Kotlin koans不工作？我疯了吗？

Question

Kotlin koan 3，“默认参数”，似乎很简单，但我无法解决它：

fun foo(name: String = "", number: Int = 42, toUpperCase: Boolean = false): String {
    if(!toUpperCase)
        return name+number
    else
        return name.toUpperCase()+number
}

fun task3(): String {
//    todoTask3()
        return ( foo("a") +
            foo("b", number = 1) +
            foo("c", toUpperCase = true) +
            foo(name = "d", number = 2, toUpperCase = true))
}

第一个“+”符号在Intellij中标记为错误，带有邪恶的长堆栈跟踪：

Error:(27, 22) Kotlin: Unresolved reference. None of the following candidates is applicable because of receiver type mismatch: 
@InlineOnly public operator inline fun BigDecimal.plus(other: BigDecimal): BigDecimal defined in kotlin
@InlineOnly public operator inline fun BigInteger.plus(other: BigInteger): BigInteger defined in kotlin
public operator fun <T> Array<???>.plus(elements: Array<out ???>): Array<???> defined in kotlin.collections
public operator fun <T> Array<???>.plus(elements: Collection<???>): Array<???> defined in kotlin.collections
public operator fun <T> Array<String>.plus(element: String): Array<String> defined in kotlin.collections
public operator fun BooleanArray.plus(element: Boolean): BooleanArray defined in kotlin.collections
public operator fun BooleanArray.plus(elements: BooleanArray): BooleanArray defined in kotlin.collections
public operator fun BooleanArray.plus(elements: Collection<Boolean>): BooleanArray defined in kotlin.collections
public operator fun ByteArray.plus(element: Byte): ByteArray defined in kotlin.collections
public operator fun ByteArray.plus(elements: ByteArray): ByteArray defined in kotlin.collections
public operator fun ByteArray.plus(elements: Collection<Byte>): ByteArray defined in kotlin.collections
@InlineOnly public operator inline fun Char.plus(other: String): String defined in kotlin.text
public operator fun CharArray.plus(element: Char): CharArray defined in kotlin.collections
public operator fun CharArray.plus(elements: CharArray): CharArray defined in kotlin.collections
public operator fun CharArray.plus(elements: Collection<Char>): CharArray defined in kotlin.collections
public operator fun DoubleArray.plus(element: Double): DoubleArray defined in kotlin.collections
public operator fun DoubleArray.plus(elements: DoubleArray): DoubleArray defined in kotlin.collections
public operator fun DoubleArray.plus(elements: Collection<Double>): DoubleArray defined in kotlin.collections
public operator fun FloatArray.plus(element: Float): FloatArray defined in kotlin.collections
public operator fun FloatArray.plus(elements: FloatArray): FloatArray defined in kotlin.collections
public operator fun FloatArray.plus(elements: Collection<Float>): FloatArray defined in kotlin.collections
public operator fun IntArray.plus(element: Int): IntArray defined in kotlin.collections
public operator fun IntArray.plus(elements: IntArray): IntArray defined in kotlin.collections
public operator fun IntArray.plus(elements: Collection<Int>): IntArray defined in kotlin.collections
public operator fun LongArray.plus(element: Long): LongArray defined in kotlin.collections
public operator fun LongArray.plus(elements: LongArray): LongArray defined in kotlin.collections
public operator fun LongArray.plus(elements: Collection<Long>): LongArray defined in kotlin.collections
public operator fun ShortArray.plus(element: Short): ShortArray defined in kotlin.collections
public operator fun ShortArray.plus(elements: ShortArray): ShortArray defined in kotlin.collections
public operator fun ShortArray.plus(elements: Collection<Short>): ShortArray defined in kotlin.collections
public operator fun String?.plus(other: Any?): String defined in kotlin
public operator fun String?.plus(other: Any?): String defined in kotlin
public operator fun <T> Collection<???>.plus(elements: Array<out ???>): List<???> defined in kotlin.collections
public operator fun <T> Collection<???>.plus(elements: Iterable<???>): List<???> defined in kotlin.collections
public operator fun <T> Collection<???>.plus(elements: Sequence<???>): List<???> defined in kotlin.collections
public operator fun <T> Collection<String>.plus(element: String): List<String> defined in kotlin.collections
public operator fun <T> Iterable<???>.plus(elements: Array<out ???>): List<???> defined in kotlin.collections
public operator fun <T> Iterable<???>.plus(elements: Iterable<???>): List<???> defined in kotlin.collections
public operator fun <T> Iterable<???>.plus(elements: Sequence<???>): List<???> defined in kotlin.collections
public operator fun <T> Iterable<String>.plus(element: String): List<String> defined in kotlin.collections
public operator fun <K, V> Map<out ???, ???>.plus(pairs: Array<out Pair<???, ???>>): Map<???, ???> defined in kotlin.collections
public operator fun <K, V> Map<out ???, ???>.plus(pair: Pair<???, ???>): Map<???, ???> defined in kotlin.collections
public operator fun <K, V> Map<out ???, ???>.plus(pairs: Iterable<Pair<???, ???>>): Map<???, ???> defined in kotlin.collections
public operator fun <K, V> Map<out ???, ???>.plus(map: Map<out ???, ???>): Map<???, ???> defined in kotlin.collections
public operator fun <K, V> Map<out ???, ???>.plus(pairs: Sequence<Pair<???, ???>>): Map<???, ???> defined in kotlin.collections
public operator fun <T> Set<???>.plus(elements: Array<out ???>): Set<???> defined in kotlin.collections
public operator fun <T> Set<???>.plus(elements: Iterable<???>): Set<???> defined in kotlin.collections
public operator fun <T> Set<???>.plus(elements: Sequence<???>): Set<???> defined in kotlin.collections
public operator fun <T> Set<String>.plus(element: String): Set<String> defined in kotlin.collections
public operator fun <T> Sequence<???>.plus(elements: Array<out ???>): Sequence<???> defined in kotlin.sequences
public operator fun <T> Sequence<???>.plus(elements: Iterable<???>): Sequence<???> defined in kotlin.sequences
public operator fun <T> Sequence<???>.plus(elements: Sequence<???>): Sequence<???> defined in kotlin.sequences
public operator fun <T> Sequence<String>.plus(element: String): Sequence<String> defined in kotlin.sequences

如果我将第一个调用从foo（“a”）更改为foo（“a”）。toString（），

我得到以下内容：

org.junit.Comparison

Failure: 
Expected :a42b1C42D2
Actual   :kotlin.Unitb1C42D2

所以它实际上正在返回“kotlin.Unit”？

我做的是多么愚蠢的事？