Don't reuse iterator var to avoid -D_GLIBCXX_DEBUG crash

For a CXXFLAGS='-D_GLIBCXX_DEBUG' build, running the
examples/corelib/ipc/sharedmemory/sharedmemory
example and clicking on the "Load Image from File..." button
would result in a crash:

> usr/include/c++/13/debug/safe_iterator.h:492:
> In function:
>     bool gnu_debug::operator!=(const
>     _Safe_iterator<std::_Rb_tree_const_iterator<std::pair<const
>     QSettingsKey, QVariant> >, std::debug::map<QSettingsKey, QVariant,
>     std::less<QSettingsKey>, std::allocator<std::pair<const QSettingsKey,
>     QVariant> > >, std::forward_iterator_tag>::_Self&, const
>     _Safe_iterator<std::_Rb_tree_const_iterator<std::pair<const
>     QSettingsKey, QVariant> >, std::debug::map<QSettingsKey, QVariant,
>     std::less<QSettingsKey>, std::allocator<std::pair<const QSettingsKey,
>     QVariant> > >, std::forward_iterator_tag>::_Self&)
>
> Error: attempt to compare a singular iterator to a
> singular (value-initialized) iterator.
>
> Objects involved in the operation:
>     iterator "lhs" @ 0x7ffe8e811550 {
>       type = std::_Rb_tree_const_iterator<std::pair<QSettingsKey const, QVariant> > (constant iterator);
>       state = singular;
>     }
>     iterator "rhs" @ 0x7ffe8e811670 {
>       type = std::_Rb_tree_const_iterator<std::pair<QSettingsKey const, QVariant> > (constant iterator);
>       state = singular (value-initialized);
>     }
> Aborted (core dumped)

This may be a libstdc++ bug, but still avoid/work around the issue
by just using two separate variables for the iterators here.

While at it, simplify the code a bit and replace the use
of const_cast and pointers with the use of const references.

Many thanks to Giuseppe D'Angelo for the analysis of
the underlying problem and reporting a bug for GCC/libstdc++ [1] !

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112477

Fixes: QTBUG-119044
Pick-to: 6.5
Change-Id: I00a8cc35033cf3ab4ba1f071cccabdef8ef52f9c
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
(cherry picked from commit 5db48d584e626a15bf436a56aa2165dfe9dfde1d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 9 insertions, 9 deletions

diff --git a/src/corelib/io/qsettings.cpp b/src/corelib/io/qsettings.cpp
index 6bd9a2c819..d8d1419a51 100644
--- a/src/corelib/io/qsettings.cpp
+++ b/src/corelib/io/qsettings.cpp
@@ -1244,17 +1244,17 @@ QStringList QConfFileSettingsPrivate::children(const QString &prefix, ChildSpec
         else
             ensureSectionParsed(confFile, thePrefix);
 
-        auto j = const_cast<const ParsedSettingsMap *>(
-                &confFile->originalKeys)->lowerBound( thePrefix);
-        while (j != confFile->originalKeys.constEnd() && j.key().startsWith(thePrefix)) {
-            if (!confFile->removedKeys.contains(j.key()))
-                processChild(QStringView{j.key().originalCaseKey()}.sliced(startPos), spec, result);
-            ++j;
+        const auto &originalKeys = confFile->originalKeys;
+        auto i = originalKeys.lowerBound(thePrefix);
+        while (i != originalKeys.end() && i.key().startsWith(thePrefix)) {
+            if (!confFile->removedKeys.contains(i.key()))
+                processChild(QStringView{i.key().originalCaseKey()}.sliced(startPos), spec, result);
+            ++i;
         }
 
-        j = const_cast<const ParsedSettingsMap *>(
-                &confFile->addedKeys)->lowerBound(thePrefix);
-        while (j != confFile->addedKeys.constEnd() && j.key().startsWith(thePrefix)) {
+        const auto &addedKeys = confFile->addedKeys;
+        auto j = addedKeys.lowerBound(thePrefix);
+        while (j != addedKeys.end() && j.key().startsWith(thePrefix)) {
             processChild(QStringView{j.key().originalCaseKey()}.sliced(startPos), spec, result);
             ++j;
         }