The custom first solution goal GenerateFirstSolution is called in the RoutingSolver::solve function.

void RoutingSolver::solve() {
  IloDispatcher dispatcher(_solver);
  IloEnv env = _solver.getEnv();
  IloDimension1 weight = IloDimension1::Find(env, "Weight");
  // Subgoal
  IloGoal instantiateCost = IloDichotomize(env,
             dispatcher.getCostVar(),
             IloFalse);
  IloGoal restoreSolution = IloRestoreSolution(env, _solution);
  IloGoal goal = GenerateFirstSolution(env, weight) && instantiateCost;

  // Solving
  if (findFirstSolution(goal)) {
    improve(instantiateCost);
    _solver.solve(restoreSolution);
  }
}