[Bug 1933018] [NEW] can't include algorithm header on clang++ with C++20

[r3muxd]

Public bug reported:

$ cat a.cpp
#include <algorithm>
$ clang++ -std=c++20 a.cpp
In file included from a.cpp:1:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/algorithm:64:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/ranges_algo.h:35:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/ranges_algobase.h:38:
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/ranges:3526:19: error: missing 'typename' prior to dependent type name 'iterator_traits<iterator_t<_Base>>::iterator_category'
            using _Cat = iterator_traits<iterator_t<_Base>>::iterator_category;
                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

expectation: program compiles
reality: it doesn't

Ubuntu Impish Indri 21.10
libstdc++-10-dev:
  Installed: 10.3.0-4ubuntu1
  Candidate: 10.3.0-4ubuntu1
  Version table:
 *** 10.3.0-4ubuntu1 500
        500 http://us.archive.ubuntu.com/ubuntu impish/main amd64 Packages
        100 /var/lib/dpkg/status

ProblemType: Bug
DistroRelease: Ubuntu 21.10
Package: libstdc++-10-dev 10.3.0-4ubuntu1
ProcVersionSignature: Ubuntu 5.11.0-18.19+21.10.1-generic 5.11.17
Uname: Linux 5.11.0-18-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu67
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: KDE
Date: Sat Jun 19 23:08:16 2021
InstallationDate: Installed on 2021-06-17 (2 days ago)
InstallationMedia: Kubuntu 21.10 "Impish Indri" - Alpha amd64 (20210615)
SourcePackage: gcc-10
UpgradeStatus: No upgrade log present (probably fresh install)

** Affects: gcc-10 (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug impish

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https://bugs.launchpad.net/bugs/1933018

Title:
  can't include algorithm header on clang++ with C++20

Status in gcc-10 package in Ubuntu:
  New

Bug description:
  $ cat a.cpp
  #include <algorithm>
  $ clang++ -std=c++20 a.cpp
  In file included from a.cpp:1:
  In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/algorithm:64:
  In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/ranges_algo.h:35:
  In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/ranges_algobase.h:38:
  /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/ranges:3526:19: error: missing 'typename' prior to dependent type name 'iterator_traits<iterator_t<_Base>>::iterator_category'
              using _Cat = iterator_traits<iterator_t<_Base>>::iterator_category;
                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  1 error generated.

  expectation: program compiles
  reality: it doesn't

  Ubuntu Impish Indri 21.10
  libstdc++-10-dev:
    Installed: 10.3.0-4ubuntu1
    Candidate: 10.3.0-4ubuntu1
    Version table:
   *** 10.3.0-4ubuntu1 500
          500 http://us.archive.ubuntu.com/ubuntu impish/main amd64 Packages
          100 /var/lib/dpkg/status

  ProblemType: Bug
  DistroRelease: Ubuntu 21.10
  Package: libstdc++-10-dev 10.3.0-4ubuntu1
  ProcVersionSignature: Ubuntu 5.11.0-18.19+21.10.1-generic 5.11.17
  Uname: Linux 5.11.0-18-generic x86_64
  NonfreeKernelModules: nvidia_modeset nvidia
  ApportVersion: 2.20.11-0ubuntu67
  Architecture: amd64
  CasperMD5CheckResult: pass
  CurrentDesktop: KDE
  Date: Sat Jun 19 23:08:16 2021
  InstallationDate: Installed on 2021-06-17 (2 days ago)
  InstallationMedia: Kubuntu 21.10 "Impish Indri" - Alpha amd64 (20210615)
  SourcePackage: gcc-10
  UpgradeStatus: No upgrade log present (probably fresh install)

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