Enhance VisitorTransforms

[dlurton]

Given:

(define (domain a (product x y::int))
(define (domain b (product x y::symbol)))
(transform a b)

You get a generated abstract cross domain AToBVisitorTransform with method like this:

abstract fun transformX(node: A.X): B.X
                        //      ^     ^

In both domains the types have the same name but are not identical, so PIG can rightly assume you want to convert A.X. to B.X.

However, if domain b does not have an x type then you won't get a similar method to convert x to anything in the B domain because PIG doens't currently make any assumptions about what the appropriate return type for such a function would be.

It would however be possible to generate an abstract method like this:

fun transformX(node: A.X): B.BNode = ...

And implementations of this function can use covariant return types to narrow the return type of transformX as needed.

(BNode is the generated superclass for all classes in the B domain.)