std=3Dc++20 sample.cpp" reproduces the i=
ssue:

{code:c++}
  // sample.cpp
  #undef __cpp_concepts
  #include <vector>
{code}

{noformat}
> g++-11 -std=3Dc++20 sample.cpp

In file included from /usr/include/c++/11/bits/stl_algobase.h:67,
                 from /usr/include/c++/11/vector:60,
                 from sample.cpp:3:
/usr/include/c++/11/bits/stl_iterator.h:158:25: error: =E2=80=98random_acce=
ss_iterator=E2=80=99
was not declared in this scope; did you mean =E2=80=98random_access_iterato=
r_tag=E2=80=99?
  158 |         =3D conditional_t<random_access_iterator<_Iterator>,
      |                         ^~~~~~~~~~~~~~~~~~~~~~
      |                         random_access_iterator_tag
/usr/include/c++/11/bits/stl_iterator.h:158:57: error: wrong number of temp=
late
arguments (1, should be 3)
  158 |         =3D conditional_t<random_access_iterator<_Iterator>,
      |                                                         ^
In file included from /usr/include/c++/11/bits/move.h:57,
                 from /usr/include/c++/11/bits/stl_pair.h:59,
                 from /usr/include/c++/11/bits/stl_algobase.h:64,
                 from /usr/include/c++/11/vector:60,
                 from sample.cpp:3:
{noformat}


The following addresses the issue for me locally:
{code:c++}
#if __cplusplus <=3D 201703L || ! defined (__cpp_lib_concepts)
{code}


Cheers,


Richard=