While working on pr100651, the following issues surfaced (not discussed there):

program main
  call test_rank1 ()
contains
  subroutine test_rank1 (msg1)
    class(*), optional, allocatable :: msg1(:)
    if (present (msg1)) stop 77
    call assert_rank1 ()        ! <- no problem here
    call assert_rank1 (msg1)    ! <- problematic code path
  end

  subroutine assert_rank1 (msg2)
    class(*), optional, allocatable :: msg2(:)
  end
end

This crashes at runtime.  The dump for the problematic code path is:

  {
    struct __class__STAR_1_0a class.0;

    if (msg1 != 0B)
      {
        class.0._len = msg1->_len;
        class.0._data = msg1->_data;
        class.0._vptr = msg1->_vptr;
      }
    assert_rank1 (msg1 != 0B ? &class.0 : 0B);
    msg1->_data = class.0._data;
    msg1->_vptr = class.0._vptr;
    msg1->_len = class.0._len;
  }

We need to protect the copy-out for an absent optional argument.

Also, the testcase provided in

https://gcc.gnu.org/pipermail/fortran/2023-November/059969.html

which is similar to the above but uses deferred-length character
shows a questionable tree dump (excerpt):

void test_rank1 (struct array01_character(kind=1) * msg1, integer(kind=8) * _msg1)
{
  integer(kind=8) D.4350;
  bitsizetype D.4351;
  sizetype D.4352;

  D.4350 = *_msg1;
  D.4351 = (bitsizetype) (sizetype) *_msg1 * 8;
  D.4352 = (sizetype) *_msg1;
  if (msg1 != 0B)
    {
      _gfortran_stop_numeric (77, 0);
    }
  L.2:;
  assert_rank1 (0B, &0);
  {
    struct array01_character(kind=1) * D.4348;
    integer(kind=8) * D.4349;

    D.4348 = msg1 != 0B ? msg1 : 0B;
    D.4349 = msg1 != 0B ? _msg1 : 0B;
    assert_rank1 (D.4348, D.4349);
  }
}

This emits warning messages with -fsanitize=undefined, likely due to the
unconditional dereferencing of _msg1 at function entry.