Re: Question about pattern matching and sealed types – redundant switch cases

[Remi Forax]

> From: "Andreas Berheim Brudin" <andreas.brudin at gmail.com>
> To: "amber-dev" <amber-dev at openjdk.org>
> Sent: Saturday, April 19, 2025 6:17:43 PM
> Subject: Question about pattern matching and sealed types – redundant switch
> cases

> Hi all,

> I'm new to the list—apologies if this has been discussed before, and thanks in
> advance for your time.

> I have a question about pattern matching with sealed types. Consider this
> example:

> sealed interface A permits B, C {}
> record B(String b1, String b2) implements A {}
> record C(String c) implements A {}

> A myVar = ...;
> if (!(myVar instanceof B(var b1, var b2))) {
> return switch (myVar) {
> case C(var c) -> c;
> case B b -> throw new IllegalStateException("should not happen");
> };
> }
> // use b1, b2

> Here, I want to keep an early-return style, but since I need to return a value
> from C, I have to use a switch. This leads to redundancy: we've already tested
> myVar is not a B, so that case should be statically unreachable.

> My questions:

> 1. Is there any consideration or ongoing work to allow the compiler to
> automatically eliminate such unreachable cases?

The short answer is no :) 

Long answer, 
(1) myVar does not change its type, it is declared as an A, it always be an A (Groovy or Kotlin behave differently, they use flow typing) 
(2) the type system of Java has no notion of exclusion, the type A but not B does not exist 
(3) instanceof and switch does not have the same semantics, instanceof has no notion of exhaustiveness while a switch on a sealed type has. 

so because of (1) the type of myVar can not be changed, because of (2) its new type inside the if can not be "A but not B" and because of (3) the new type can not be only C. 

> 2. If only one case remains (in this case, C), could it be possible to treat the
> variable as a C directly without requiring an explicit switch?

No, 

> Again, apologies if this has already been discussed. I'd appreciate any pointers
> to relevant threads or JEPs if so.

The relevant JEPs are JEP 394 (for instanceof) and 441 (for switch). 

> Thanks,
> Andreas

regards, 
Rémi 
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