Using not yet defined variables is allowed

[afrieder]

This might be the same issue as #937 but this raises no errors:

s += 1
s: int = 0

or

x: int
x += 1

[JukkaL]

Mypy will catch the first case if it's within a function:

def f() -> None:
    s += 1   # Name 's' is not defined
    s: int = 0

This should be rejected at the top level of a file as well. The second case is arguably different and would require keeping track of when variables are defined or not. I remember having seen an issue about this but I can't find it right now.

[KotlinIsland]

Duplicate of #686?