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<font size="4" face="monospace">It's not as bad as you say, but the
answer is still the same :)<br>
<br>
It is possible to do flow-specific refinement without actually
changing how we track the type. So it would be possible to say
that the type of myVar is still A, while maintaining a lattice of
type inclusion/exclusion refinements that would not actually
change semantics (such as method overload selection), but could be
used to detect unreachable code. <br>
<br>
But the return on that effort seems pretty squarely in the "there
are much better things we could work on" bucket.<br>
</font><br>
<div class="moz-cite-prefix">On 4/19/2025 1:08 PM, Remi Forax wrote:<br>
</div>
<blockquote type="cite" cite="mid:1920807543.235395784.1745082483075.JavaMail.zimbra@univ-eiffel.fr">
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<hr id="zwchr" data-marker="__DIVIDER__">
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<blockquote style="border-left:2px solid #1010FF;margin-left:5px;padding-left:5px;color:#000;font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt;"><b>From:
</b>"Andreas Berheim Brudin"
<a class="moz-txt-link-rfc2396E" href="mailto:andreas.brudin@gmail.com"><andreas.brudin@gmail.com></a><br>
<b>To: </b>"amber-dev" <a class="moz-txt-link-rfc2396E" href="mailto:amber-dev@openjdk.org"><amber-dev@openjdk.org></a><br>
<b>Sent: </b>Saturday, April 19, 2025 6:17:43 PM<br>
<b>Subject: </b>Question about pattern matching and sealed
types – redundant switch cases<br>
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<div>Hi all,<br>
<br>
I'm new to the list—apologies if this has been discussed
before, and thanks in advance for your time.<br>
<br>
I have a question about pattern matching with sealed
types. Consider this example:<br>
<br>
sealed interface A permits B, C {}<br>
record B(String b1, String b2) implements A {}<br>
record C(String c) implements A {}<br>
<br>
A myVar = ...;<br>
if (!(myVar instanceof B(var b1, var b2))) {<br>
return switch (myVar) {<br>
case C(var c) -> c;<br>
case B b -> throw new
IllegalStateException("should not happen");<br>
};<br>
}<br>
// use b1, b2<br>
<br>
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.<br>
<br>
My questions:<br>
<br>
1. Is there any consideration or ongoing work to allow
the compiler to automatically eliminate such unreachable
cases?</div>
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<div><br>
</div>
<div>The short answer is no :)</div>
<div><br data-mce-bogus="1">
</div>
<div>Long answer,</div>
<div> (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)</div>
<div> (2) the type system of Java has no notion of exclusion,
the type A but not B does not exist</div>
<div> (3) instanceof and switch does not have the same
semantics, instanceof has no notion of exhaustiveness while
a switch on a sealed type has.</div>
<div><br data-mce-bogus="1">
</div>
<div>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. </div>
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<div><br>
<br>
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?</div>
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<div><br>
</div>
<div>No, <br>
</div>
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<div><br>
<br>
Again, apologies if this has already been discussed. I'd
appreciate any pointers to relevant threads or JEPs if
so.</div>
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<div>The relevant JEPs are JEP 394 (for instanceof) and 441
(for switch).</div>
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<div><br>
<br>
Thanks,<br>
</div>
Andreas</div>
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<div><br>
</div>
<div>regards,</div>
<div>Rémi</div>
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