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From: "fcontact at cuveland dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/108554] New: Warning "null pointer dereferece" raised
 when extracting a unique_ptr from a map and any "-O" flag
Date: Thu, 26 Jan 2023 09:13:35 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 12.2.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108554

            Bug ID: 108554
           Summary: Warning "null pointer dereferece" raised when
                    extracting a unique_ptr from a map and any "-O" flag
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fcontact at cuveland dot de
  Target Milestone: ---

I found what I believe might be a false positive with g++ 12 when using the
"-Wnull-dereference" warning and a map of unique_ptr with any level of
optimisation.

This behavior does not show up in GCC 9, 10, and 11.

Small but complete example:

#include <map>
#include <memory>
#include <string>
#include <utility>

int main()
{
  // create and initialize a map
  std::map<std::string, std::unique_ptr<int>> my_map;
  my_map["my_key"] =3D std::make_unique<int>(1);

  auto it =3D my_map.find("my_key");
  if (it !=3D my_map.end()) {
    // extract an item from the map
    auto item =3D std::move(my_map.extract(it).mapped());
    return *item;
  }
  return 0;
}

Compiled with:
g++-12 -O -Wnull-dereference <filename>

Compiler version:
g++ (Compiler-Explorer-Build-gcc--binutils-2.38) 12.2.0
(see https://godbolt.org/z/E9KhTT4f6)=