Why won't this compile? ``` fun String.differingCh...

# getting-started

[benleggiero]

Why won't this compile?

fun String.differingCharacters(other: String): List<Char> {
    var endList: MutableList<Char> = mutableListOf()
    (0..min(this.length, other.length)).forEach { index ->
        val thisChar = this[index]
        val otherChar = other[index]
        if (thisChar != otherChar) {
            endList += thisChar
        }
    }
    if (this.length < other.length) {
        endList += other.substring(this.length).toCharArray().asList()
    } else if (other.length > this.length) {
        endList += other.substring(this.length).toCharArray().asList()
    }
    return endList
}

Errors:

Error:(123, 21) Kotlin: Assignment operators ambiguity:

public operator fun <T> Collection<Char>.plus(element: Char): List<Char> defined in kotlin.collections

@InlineOnly public operator inline fun <T> MutableCollection<in Char>.plusAssign(element: Char): Unit defined in kotlin.collections

Error:(127, 17) Kotlin: Assignment operators ambiguity:

public operator fun <T> Collection<Char>.plus(elements: Iterable<Char>): List<Char> defined in kotlin.collections

@InlineOnly public operator inline fun <T> MutableCollection<in Char>.plusAssign(elements: Iterable<Char>): Unit defined in kotlin.collections

Error:(129, 17) Kotlin: Assignment operators ambiguity:

public operator fun <T> Collection<Char>.plus(elements: Iterable<Char>): List<Char> defined in kotlin.collections

@InlineOnly public operator inline fun <T> MutableCollection<in Char>.plusAssign(elements: Iterable<Char>): Unit defined in kotlin.collections

[ilya.gorbunov]

+= operator on var MutableList can be interpreted either as list.plusAssign(char) or as list = list.plus(char), therefore it's an ambiguity

[ilya.gorbunov]

So either make the list read-only or declare variable as val

[benleggiero]

that's... wow, really? So if plusAssign wasn't there what would happen?