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

[Maurizio Cimadamore]

On Fri, 21 Apr 2023 15:17:37 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Flow.java line 915:
>> 
>>> 913:          */
>>> 914:         private List<PatternDescription> reduceBindingPatterns(Type selectorType, List<PatternDescription> patterns) {
>>> 915:             Set<Symbol> existingBindings = patterns.stream()
>> 
>> Playing some more - I found this example:
>> 
>> 
>> class Test {
>>     sealed interface I permits C, D { }
>>     non-sealed interface C extends I { }
>>     non-sealed interface D extends I { }
>> 
>>     interface F { }
>> 
>>     <Z extends I & F> int test(Z o) {
>>         return switch (o) {
>>             case C c -> 1;
>>             case D d -> 2;
>>         };
>>     }
>> }
>> 
>> 
>> Which compiles correctly, but it doesn't look exhaustive to me (because of F) ?
>
> 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).

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/13074#discussion_r1173912165