Linker.nativeLinker().downcallHandle(
        Linker.nativeLinker().defaultLookup().find("printf").get(),
        FunctionDescriptor.of(JAVA_INT, ADDRESS, JAVA_INT, JAVA_INT, JAVA_INT),
        Linker.Option.firstVariadicArg(4)
);

Here argument index 4 is invalid for the given function descriptor. Hence expect IllegalArgumentException to be thrown, but is not thrown.

When tried with index 5, exception is thrown indeed.