Initial draft of puzzler about default args for case classes

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Based on a suggestion by @didierd in https://groups.google.com/forum/#!topic/scala-language/DDpvGcrs3QQ

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So many choices here! I found it really hard to decide between this one (which is probably the reader thinking "how can it compile with a dangling this?) and "The first statement prints true, the second one fails to compile", which would be the user wondering whether default args carry over to the apply factory method.

I was thinking of a different code sample that would allow for both options:

case class Part(serialNr: Int = this.hashCode)
println(Part(4322551) == new Part(4322551))
println(Part() == new Part())

Here, the candidate answer with the runtime exception (i.e. endless loop) can be dropped and we can have both possible "fails to compile" answers. But somehow this variant didn't seem quite as "puzzling."

@didierd: What do you think?

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Thanks for the comments and references, @som-snytt!

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Merged to master: 7176928