cxx::optional and cxx::expected adjustments

[elfenpiff]

Brief feature description

• The methods of cxx::expected on_success and on_error should be renamed to and_then and or_else.
• It should be possible to create an cxx::optional directly from an cxx::expected with a method called .to_optional() which returns the value of the cxx::expected packed inside an cxx:optional if it contains a value otherwise it returns the cxx::nullopt
• If a value is moved outside the cxx::optional the method has_value should no longer return true since we would like to avoid undefined behavior

cxx::optional<int> a = 5;
if ( a.has_value() ); // returns true
int b = std::move(*a);
if ( a.has_value() ); // should now return false since the value is moved outside the optional

// this case would false assume that the optional has no more contained value but at the moment I do not see a problem with that.
std::move(*a);

• If a value is moved outside the cxx::expected it should no longer be contained. We need the methods has_value() and has_error() which both should return false in that case. If the methods and_then() or or_else() are called on an cxx::expected in that state they should terminate.

Detailed information

We would further reduce undefined behavior.

[elfenpiff]

@elBoberido sadly it is not possible to invalidate an optional when the value is moved out since
int b = std::move(*a); calls the dereferencing operator which returns T& and the std::move is transforming it into an rvalue. This could only be done for the method T&& value() && but then it would be inconsistent in the behavior. We have the same problem also in the expected

[dkroenke]

Would you please create a link to the according C++ standards where the adjustments are based on. That makes comparing easier.