Comment by kuratkull

Because to me this seems like a fantastic example of a highly possible mistake that a typechecker _should_ catch. Without defined types in this situation a couple of things could happen: 1) it gets printed or passed to some other Any method and the typechecker never yells at you and it crashes in production 2) the typechecker catches the error somewhere long down the line and you have to backtrack to find where you might be appending a str to a list[int].

Instead it could mark it as an error (as all the other checkers do), and if that's what the user really intended they can declare the type as list[str | int] and everything down the line is checked correctly.

So in short, this seems like a great place to start pushing the user towards actually (gradually) typing their code, not just pushing likely bugs under the rug.