cannot compile on monterey macos

[sanruby77]

class IdentityCert : public IdentityCertBase { public: explicit IdentityCert(std::string identity); ~IdentityCert() override = default;

std::string getIdentity() const override;

std::optionalstd::string getDER() const override;

private: std::string identity_; }; why the compiler complain the error "cannot use override for the std::optionalstd::string getDER() const override;"