(Resolved) Creating a generic extension is behaving weirdly. I have a boolean list and its allowing ...

# compiler

[Ashish Suman]

(Resolved) Creating a generic extension is behaving weirdly. I have a boolean list and its allowing me to add a integer, no compilation issue (line 15). What am I missing here?
Kotlin playground link: https://pl.kotl.in/eSKHXxqzq

[Youssef Shoaib [MOD]]

List is declared with <out T>, and so the add call upcasts List<Boolean> to List<Any> and hence allows the addition. Solution here is kinda complex. Either you just use MutableList, or you can rely on an internal compiler annotation like so (playground):

fun main() {
    val list: List<Boolean> = mutableListOf(false)
    list.add(1) // Compiler error: The integer literal does not conform to the expected type Boolean
    println(list)
}

// NoInfer prevents the type of [item] from affecting the inferred type of T
@Suppress("INVISIBLE_MEMBER", "INVISIBLE_REFERENCE")
fun <T> List<T>.add(item: @kotlin.internal.NoInfer T) {
    if(this is MutableList) {
        add(item)
    } else {
        error("Not a mutable list.")
    }
}

[Youssef Shoaib [MOD]]

You can also define the NoInfer annotation in your own project so that there's no need for the suppression

[Ashish Suman]

Thanks @Youssef Shoaib [MOD] for the clarification, this was very helpful.

[Ashish Suman]

LMK if this should be removed, as this doesn't really concern about compiler.

[Youssef Shoaib [MOD]]

Hmm, type inference is part of the compiler frontend, so I think it fits? I don't know if there's anywhere else it'd fit.

It seems like, scrolling up, that people do ask questions here related to resolution and inference, so this question seems to be on topic.

[Olaf Gottschalk]

You are basically cheating to the compiler. A list is a list and can contain any object, technically. Generics are always gone after compilation.
And because your generic fun accepts any type T and then just assures it's seeing a MutableList, it happily adds an object of type Any to it.