RFR: 8300543 Compiler Implementation for Pattern Matching for switch [v4]

[Rémi Forax]

On Fri, 21 Apr 2023 15:30:24 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Also, surprisingly, if I make C and D classes (instead of interfaces):
>> 
>> class Test {
>>     sealed interface I permits C, D { }
>>     final class C implements I { }
>>     final class D implements I { }
>> 
>>     interface F { }
>> 
>>     <Z extends I & F> int test(Z o) {
>>         return switch (o) {
>>             case C c -> 1;
>>             case D d -> 2;
>>         };
>>     }
>> }
>> 
>> I get errors like:
>> 
>> 
>> Foo.java:10: error: incompatible types: Z cannot be converted to Test.C
>>             case C c -> 1;
>>                  ^
>>   where Z is a type-variable:
>>     Z extends I,F declared in method <Z>test(Z)
>> 
>> 
>> Which seems odd?
>
> Nevermind - these issues are due to a misunderstanding of the rules with intersection types - e.g. given A & B, if a pattern covers _any_ of A, B, then it covers A & B.
> 
> Similarly, the second issue I reported is in reality caused by the fact that in the snippet with `class`, I also added `final` which then makes I & F a type with no concrete witnesses (so cast will always fail from there to either C/D).

> Which compiles correctly, but it doesn't look exhaustive to me (because of F) ?

Can you expand a little more, because for me a Z either implement C or D or both C and D.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13074#discussion_r1173916081