False positive java/local-variable-is-never-read for Kotlin data object-generated equals temporaries

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Bewertung

Schwierigkeit
4/5
Geschätzter Aufwand
3-5 Tage
Anfängerfreundlichkeit
35/100
Issue-Typ
Bug
Klarheit
Größtenteils klar
Aktivitätsstatus
Aktiv
Tech-Stack
java, kotlin
Bereich
devtools, security

Rechercherichtung

The false positive comes from the java/local-variable-is-never-read query (UnreadLocal.ql) reporting compiler-generated other_with_cast locals in zero-property Kotlin data object equals methods. Start by reproducing with the reduced LoadState source in a CodeQL database, then check how the query and the Kotlin extractor model generated locals. Done means these temporaries are excluded while genuinely unused user locals are still reported.

Vom Indexierungsmodell aus dem Issue-Text verfasst.

Beschreibung

Description of the false positive

java/local-variable-is-never-read reports Kotlin compiler-generated other_with_cast temporaries for zero-property data object declarations. The diagnostic is attached to the Kotlin file without a source region; GitHub displays line1. The application does not declare these variables or an equals implementation.

Environment actually observed

  • CodeQL CLI 2.27.3, as selected by github/codeql-actionv4.39.1 at 9b3254085494b55a0f7c4c2b3d2d52cc712ce370.
  • Kotlin 2.4.21, JVM 27, Java/Kotlin manual build, security-extended with the remote threat model.
  • The observations below come from a completed Android/Compose analysis and its SARIF, not an independently rerun CodeQL database for the reduced source below.

Reduced source

private sealed interface LoadState {
    data object Loading : LoadState
    data object Failed : LoadState
}

fun main() {
    val states: List<LoadState> = listOf(LoadState.Loading, LoadState.Failed)
    states.forEach { println(it) }
}

Observed SARIF diagnostic

Rule: java/local-variable-is-never-read
Level: note
Variable 'Loading tmp0_other_with_cast' is never read.
Variable 'Failed tmp0_other_with_cast' is never read.
Physical location: Kotlin source artifact, without a source region

The same analysis reports the same temporary-name pattern for two other zero-property data objects in another Kotlin source file.

Compiler and bytecode evidence

Kotlinv2.4.21 DataClassMembersGenerator.kt creates other_with_cast before iterating through properties. Reads occur within that loop. For a zero-property data object, the compiler-generated local therefore has no property read.

The Kotlin data-object contract specifies generated equals/hashCode implementations and prohibits supplying custom implementations. javap -p -c -l -v on both affected compiled classes confirms that equals(Object) exists despite being absent from application source; its LocalVariableTable contains only this and other after JVM lowering. No application declaration named tmp0_other_with_cast exists.

Expected behavior

Exclude this generated data-object equality temporary from the unread-source-local diagnostic, while retaining detection of genuinely unused user declarations.

Version-check limit

I did not execute a new CodeQL analysis with 2.27.4. Its official query is byte-identical to2.27.3 (UnreadLocal.ql SHA256 aae0d6fd7a2df5291352a3b2597cdff9e92a41f264e413801bb57ffbee071894), and the official Kotlin-extractor source subtree is identical (17c4961985b50180674d18fb4b12d64d3fd36b23). This is not a claim that an upgrade fixes or reproduces the issue; it records the inspected source boundaries.

The application has not been rewritten and the query has not been suppressed. The public example above contains no application-specific code, credentials or private repository references.

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