22 General utilities library [utilities]
22.8.5 Class template specialization bad_expected_access<void> [expected.bad.void]
namespace std {
  template<>
  class bad_expected_access<void> : public exception {
  protected:
    bad_expected_access() noexcept;
    bad_expected_access(const bad_expected_access&);
    bad_expected_access(bad_expected_access&&);
    bad_expected_access& operator=(const bad_expected_access&);
    bad_expected_access& operator=(bad_expected_access&&);
    ~bad_expected_access();
  public:
    const char* what() const noexcept override;
  };
}
const char* what() const noexcept override;
Returns: An implementation-defined 
ntbs.