unsound cyclic initialization of value constructors

[jvasileff]

The following prints <null> on the JVM:

shared void run() {
    print(X.x.y.x); // JVM: <null>
}
class X {
    shared Y y;
    shared new x {
        y = Y.y;
    }
}
class Y {
    shared X x;
    shared new y {
        x = X.x;
    }
}

4474 addressed a specific, detectable case of cyclic initialization of value constructors, but for the example above, it seems a runtime check will be necessary.

I suggest that the rules in 8.2.10. Initialization of toplevel references be generalized to cover value constructors, particular with respect to throwing an InitializationError when cycles occur.

Note that I'm advocating for the same for statics in https://github.com/ceylon/ceylon/issues/6821#issuecomment-266244728 and https://github.com/ceylon/ceylon/issues/6720.

[jvasileff]

I think this is only an issue for value constructors of toplevel classes, at least once https://github.com/ceylon/ceylon/issues/6728 is resolved.