I'm getting Overload resolution ambiguity erro

问题:

I have a nullable string variable ab. If I call toUpperCase via safe call operator after I assign null to it, kotlin gives error.

fun main(args: Array<String>){
    var ab:String? = "hello"
    ab = null
    println(ab?.toUpperCase())
}

Error:(6, 16)
Overload resolution ambiguity:
@InlineOnly public inline fun Char.toUpperCase(): Char defined in kotlin.text
@InlineOnly public inline fun String.toUpperCase(): String defined in kotlin.text

What's the problem here?

回答1:

As stated in this doc about smart-casts:

x = y makes x of the type of y after the assignment

The line ab = null probably smart casts ab to Nothing?. If you check ab is Nothing? it is indeed true.

var ab: String? = "hello"
ab = null
println(ab?.toUpperCase())
println(ab is Nothing?) // true

Since Nothing? is subtype of all types (including Char? and String?), it explains why you get the Overload resolution ambiguity error. The solution for this error will be what Willi Mentzel mentioned in his answer, casting ab to the type of String before calling toUpperCase().

---

Remarks: This kind of error will occur when a class implements two interfaces and both interface have extension function of the same signature:

//interface
interface A {}
interface B {}

//extension function
fun A.x() = 0
fun B.x() = 0

//implementing class
class C : A, B {}

C().x()    //Overload resolution ambiguity
(C() as A).x()    //OK. Call A.x()
(C() as B).x()    //OK. Call B.x()

回答2:

It really seems like a bug. The type String? is lost somehow upon assigning null, so you have to tell the compiler explicitely that it should deal with a String?.

fun main(args: Array<String>){
    var ab: String? = "hello"
    ab = null
    println((ab as String?)?.toUpperCase()) // explicit cast

    // ...or

    println(ab?.let { it.toUpperCase() }) // use let
}

回答3:

I'm not sure but that seems to be a bug due to smart casting (to Nothing?, subtype of every nullable type). This one works:

fun main(args: Array<String>) {
    var ab: String? = "hello"
    ab = makeNull()
    println(ab?.toUpperCase())
}

fun makeNull(): String? = null

The only difference: The compiler does not know the null assignment directly, which seems to cause the error in your example. But still, yours should probably work too.

回答4:

I believe this is due to smart casts used by Kotlin. In other words, Kotlin is able to infer that after this line of code:

ab = null

type of variable ab is simply null (this is not actual type you can use in Kotlin - I am simply referring to range of allowed values), not String? (in other words, there is no way ab might contain a String).

Considering that toUpperString() extension function is defined only for Char and String (and not Char? or String?), there is no way to choose between them.

To avoid this behaviour see answers proposed by other guys (e.g. explicit casting to String?), but this definitely looks like a feature (and quite a useful one) rather than a bug for me.

回答5:

I decompiled your function and I figured: after the moment you make ab = null the compiler will smartcast it, putting null (ACONST_NULL) in every ocurrence of ab. Then as null has no type. you can't infer the type for the receiver of toUpperCase().

This is the java equivalent code generated from the kotlin byte code:

public final void main(@NotNull String[] args) {
   Intrinsics.checkParameterIsNotNull(args, "args");
   String ab = "hello";
   ab = (String)null;
   Object var3 = null;
   System.out.println(var3);
}

It looks as an issue that should be resolved by the kotlin team.