Method swap

common/expr.h:223–228

  void set_test_only(bool test_only) { test_only_ = test_only; }

  friend void swap(SelectExpr& lhs, SelectExpr& rhs) noexcept {
    using std::swap;
    swap(lhs.operand_, rhs.operand_);
    swap(lhs.field_, rhs.field_);
    swap(lhs.test_only_, rhs.test_only_);
  }

 private:
  friend class Expr;