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From: "pkeir at outlook dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110714] New: constexpr lifetime error: base class this
 pointer
Date: Tue, 18 Jul 2023 11:39:38 +0000
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X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110714

            Bug ID: 110714
           Summary: constexpr lifetime error: base class this pointer
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pkeir at outlook dot com
  Target Milestone: ---

Compiling the C++20 MRE code below fails:

struct Base
{
  constexpr virtual ~Base() {}
  constexpr Base* get_this() { return this; }
  int x;
};

struct Derived : public Base {};

constexpr bool test()
{
  Derived* pf =3D new Derived;
  delete pf->get_this();
  return true;
}

constexpr bool b =3D test();

...with the following error message:

2$ /opt/gcc-latest/bin/g++ -std=3Dc++20 -c ce_base_alloc2.cpp=20
ce_base_alloc2.cpp:17:24:   in =E2=80=98constexpr=E2=80=99 expansion of =E2=
=80=98test()=E2=80=99
ce_base_alloc2.cpp:13:23:   in =E2=80=98constexpr=E2=80=99 expansion of
=E2=80=98pf->Derived::<anonymous>.Base::get_this()->Base::~Base()=E2=80=99
ce_base_alloc2.cpp:8:8: error: deallocation of storage that was not previou=
sly
allocated
    8 | struct Derived : public Base {};
      |        ^~~~~~~

I have tried with GCC trunk (14.0.0) and also version 12.2.0.

I suspect that the this pointer in the base class is not tracking the const=
expr
dynamic allocation. Clang and MSVC both compile successfully. Clang requires
the virtual destructor.=