авторfun sum(x: Number, y: Number) = x + y
println(sum(2.2, 2))

Сообщение об ошибке впечатляет :)

авторerror: unresolved reference. None of the following candidates is applicable because of receiver type mismatch:
@InlineOnly public inline operator fun BigDecimal.plus(other: BigDecimal): BigDecimal defined in kotlin
@InlineOnly public inline operator 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<Number>.plus(element: Number): Array<Number> 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 inline operator 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
@SinceKotlin @ExperimentalUnsignedTypes @InlineOnly public inline operator fun UByteArray.plus(element: UByte): UByteArray defined in kotlin.collections
@SinceKotlin @ExperimentalUnsignedTypes @InlineOnly public inline operator fun UByteArray.plus(elements: UByteArray): UByteArray defined in kotlin.collections
@SinceKotlin @ExperimentalUnsignedTypes public operator fun UByteArray.plus(elements: Collection<UByte>): UByteArray defined in kotlin.collections
@SinceKotlin @ExperimentalUnsignedTypes @InlineOnly public inline operator fun UIntArray.plus(element: UInt): UIntArray defined in kotlin.collections
@SinceKotlin @ExperimentalUnsignedTypes @InlineOnly public inline operator fun UIntArray.plus(elements: UIntArray): UIntArray defined in kotlin.collections
@SinceKotlin @ExperimentalUnsignedTypes public operator fun UIntArray.plus(elements: Collection<UInt>): UIntArray defined in kotlin.collections
@SinceKotlin @ExperimentalUnsignedTypes @InlineOnly public inline operator fun ULongArray.plus(element: ULong): ULongArray defined in kotlin.collections
@SinceKotlin @ExperimentalUnsignedTypes @InlineOnly public inline operator fun ULongArray.plus(elements: ULongArray): ULongArray defined in kotlin.collections
@SinceKotlin @ExperimentalUnsignedTypes public operator fun ULongArray.plus(elements: Collection<ULong>): ULongArray defined in kotlin.collections
@SinceKotlin @ExperimentalUnsignedTypes @InlineOnly public inline operator fun UShortArray.plus(element: UShort): UShortArray defined in kotlin.collections
@SinceKotlin @ExperimentalUnsignedTypes @InlineOnly public inline operator fun UShortArray.plus(elements: UShortArray): UShortArray defined in kotlin.collections
@SinceKotlin @ExperimentalUnsignedTypes public operator fun UShortArray.plus(elements: Collection<UShort>): UShortArray defined in kotlin.collections
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<Number>.plus(element: Number): List<Number> 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<Number>.plus(element: Number): List<Number> 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<Number>.plus(element: Number): Set<Number> 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<Number>.plus(element: Number): Sequence<Number> defined in kotlin.sequences
fun sum(x: Number, y: Number) = x + y