Use type context to infer type of empty collections in and/or expressions

[zunger-humu]

Underlying issue: for any generic type, mypy considers GenType[] and GenType[X] to be incompatible classes. These should instead be considered to be of type GenType[X]. (Observed with mypy 0.670 / Python 3.6, no flags passed to mypy)

To illustrate why, consider two common Python idioms where this happens. One is in a function declaration, where None is passed as a default value rather than a class-scoped mutable variable:

def fn(arg: Set[str] = None) -> None:
    arg = arg or set()
    ...

This is reported as an error, "Incompatible types in assignment (expression has type "Union[Set[str], Set[]]", variable has type "Optional[Set[str]]")"

Likewise, if an abstract superclass declares a method which yields an iterator,

def abstractMethod(self) -> Iterator[...]:
    pass

Because the abstract declaration passes, it is actually returning an optional iterator (which mypy correctly understands). Callers of the abstract method then cope with this easily:

for value in (thing.abstractMethod() or []):
   ...

which yields the same error as above, or

yield from (thing.abstractMethod() or [])

which fails with "error: "yield from" can't be applied to "Union[Iterator[type], List[]]".

The expected behavior in all cases is no error: empty iterables should be considered to be valid instances of iterables of any type, without further annotation.

[ilevkivskyi]

List[<nothing>] is not a subtype of List[X] because it is invariant, for example

x = []  # mypy would complain here, but assume we ignore it, so 'x' is 'List[<nothing>]'

y: List[str] = x  # this is dangerous
y.append('boom')

z: List[int] = x
z[0] + 42  # TypeError here

Instead mypy should use the outer type context to infer the correct type of an empty collection. I will edit the title accordingly.

[ilevkivskyi]

Also this is low priority since it may be tricky to implement one can instead simply write:

def func(lst: Optional[List[int]] = None) -> None:
    if lst is None:
        lst = []

(which is also more readable IMO)

[JukkaL]

It might be reasonable to make List[<nothing>] a subtype of List[X] though (#6613).

[AlexWaygood]

Cannot reproduce any of these false-positive errors on mypy 0.941; mypy seems to cope with these fine now.