Should stub functions fail?

[samueljsb]

Currently it's possible to write a function that has a non-None return type but returns None, e.g:

def f() -> int: pass
def g() -> str: ...

This is considered fine by mypy, but will cause type-related problems at runtime:

>>> def f() -> int: pass
...
>>> f() + 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'

Why does mypy allow these functions?

[hauntsaninja]

On mypy master, we've modified heuristics to complain about this. See:
python/mypy#13729
python/mypy#12118
python/mypy#2350