[false negative] DoubleCheckedLocking: not reported when the outer null check is written as !(x != null)

[MarkLee131]

Versions: Error Prone error_prone_core 2.41.0, JDK 25, invoked via javac (CLI).

Description

DoubleCheckedLocking matches the guard by its syntactic form. Rewriting the outer null check from baz == null to the logically-equivalent !(baz != null) makes the check stop firing, even though the double-checked locking on the non-volatile field — and its unsafe publication — is entirely unchanged. The two programs are behaviourally identical.

Reproducer (reported):

public class DclSeed {
    Object baz;

    Object bar() {
        if (baz == null) {                        // reported here (line 5)
            synchronized (this) {
                if (baz == null) {
                    baz = new Object();
                }
            }
        }
        return baz;
    }
}

Reproducer (NOT reported — the only change is the outer guard):

public class DclVariant {
    Object baz;

    Object bar() {
        if (!(baz != null)) {                     // equivalent to `baz == null`; no diagnostic
            synchronized (this) {
                if (baz == null) {
                    baz = new Object();
                }
            }
        }
        return baz;
    }
}

How to run (the -J--add-exports/--add-opens and -XDcompilePolicy=simple --should-stop=ifError=FLOW flags are required for Error Prone on a modern JDK; without them the plugin does not run):

javac \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.model=ALL-UNNAMED \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED \
  -J--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED \
  -J--add-opens=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED \
  -J--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED \
  -XDcompilePolicy=simple --should-stop=ifError=FLOW \
  -processorpath error_prone_core-2.41.0-with-dependencies.jar -Xplugin:ErrorProne \
  -d out DclSeed.java
# DclSeed.java:5: warning: [DoubleCheckedLocking] Double-checked locking on non-volatile fields is unsafe
# (running the same command on DclVariant.java produces no DoubleCheckedLocking diagnostic)

Expected: Error Prone should emit [DoubleCheckedLocking] on DclVariant.java line 5, but does not.