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From: "overdijk at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/114253] New: False positive
 maybe-uninitialized with std::optional and ternary
Date: Wed, 06 Mar 2024 11:27:12 +0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 8.3.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114253

            Bug ID: 114253
           Summary: False positive maybe-uninitialized with std::optional
                    and ternary
           Product: gcc
           Version: 8.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: overdijk at gmail dot com
  Target Milestone: ---

The following warning is reported:
> mwe.cpp: In function =E2=80=98int main()=E2=80=99:
> mwe.cpp:26:13: warning: =E2=80=98pid=E2=80=99 may be used uninitialized i=
n this function [-Wmaybe-uninitialized]
>      kill_pid(*pid);
>      ~~~~~~~~^~~~~~

When compiling this minimum working example:
````````````````````
#include  <string>
#include  <optional>

std::optional<int> get_pid()
{
  return std::nullopt;
}

void kill_pid(int)
{
}

int main()
{
  std::string s;

  volatile bool is_running =3D true;
  std::optional<int> pid =3D is_running ? get_pid() : std::nullopt;

  kill_pid(0);

  if (pid)
    kill_pid(*pid); // no warning

  if (pid)
    kill_pid(*pid); // warning: 'pid' may be used uninitialized in this
function [-Wmaybe-uninitialized]
}
````````````````````

With the following compiler command line:
$ g++ -std=3Dc++17 -O1 -fPIC -Wmaybe-uninitialized mwe.cpp

I think it is a false positive, since *pid is only used after verifying tha=
t it
is valid.

If any of these changes are made, the warning is no longer reported:
* Replace std::optional<int> by a plain int*
* Remove the std::string variable (or replace it by something simple such as
int)
* Remove the first or second call to kill_pid
* Changing the ternary (e.g. `is_running ? std::optional<int>{999} :
std::nullopt`)
* Removing -O1 or -fPIC from the command line (a higher optimization level =
is
fine)

I observed it with gcc 8.3.0, but it can be reproduced with gcc 8.1 through
13.2 as can be seen on godbolt:
https://godbolt.org/z/TYqYdfc77=