RFR: 8273544: Increase test coverage for snippets [v3]

[Jonathan Gibbons]

On Fri, 19 Nov 2021 18:07:50 GMT, Pavel Rappo <prappo at openjdk.org> wrote:

>> This is embarrassing. The commit clearly contradicts its message, and I don't immediately understand why. I'll try to figure it out before integrating. Thanks for noticing!
>
> I vaguely remember that at some stage I seem to have realized that while `sealed` works for Style, it does not work for Attribute. This is because the attribute is typically queried by the most specific `Class<T extends Attribute>`, which means that the client knows the exact type of the attribute upon return from Attributes.get (note plural attributeS).
> 
> Here's another issue that might have contributed to this mess: although the [sealed classes](https://openjdk.java.net/jeps/409) are available, the [exhaustive switch for sealed hierarchies](https://openjdk.java.net/jeps/420) is not.
> 
> That said, here's what I will do. I will push a commit to make Style `sealed`, which seems a reasonable thing to do. Do you have any objections, Jon? (Asking because you have already reviewed the change.)

Thanks for the explanation.   Mostly, I was curious about what had happened in your thinking in this area.  I have no strong preferences either way, with a mild preference for leaving it alone, and improve it later when you may be able to make better use of the declaration.

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PR: https://git.openjdk.java.net/jdk/pull/6359