cell.c:944:12: error: variable 'M_000' set but not used [-Werror,-Wunused-but-set-variable]

Looking at the code, the variable is indeed only written to, but not used:

void cell_check_foreign_multipole(const struct cell *c) {
#ifdef SWIFT_DEBUG_CHECKS

  if (c->split) {
    double M_000 = 0.;
    long long num_gpart = 0;

    for (int k = 0; k < 8; k++) {
      const struct cell *cp = c->progeny[k];

      if (cp != NULL) {
        /* Check the mass */
        M_000 += cp->grav.multipole->m_pole.M_000;

        /* Check the number of particles */
        num_gpart += cp->grav.multipole->m_pole.num_gpart;

        /* Now recurse */
        cell_check_foreign_multipole(cp);
      }
    }

    if (num_gpart != c->grav.multipole->m_pole.num_gpart)
      error("Sum of particles in progenies does not match");
  }

#else
  error("Calling debugging code without debugging flag activated.");
#endif
}

Did somebody forget to add a test here?