CodeQL library for Java/Kotlin
codeql/java-all 0.11.0 (changelog, source)

Member predicate ExtensionMethod::getExtensionReceiverParameterIndex

Gets the index of the parameter that is the extension receiver. This is typically index 0. In case of $default extension methods that are defined as members, the index is 1. Index 0 is the dispatch receiver of the $default method.

int getExtensionReceiverParameterIndex()