Inconsistent inference of `sum` return type when coupled with nullability.

[Sean1708]

Please provide more information to help us understand the issue:

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

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

  from typing import Optional
  
  
  def foo() -> int:
      return sum([])
  
  
  reveal_type(sum([]))
  
  
  def test1(a: Optional[int]) -> None:
      b = a
      if b is None:
          b = sum([])
  
      reveal_type(b)
  
  def test2(a: Optional[int]) -> None:
      b = a
      if b is None:
          b = foo()
  
      reveal_type(b)
  
  def test3() -> None:
      b = None
      if b is None:
          b = sum([])
  
      reveal_type(b)
  
  def test4(a: Optional[int]) -> None:
      if a is None:
          b = sum([])
      else:
          b = a
  
      reveal_type(b)

• What is the actual behavior/output?

  $ mypy test.py
  test.py:8: note: Revealed type is 'builtins.int'
  test.py:16: note: Revealed type is 'Union[builtins.int, None]'
  test.py:23: note: Revealed type is 'builtins.int'
  test.py:30: note: Revealed type is 'builtins.int'
  test.py:38: note: Revealed type is 'builtins.int'
  

• What is the behavior/output you expect? I expect b to be inferred as an int in all cases, i.e.:

  $ mypy test.py
  test.py:8: note: Revealed type is 'builtins.int'
  test.py:16: note: Revealed type is 'builtins.int'
  test.py:23: note: Revealed type is 'builtins.int'
  test.py:30: note: Revealed type is 'builtins.int'
  test.py:38: note: Revealed type is 'builtins.int'
  

• What are the versions of mypy and Python you are using?

  $ python3.7 --version
  Python 3.7.2
  $ mypy --version
  mypy 0.720
  

  Do you see the same issue after installing mypy from Git master? Yes (mypy 0.730+dev.eb34a7bdcceb4507459c0ef5472117f65bd0855d)

This might be a duplicate of #6697, but the details differ enough that I thought I should open a ticket just in case.

[ilevkivskyi]

This is quite subtle. It looks like in the problematic case the "declared" type of b is used as an external context to infer the type variable of sum(). I don't have ideas for how to fix this and there are several alternative ways to write this that work. I am therefore setting priority to low.

[ilevkivskyi]

(Note that if in the actual code the list was non-empty, then this is a known issue of mypy being too eager about external context.)

[Sean1708]

Thanks for having a look. I can't think of any situations that don't have workarounds, so I think low priority is sensible.

[hynekcer]

This was fixed even before mypy 0.950. Can be closed