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From: "vanyacpp at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107529] New: constexpr evaluator doesn't check for
 destroyed objects
Date: Sat, 05 Nov 2022 01:04:23 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: vanyacpp at gmail dot com
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107529

            Bug ID: 107529
           Summary: constexpr evaluator doesn't check for destroyed
                    objects
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vanyacpp at gmail dot com
  Target Milestone: ---

I believe this function contains undefined behavior and should not be allow=
ed
to evaluate at compile-time.

The call to `std::destroy_at(p)` should end the lifetime of `*p` and access=
es
to `*p` after that should be invalid.

#include <memory>

struct mytype
{
    constexpr mytype() : x(42) {}
    constexpr ~mytype() {}

    int x;
};

constexpr int foo()
{
    std::allocator<mytype> alloc;
    mytype* p =3D alloc.allocate(1);
    std::construct_at(p);
    std::destroy_at(p);       // destroy *p
    int result =3D p->x;        // access
    alloc.deallocate(p, 1);
    return result;
}

static_assert(foo() =3D=3D 42);=