[Bug c++/104492] New: Bogus dangling pointer warning (dangling pointer to ‘candidates’ may be used [-Werror=dangling-pointer=])

["thiago at kde dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104492

            Bug ID: 104492
           Summary: Bogus dangling pointer warning (dangling pointer to
                    ‘candidates’ may be used [-Werror=dangling-pointer=])
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thiago at kde dot org
  Target Milestone: ---

Created attachment 52409
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52409&action=edit
Preprocessed sources for qlibrary.cpp

Workaround: https://codereview.qt-project.org/c/qt/qtbase/+/394894
Original sources: 
https://code.qt.io/cgit/qt/qtbase.git/tree/src/corelib/plugin/qlibrary.cpp?id=5e378aaff61c9708e0657f3ee29517c06cb075fa#n618
https://github.com/qt/qtbase/blob/5e378aaff61c9708e0657f3ee29517c06cb075fa/src/corelib/plugin/qlibrary.cpp#L618-L677

Code snippet:

    auto isValidSuffix = [](QStringView s) {
        const QLatin1String candidates[] = {
            QLatin1String("so"),
        };
        return std::find(std::begin(candidates), std::end(candidates), s) !=
std::end(candidates);
    };
    auto suffixes = qTokenize(completeSuffix, u'.');
    auto it = suffixes.begin();
    const auto end = suffixes.end();
    while (it != end) {
        if (isValidSuffix(*it++))
          return true;
    }
    return false;

This code above produces the warning:
qlibrary.cpp:114:9: error: dangling pointer to ‘candidates’ may be used
[-Werror=dangling-pointer=]          
  114 |         if (isValidSuffix(*it++))                                       
      |         ^~                                                              
qlibrary.cpp:76:29: note: ‘candidates’ declared here                            
   76 |         const QLatin1String candidates[] = {                            
      |                             ^~~~~~~~~~                                  

The 'candidates' variable has indeed gone out of scope, but there's no use of
its pointer where the lambda is used. The lambda does a std::find, which
returns a pointer into the array, but that pointer is compared to std::end and
the lambda returns a boolean.

Compile the attached preprocessed sources with:
 g++ -march=x86-64-v3 -Wall -Wextra -O3 -o /dev/null -c qlibrary.cpp.ii

GCC commit 1ce5395977f37e8d0c03394f7b932a584ce85cc7 (master branch dated
2022-02-09).