match<...>.value() should return the error object, not the matched value

[Stephen Kelly]

match<E, V...>::value() currently returns the matches value, which I wrote in my lambda, but it should return the error object which matched.

Consider https://godbolt.org/z/it5qjZ

We have

enum ErrorKind
{
    NoError,
    SomeError,
    OtherError
};

struct MyErrorType
{
    ErrorKind value;
    std::string Payload;
};

and

        ...
        [] (leaf::match<MyErrorType, ErrorKind::OtherError> Err) {
            std::cout << "Matched: " << Err.value() << std::endl;
            return -1;
        },
        ...

Currently Err.value() returns exactly what I wrote on the line above: ErrorKind::SomeError.

In the other case with leaf::match<MyErrorType, ErrorKind::SomeError, ErrorKind::MoreError> Err), Err.value() returns whichever of ErrorKind::SomeError or ErrorKind::MoreError actually matched.

In all cases, the actual error object is not accessible. As I am matching on the type of the error object, I am aware of the MyErrorType itself and it is probably public API and I can know the members of it. That means I can use Err.value().value to access what is currently available as Err.value but I can also Err.value().Payload to access the payload in the above case.

There is no loss of functionality by making the error instance available in error handlers. There is a gain in functionality.

This is true for any error type. I encountered it with std::error_code because I needed to access the message of the error_code. If match<E, V...>::value() returned the std::error_code itself, I could write:

        [] (boost::leaf::match<std::error_code, FileResult::FileNotFound> err)
        {
            std::error_code errc = err.value();
            std::cout << "File error: " << errc.message() << std::endl;
            return -1;
        },

As it is err.value() returns me FileResult::FileNotFound. It is not useful for the framework to tell me what I just told it.