On Fri, 6 May 2022 14:09:24 GMT, Jan Lahoda <jlah...@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). ------------- PR: https://git.openjdk.java.net/jdk/pull/8516
