Lifting operations from inline classes to reference projections

Brian Goetz brian.goetz at
Fri Dec 6 19:56:24 UTC 2019

If we have an inline class:

    inline class V { 
        public void m() { }

then, at least for the public members of V (if not more), we would like to lift these onto the reference projection:

    V.ref vr = …
    vr.m()  // succeeds

One way to get there is to simply have the compiler desugar these members onto the reference projection.  This is simple for the VM, in that these become ordinary interface calls and no additional VM help is needed, but on the other hand, we are now duplicating members between V and V.ref, which VM folks don’t like.  The VM folks would prefer that V.ref be an “empty” interface.  

Another would be to appeal to casting; desugar `vr.m()` as `((V) vr).m()` in the static compiler.  This works, until we want to make V private (and the combination of public reference projection and private implementation is expected to be a common one.)  If we do this, we’ll get a linkage error trying to resolve the cast, since it names a class not accessible to the client.  

A third way is to soften up the linkage requirements for `invokeinterface` when in the presence of a reference projection.  Here, when called upon to link an invocation, if the method is not present in the interface, and the interface is a reference projection for V, we can try to link to the corresponding method on V, _ignoring access control for V itself but still doing access control for the method_. (This also has the benefit of working for non-public members too.)  

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