Dominance in pattern matching for switch: Spec and Javac inconsistency

[Remi Forax]

> From: "Ilyas Selimov" <ilyas.selimov at jetbrains.com>
> To: "compiler-dev" <compiler-dev at openjdk.java.net>
> Sent: Vendredi 3 Septembre 2021 07:27:42
> Subject: Dominance in pattern matching for switch: Spec and Javac inconsistency

> Hello!

> The next code compiles correctly, but it seems to contradict the dominance
> rules:

> void test(Integer i) {
> switch (i) {
> case Integer in && in != null:
> break;
> case 1:
> break;
> case default:
> break;
> }
> }

>> A switch label that has a pattern case label element p dominates another switch
>> label that has a constant case label element c if either of the following is
> > true:
>> - the type of c is a primitive type and its wrapper class (5.1.7) is a subtype
> > of the erasure of the type of p.

> Maybe the type of p should also be total for the type of selector expression
> like in the rules for pattern-over-null dominance?

For me, the problem comes from the guard, if you write the same code without the guard, the compiler correctly emits an error 
void test(Integer i) { 
switch (i) { 
case Integer in: 
break; 
case 1: 
break; 
case default: 
break; 
} 
} 

for me, this rule should apply even if p is a guard, the type of p when p is a guard is the type of the type part (the left part) of a guard. 

So it's more a bug in the implementation than a spec issue. 
But given that Eclipse has the same issue, the spec should be clarified. 

> Thanks,
> Ilyas

regards, 
Rémi 
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