RFR: 8294943: Implement record patterns in enhanced for

[Aggelos Biboudis]

On Thu, 20 Oct 2022 16:39:51 GMT, Aggelos Biboudis <abimpoudis at openjdk.org> wrote:

> This PR enables the ability to use record patterns in the enhanced-for, initializing the record components accordingly: 
> 
> 
> record Complex(double real, double img) {}
> 
> List<Complex> list = ...;
> 
> for (Complex(var real, var img) : list) {
>     // can use “real” and “img” directly
> }
> 
> 
> This PR proposes an implementation for the "Record Patterns in Enhanced For" [subtask](https://bugs.openjdk.org/browse/JDK-8294943) regarding the following [CSR](https://bugs.openjdk.org/browse/JDK-8294944) (note the different JBS entries), summarised by the following:
> 
> - It enhances the grammar for the `EnhancedForStatement` to support record patterns too, alongside `LocalVariableDeclarations`.
> - Any pattern variables introduced by the record pattern in the header of the pattern are definitely matched in the statement block of the enhanced for. 
> - The record patterns are only permitted when the pattern is exhaustive over the enhanced for's expression.
> - In the case that the element of the iteration is `null`, the switch raises a `MatchException` wrapping the `NullPointerException`.
> - The enhanced for, supports record patterns for both arrays and reference types.
> 
> Currently, the precise meaning of the enhanced for statement is given by translation into a basic for statement. By introducing record patterns in the pattern header, the new meaning is defined by the new translation which incorporates a switch whose selector expression is the enhanced for's expression, and whose singleton case has the given record pattern as a sole label would be exhaustive. Note, that in cases where the imaginary switch would reach the default clause and end abruptly, the enhanced for each will end abruptly for the same reason.
> 
> For more information on the changes please see:
> 
> - the JEP: [JEP 432](https://openjdk.org/jeps/432) 
> - the CSR: [JEP 432 - JDK-8294944](https://bugs.openjdk.org/browse/JDK-8294944) 
> - the current [specification draft](https://cr.openjdk.java.net/~gbierman/jep432%2b433/jep432+433-20221018/specs/patterns-switch-record-patterns-jls.html#jls-14.14.2) 
> 
> Looking forward for your review.

Thank you Alex! Comments on spec are welcome, good points raised. We will address them and circulate the updated draft spec separately.

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PR: https://git.openjdk.org/jdk/pull/10798