why?

Because extension functions are resolved statically

in what scenario would that function (for

MutableList

) lead to an error? Whether I declare a

MutableList<Int>

and concat it with a

MutableList<Double>

to get a

MutableList<Number>

or declare it as a

MutableList<Number>

initialized with only `Int`s, then insert a bunch of `Double`s doesn't make a difference

If you concat

MutableList<Int>

and

MutableList<Double>

to get a

MutableList<Number>

, what do you expect to happen when you add a

BigDecimal

to it?

I still get a

MutableList<Number>

since

BigDecimal

is a

Number

. The list only stores references and the generic type is erased, so in fact I can put any object in it, as long as I don't declare it as a `MutableList<Int>`:

val list1: MutableList<Number> = MutableList<Int>(1,2,3,4,5).concat(MutableList<Double>(1.1, 2.2, 3.3)) // ok

val list2 = MutableList<Int>(1,2,3,4,5) // ok
(list2 as MutableList<Double>) += 1.1 // ok
list2.map(::functionForInts) // not ok

this only works because you happen to be using a MutableList implementation that doesn't check the runtime type of its contents --

(list2 as MutableList<Double>) += 1.1

is not really ok, it doesn't make sense, just the runtime is deficient and allows it.