From 5656a60dd067a69f9e864a33068ec300124d4e05 Mon Sep 17 00:00:00 2001
From: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Date: Thu, 15 Apr 2021 16:10:12 +0200
Subject: [PATCH] QList::(const_)iterator: protect element_type on GCC < 11
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GCC 10 in C++20 mode will still try to use
std::indirectly_readable_traits on QList iterators, and since it
does not have the fixes for LWG 3346 / 3541, it will fail to build:

    /usr/include/c++/10/bits/iterator_concepts.h: In substitution of ‘template<class _Tp> using __iter_value_t = typename std::__detail::__iter_traits_impl<_Tp, std::indirectly_readable_traits<_Iter> >::type::value_type [with _Tp = QList<std::pair<int, int> >::const_iterator]’:
    /usr/include/c++/10/bits/iterator_concepts.h:248:11:   required by substitution of ‘template<class _Tp> using iter_value_t = std::__detail::__iter_value_t<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type> [with _Tp = QList<std::pair<int, int> >::const_iterator]’
    /usr/include/c++/10/bits/iterator_concepts.h:468:11:   required from ‘class std::reverse_iterator<QList<std::pair<int, int> >::const_iterator>’
    ../src/corelib/itemmodels/qsortfilterproxymodel.cpp:669:43:   required from here
    /usr/include/c++/10/bits/iterator_concepts.h:243:13: error: ambiguous template instantiation for ‘struct std::indirectly_readable_traits<QList<std::pair<int, int> >::const_iterator>’
      243 |       using __iter_value_t = typename
          |             ^~~~~~~~~~~~~~
    /usr/include/c++/10/bits/iterator_concepts.h:231:12: note: candidates are: ‘template<class _Tp>  requires requires{typename _Tp::value_type;} struct std::indirectly_readable_traits<_Iter> [with _Tp = QList<std::pair<int, int> >::const_iterator]’
      231 |     struct indirectly_readable_traits<_Tp>
          |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /usr/include/c++/10/bits/iterator_concepts.h:236:12: note:                 ‘template<class _Tp>  requires requires{typename _Tp::element_type;} struct std::indirectly_readable_traits<_Iter> [with _Tp = QList<std::pair<int, int> >::const_iterator]’
      236 |     struct indirectly_readable_traits<_Tp>
          |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

So hide element_type as well.

The CI didn't catch this because the CI doesn't build in C++20 mode.

Amends 595b4e1a9b4.

Change-Id: I5c8e47d693ca584571cd89f856d08ba249dd05ab
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
---
 src/corelib/tools/qlist.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/corelib/tools/qlist.h b/src/corelib/tools/qlist.h
index 61a02f795ba..6aafb86c01e 100644
--- a/src/corelib/tools/qlist.h
+++ b/src/corelib/tools/qlist.h
@@ -137,10 +137,10 @@ public:
         // libstdc++ shipped with gcc < 11 does not have a fix for defect LWG 3346
 #if __cplusplus >= 202002L && (!defined(_GLIBCXX_RELEASE) || _GLIBCXX_RELEASE >= 11)
         using iterator_category = std::contiguous_iterator_tag;
+        using element_type = value_type;
 #else
         using iterator_category = std::random_access_iterator_tag;
 #endif
-        using element_type = value_type;
 
         using pointer = T *;
         using reference = T &;
@@ -179,10 +179,10 @@ public:
         // libstdc++ shipped with gcc < 11 does not have a fix for defect LWG 3346
 #if __cplusplus >= 202002L && (!defined(_GLIBCXX_RELEASE) || _GLIBCXX_RELEASE >= 11)
         using iterator_category = std::contiguous_iterator_tag;
+        using element_type = const value_type;
 #else
         using iterator_category = std::random_access_iterator_tag;
 #endif
-        using element_type = const value_type;
         using pointer = const T *;
         using reference = const T &;