// Force boxing if the function overrides function with different type modulo nullability ignoring type parameters
    if ((this as? IrSimpleFunction)?.let {
            it.overriddenSymbols.all { overridden ->
                overridden.owner.returnType.isNullable() && overridden.owner.returnType.underlyingType()?.isNullable() == true
                overridden.owner.returnType.makeNotNull().classOrNull == returnType.makeNotNull().classOrNull
            }
        } != true) return null

operator fun StringBuilder.plus(a: Any): StringBuilder {
    append(a)
    return this
}

fun main() {
    var sb = StringBuilder()
    sb += 'c' + "String"
    println(sb.toString())
}