Is there a reason I cannot write `if (list is List<MyClass>)`? Compiler forces me to write `if...

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[Grigorii Yurkov]

Is there a reason I cannot write if (list is List<MyClass>)? Compiler forces me to write if (list is List<*>). So I have to write if (list as? List<MyClass> != null) to smartcast my list

[Roukanken]

JVM does not remember type parameters at runtime (called "type erasure")
So this check is impossible to do

And not sure why the second thing works, but I think it will do funny things if your list is smth like List<OtherClass> lemme check

[Roukanken]

yep:

data class ClassA(val v: Int)
data class ClassB(val v: String)

val list: List<Any> = listOf(ClassA(1), ClassA(2), ClassA(2))

if (list as? List<ClassB> != null) {
    println(list)       // [ClassA(v=1), ClassA(v=2), ClassA(v=2)]
    println(list[0].v)  // throws ClassCastException
}

[Grigorii Yurkov]

I don't ask compiler to check that this is List of MyClass. I want to check that this is List and cast it to List<MyClass>, because in this particular line of code I know, that it can't be list of any other class but MyClass

[Roukanken]

except that a is B is made for checking if a is of B type

and since we consider List<A> different type from List<B>
then a is List<A> should not succeed if a is of type List<B>

except that compiler/runtime can't implement this, so it just forbids you from doing these kind of checks