Is this intended behaviour? <https://twitter.com/_vsmenon/status/1566851905913241600>

# stdlib

[xxfast]

Is this intended behaviour? https://twitter.com/_vsmenon/status/1566851905913241600

[xxfast]

seems to checkout

private fun <T> List<T>.add(element: T){
  if(this is MutableList<T>) {
    this.add(element)
  }
}

@Test
fun test() {
  val list: List<Int> = mutableListOf(1, 2)
  list.add("NaN")
  list.add(null)
  assertEquals(list, listOf(1, 2, "NaN", null))
}

[xxfast]

changing the receiver to MutableList<> seem to work as intended

[dmitriy.novozhilov]

This is definitely a bug
I've created an issue for it

[Eric Ou]

I suppose is due to the call being on List and it creating a new list, and because it tried to add a String, the resulting List from add became List<Any>, thus allowing the String to be added?

[xxfast]

howd the assertion pass though?

[Eric Ou]

Sorry, I dont know much about the code generation*, this is just a guess from the screenshot and how its behaving.

[Eric Ou]

Nevermind, I just did some testing, and it turns out the List stays Int, it's just somehow a method which only accepts Int can take Strings and nullable values. Which I think is the real problem here.

For a List<Int>, List<T>.add(element: T , the method should only accept Int and not what it seems to practically accept, Any?, I think this is a compiler issue for not checking it properly.

import java.awt.Color

fun <T> List<T>.add1(element: T) {
    if (this is MutableList<T>) { // no error
        this.add(element)
    }
}

val list: List<Int> = mutableListOf(1, 2)
list.add1("NaN")
list.add1(null)
list.add1(Color.BLACK)

This code should not properly compile and run, but currently does, and is somehow List<Int>, even though the list evaluates to [1, 2, NaN, null, java.awt.Color[r=0,g=0,b=0]]