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

[Jean-Noël Rouvignac]

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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