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From: "zed.three at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/109568] New: [12/13 Regression] Spurious "potential
 null pointer dereference" in shared_ptr_base.h with "-O1"
Date: Thu, 20 Apr 2023 10:13:44 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109568

            Bug ID: 109568
           Summary: [12/13 Regression] Spurious "potential null pointer
                    dereference" in shared_ptr_base.h with "-O1"
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zed.three at gmail dot com
  Target Milestone: ---

The following MVCE gives a warning about a potential null dereference in
operator=3D=3D in shared_ptr_base.h:


    #include <memory>

    struct Base {
        virtual ~Base() =3D default;
        bool empty() { return ptr =3D=3D nullptr; }
        std::shared_ptr<int> ptr{nullptr};
    };

    struct Derived : public Base {};

    bool empty(Base* var) {
        auto* var_ref =3D dynamic_cast<Derived*>(var);
        if (var_ref->empty()) return false;
        return true;
    }

Compiler Explorer link: https://godbolt.org/z/Tqrdo61Ks

Warning:

  In file included from
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/bits/shared_ptr.h:53,
                   from
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/memory:77,
                   from <source>:1:
  In member function 'std::__shared_ptr<_Tp, _Lp>::operator bool() const [w=
ith
_Tp =3D int; __gnu_cxx::_Lock_policy _Lp =3D __gnu_cxx::_S_atomic]',
      inlined from 'bool std::operator=3D=3D(const shared_ptr<_Tp>&, nullpt=
r_t)
[with _Tp =3D int]' at
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/bits/shared_ptr.h:562:=
14,
      inlined from 'bool Base::empty() const' at <source>:5:46,
      inlined from 'bool empty(Base*)' at <source>:12:23:
=20
/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/bits/shared_ptr_base.h=
:1670:16:
error: potential null pointer dereference [-Werror=3Dnull-dereference]
   1670 |       { return _M_ptr !=3D nullptr; }
        |=20=20=20

This requires at least -O1 and appears in GCC 12.1 onwards. The `dynamic_ca=
st`
also appears to be necessary, as `std::shared_ptr<int>{nullptr} =3D=3D null=
ptr`
doesn't trigger the warning.=