Copy with slicing of a list[subclass] should be ok for list[superclass]

• Are you reporting a bug, or opening a feature request?
  As I expected this to work, I assume it is a 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 List

class Upi(object):
    pass


class Subi(Upi):
    pass


subs = [Subi(), Subi()]
ok1:List[Upi] = [s for s in subs]
ok2:List[Upi] = list(subs)
notOk:List[Upi] = subs[:] # this should also be ok IMHO

• What is the actual behavior/output?
  notOk list is not ok for mypy.

• What is the behavior/output you expect?
  Should be ok. Actually saw this in a function return value so should be ok there also when function is returning list of superclass things.

• What are the versions of mypy and Python you are using?
  mypy 0.720, Python 3.7.3

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

El issue no menciona archivos de implementación ni pruebas. Empieza ejecutando la reproducción proporcionada con mypy y, después, rastrea cómo se tipa el slicing de listas en comparación con una comprensión y list(subs). Se considera terminado cuando la asignación de subs[:] a List[Upi] se acepta sin flags y existe cobertura para el caso reportado.