RFR: 8262889: Compiler implementation for Record Patterns [v2]

[Gavin Bierman]

On Fri, 6 May 2022 14:09:24 GMT, Jan Lahoda <jlahoda at openjdk.org> wrote:

>> 8262889: Compiler implementation for Record Patterns
>> 
>> A first version of a patch that introduces record patterns into javac as a preview feature. For the specification, please see:
>> http://cr.openjdk.java.net/~gbierman/jep427+405/jep427+405-20220426/specs/patterns-switch-record-patterns-jls.html
>> 
>> There are two notable tricky parts:
>> -in the parser, it was necessary to improve the `analyzePattern` method to handle nested/record patterns, while still keeping error recovery reasonable
>> -in the `TransPatterns`, the desugaring of the record patterns is very straightforward - effectivelly the record patterns are desugared into guards/conditions. This will likely be improved in some future version/preview
>> 
>> `MatchException` has been extended to cover additional cases related to record patterns.
>
> Jan Lahoda has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Reflecting review feedback.

> From the JLS specdiff
> 
> > If the type R names a generic record class then it is a compile-time error if R is not a parameterized type.
> 
> The following snippet raises a `MatchException`. Shouldn't it be a compile-time error?
> 
> ```
>         Box<String> r = new Box<>(null);
> 
>         switch (r) {
>             case Box(String s):
>                 System.out.println("match");
>         }
> ```
> 
> If this is Ok and my understanding is wrong, then why that raises an exception at all? I can make it work (as an unconditional) if I define the Box as `record Box<T extends String>` and now I am confused...
> 
> ping @GavinBierman @lahodaj

A couple of issues here. (1) This should be a compile-time error. (2) upon investigation I think there is a bug with the pattern matching code, because the compiler is currently saying that the pattern match here: `Box<String> bs = new Box<>(null); if (bs instanceof Box<String>(String s)) { System.out.println("match!"); }` does not succeed. (It should do). The `MatchException` you are seeing is that the exhaustive pattern switch has no matching label (if you put in a default, you don't get the exception).

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PR: https://git.openjdk.java.net/jdk/pull/8516