[Bug c++/113021] New: [constexpr] gcc rejects initializing struct containing vector during constant evaluation depending if the struct also contains other member

["miro.palmu at helsinki dot fi" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113021

            Bug ID: 113021
           Summary: [constexpr] gcc rejects initializing struct containing
                    vector during constant evaluation depending if the
                    struct also contains other member
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: miro.palmu at helsinki dot fi
  Target Milestone: ---

Following code fails to compile on gcc (GCC) 13.2.1 20230801 but on clang
(16.0.6 or 17.0.1). 

```
#include <vector>

struct A {
  std::vector<int> vec;
  int a;
};

consteval auto foo() { const A tmp1{{1, 2}, 42}; }

int main() {
  foo();
  return 0;
}

```

With following error message:

```
> g++ -Wall -Wextra -std=c++20 main.cpp
In file included from /usr/include/c++/13.2.1/vector:66,
                 from main2.cpp:1:
/usr/include/c++/13.2.1/bits/stl_vector.h: In function ‘int main()’:
main2.cpp:11:6:   in ‘constexpr’ expansion of ‘foo()’
main2.cpp:8:49:   in ‘constexpr’ expansion of
‘std::vector<int>(std::initializer_list<int>{((const int*)(& const int [2]{1,
2})), 2}, ((const std::vector<int>::allocator_type&)(&
std::allocator<int>())))’
/usr/include/c++/13.2.1/bits/stl_vector.h:679:21:   in ‘constexpr’ expansion of
‘((std::vector<int>*)this)->std::vector<int>::_M_range_initialize<const
int*>(__l.std::initializer_list<int>::begin(),
__l.std::initializer_list<int>::end(),
(std::random_access_iterator_tag{std::bidirectional_iterator_tag()},
std::forward_iterator_tag()))’
/usr/include/c++/13.2.1/bits/stl_vector.h:1689:13: error: modifying a const
object
‘((std::vector<int>*)this)->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’ is not allowed in a constant expression
 1688 |           this->_M_impl._M_start
      |           ~~~~~~~~~~~~~~~~~~~~~~
 1689 |             = this->_M_allocate(_S_check_init_len(__n,
_M_get_Tp_allocator()));
      |            
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
main2.cpp:8:32: note: originally declared ‘const’ here
    8 | consteval auto foo() { const A tmp1{ {1, 2}, 42 }; }
      |                                ^~~~
```

If member variable `a` is removed it compiles:

```
#include <vector>

struct A {
  std::vector<int> vec;
};

consteval auto foo() { const A tmp1{ {1, 2} }; }

int main() {
  foo();
  return 0;
}
```