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kotlinlang

Because extension functions are resolved statically

in what scenario would that function (for `MutableList`) lead to an error? Whether I declare a `MutableList&lt;Int&gt;` and concat it with a `MutableList&lt;Double&gt;` to get a `MutableList&lt;Number&gt;` or declare it as a `MutableList&lt;Number&gt;` initialized with only `Int`s, then insert a bunch of `Double`s doesn't make a difference

If you concat `MutableList&lt;Int&gt;` and `MutableList&lt;Double&gt;` to get a  `MutableList&lt;Number&gt;`, what do you expect to happen when you add a `BigDecimal` to it?

(well, assuming you had a definition for what adding to a concatenated mutable list would do if the types *did* line up)

I still get a `MutableList&lt;Number&gt;` 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&lt;Int&gt;`:
`val list1: MutableList&lt;Number&gt; = MutableList&lt;Int&gt;(1,2,3,4,5).concat(MutableList&lt;Double&gt;(1.1, 2.2, 3.3)) // ok`
```val list2 = MutableList&lt;Int&gt;(1,2,3,4,5) // ok
(list2 as MutableList&lt;Double&gt;) += 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&lt;Double&gt;) += 1.1` is not really ok, it doesn't make sense, just the runtime is deficient and allows it.