[Bug c++/104653] New: Derived class looks for a definition of its inherited constexpr virtual destructor

[lichray at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104653

            Bug ID: 104653
           Summary: Derived class looks for a definition of its inherited
                    constexpr virtual destructor
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lichray at gmail dot com
  Target Milestone: ---

The following -std=c++20 code fails to compile:

#include <cstddef>

struct B {
    virtual ~B() = default;
};

struct D : B {};

struct C : B {};

bool foo(std::byte *p) {
    constexpr D obj;
    return __builtin_memcmp(p, &obj, sizeof(void *)) == 0;
}

<source>: In function 'bool foo(std::byte*)':
<source>:12:17: error: 'virtual constexpr D::~D()' used before its definition
   12 |     constexpr D obj;
      |                 ^~~

The code compiles in MSVC 19.29 and Clang 10.

GCC seems to understand that D inherited a defaulted, constexpr virtual
destructor, but reports that it needs a definition. The problem doesn't arise
if D is dependent. A workaround is the following:

struct D : B {
    virtual constexpr ~D() override;
};

constexpr D::~D() = default;