Report error when variable is used uninitialized in a particular scope

[mthuurne]

• Are you reporting a bug, or opening a feature request?
  Feature request.

The following code does not trigger any warnings in mypy 0.711:

def f(b: bool) -> int:
    if b:
        x = 123
        return x
    else:
        return x

I'm not sure 'uninitialized' qualifies as a type, but mypy does track execution paths to narrow types of variables within certain scopes, so it may not be too hard to detect errors like in the example above.

[msullivan]

Yeah, this would be useful. Duplicate of #4019, I think.