I'm getting Overload resolution ambiguity error on kotlin safe call

Question

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 m         


        

Answer 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().

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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()