"Cannot resolve symbol al" after `else if (! (l instanceof ArrayList al))`

Sun Jun 8 00:47:22 UTC 2025

Hi all,

First, note the original example is equivalent to below:
  if (l instanceof LinkedList) {
    throw new IllegalArgumentException("Use an efficient list implementation");
  } else {
    if (!(l instanceof ArrayList al)) {
      return;
    }
  }
Which should be more clarifying about why the language has this behavior. Variables defined in a block is not available outside of that block.

From the specification perspective, currently, JLS 6.3.2.2 "if Statements" says:
A pattern variable is introduced by if (e) S else T iff either: ... It is introduced by e when false, and S cannot complete normally, and T can complete normally.

This means that a pattern variable introduced by statement T (here the nested if statement after else) is not introduced by the (main) if statement, and this is working as intended per JLS.

I personally think the status quo is fine - allowing variables to leak beyond blocks is risky and may introduce source incompatibility; even if we commit to such a change, we have too many statements to update (for, if, do, while, etc...) and is also error-prone.

Regards,
Chen Liang
________________________________
From: amber-dev <amber-dev-retn at openjdk.org> on behalf of David Alayachew <davidalayachew at gmail.com>
Sent: Saturday, June 7, 2025 9:55 AM
To: Jean-Noël Rouvignac <jean-noel.rouvignac at pingidentity.com>; Angelos Bimpoudis <angelos.bimpoudis at oracle.com>
Cc: amber-dev <amber-dev at openjdk.org>
Subject: Re: "Cannot resolve symbol al" after `else if (! (l instanceof ArrayList al))`

This definitely looks like a bug. Maybe some of the amber folks are busy with Valhalla or something, but I can't see this NOT being a bug.

@Angelos Bimpoudis<mailto:angelos.bimpoudis at oracle.com> poke poke poke

On Sat, Jun 7, 2025, 3:14 AM Jean-Noël Rouvignac <jean-noel.rouvignac at pingidentity.com<mailto:jean-noel.rouvignac at pingidentity.com>> wrote:
Hello folks,

JDK 25 rampdown phase 1 is now behind us, congrats and thanks for all the work done to get there!

I am reviving this use case to see if anyone agrees that this may be a bug?

Thank you,
Jean-Noel

On Wed, Apr 16, 2025 at 11:37 AM Jean-Noël Rouvignac <jean-noel.rouvignac at pingidentity.com<mailto:jean-noel.rouvignac at pingidentity.com>> wrote:
Hello amber-dev folks!

While using pattern matching for instanceof I noticed a small incoherent oddity.

Take this code:
```
static void ensureCapacity(List l) {
  if (l instanceof LinkedList) {
    throw new IllegalArgumentException("Use an efficient list implementation");
  } else if (!(l instanceof ArrayList al)) {
    return;
  }
  al.ensureCapacity(10);
  System.out.println("done");
}

ensureCapacity(new ArrayList());
```

The compiler rejects this code with: `Cannot resolve symbol 'al'`.
(I have tested that code with JDK 24 in the playground (https://dev.java/playground) but also in my IDE with JDK 21)

However #1, removing the `else` keyword makes the code successfully compile:
```
  }
  if (!(l instanceof ArrayList al)) {
    return;
  }
```

However #2, adding an additional `else` keyword makes the code successfully compile:
```
        } else if (!(l instanceof ArrayList al)) {
            return;
        } else {
            al.ensureCapacity(10);
            System.out.println("done");
        }
```

While option #1 and #2 are acceptable workarounds, it looks to me like there could be a bug in javac? Therefore I am bringing this to your attention so you can decide if it is a bug or not.

I searched the JDK's bug tracker for this specific case, but could not find anything related.

Thanks!
Jean-Noël

--
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