Unreachable (not actually) code allows type errors

[andy-hanson]

Please provide more information to help us understand the issue:

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

def f() -> int:
    if False:
        return "".fake_method()

    res = None
    for x in (1, 2, 3):
        if res is not None:
            return ""
        res = x
    return 123


print(f() ** 2)  # Fails at runtime

• What is the actual behavior/output?

No errors.

• What is the behavior/output you expect?

At least, an error at return "" since this code is reachable (the second time through the loop).
An error in the genuinely unreachable code would be nice too.

• What are the versions of mypy and Python you are using? mypy 0.670+dev.07440753dd80b6c5144031b4f614f3e7dabe7fcf (installed from master just now)

• What are the mypy flags you are using? (For example --strict-optional) None

[ilevkivskyi]

Duplicate of #5423