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From: "gimira8592 at aramask dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110244] New: False error of vector memory leak
Date: Wed, 14 Jun 2023 01:34:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110244

            Bug ID: 110244
           Summary: False error of vector memory leak
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gimira8592 at aramask dot com
  Target Milestone: ---

Created attachment 55320
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D55320&action=3Dedit
the preprocessed file (*.i*) that triggers the bug, generated by adding
-save-temps

Version of GCC: (Ubuntu 12.2.0-17ubuntu1) 12.2.0
OS and Version: Ubuntu 23.04, Linux 6.2.0-20-generic
GCC configuration: -fanalyzer std=3Dc++20

To trigger bug run this code

int main()
{
    std::vector<int> vec(2);
}

and when you try to compile it, you should see this error

leak of =E2=80=98vec.std::vector<int>::<anonymous>.std::_Vector_base<int,
std::allocator<int> >::_M_impl.std::_Vector_base<int, std::allocator<int>
>::_Vector_impl::<anonymous>.std::_Vector_base<int, std::allocator<int>
>::_Vector_impl_data::_M_start=E2=80=99 [CWE-401] [-Werror=3Danalyzer-mallo=
c-leak]

Looks like a bug in GCC's static analyzer, because that code certainly does=
 not
leak. Interestingly it only happens with C++20

It still happens with the latest GCC versions: https://godbolt.org/z/o6GbGc=
MaP=20

Apparently its trigger by the addition of constexpr to the ctor/dtor in C++=
20
(and beyond).=20

Apparently this error does not seem to occur with std::string but it do occ=
ur
with data type like int, double, long and char and fails to compile.

If you just create an uninitialized vector or default initialize with brace=
, it
will compile without any error but if you set it size or initialize with a
value then it will give memory leak error.

To show that this is related to constexpr on the ctor/dtor, you can #define
_GLIBCXX20_CONSTEXPR before #including <vector> in the C++20 version.

If you do that to effectively "define away" the constexpr in the C++20 vers=
ion,
the static analyzer wont fail. Of course that is very much UB and pretty
dangerous (as you are actively meddling with the internals of the library a=
nd
the compilers assumptions), but it shows the point.=