Records: supertype?

[Brian Goetz]

>     We lose out on some future flexibility to add new methods, which
>     might amount to nothing, or might be a big deal.
>
>
> Wouldn't we just introduce the type then once we needed it? It would 
> be awkward, but would it be impossible or inadvisable?

That would not be binary-compatible.

Let's say we had

     record Foo(int x) {}

which was translated without a supertype.  Now we later try to add in 
the supertype, say with a `.toJson()` method, but don't recompile Foo.  
Then:

     Record r = (Record) aFoo;
     String s = r.toJson();

compiles, but throws some form of IncompatibleClassChangeError, since a 
Foo does not extend Record at runtime.


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