I would assume an expected<T, E1, .., En> could only return a std::variant<E1, ..., En> from its .error(). I can't think what else it could do.
As the foremost authority on the nonexistent expected<T, E...>, I can tell you what it does:
// F shall be in E... template<class F> bool has_error() const; template<class F> F error() const;
// if sizeof...(E) == 1 bool has_error() const; E1 error() const;
It returns the equivalent of variant<E...> not from error(), but from
unexpected<E...> unexpected() const;
which allows you to
expected<T, E1, E2, E3> function() { expected<U, E1, E2> e1 = function1(); if( !e1 ) return e1.unexpected();
expected<V, E3> e2 = function2(); if( !e2 ) return e2.unexpected();
return e1.value() + e2.value(); }
That's a second approach. A third approach could be implementing .index() and std::get<>() for expected. Niall -- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/