Adding variable annotation causes incompatible type when Optional and TypeVars are used

[euresti]

Found while playing around with attrs (now in the typeshed) and using --strict-optional:

$ mypy --version
mypy 0.570-dev-f3491a320ffa36e6574c4370489d9f4819d7fa83 (master)

$ python --version
Python 3.6.3

# Run with --strict-optional
from typing import TypeVar, Callable, Optional, List
_T = TypeVar('_T')
def make(typ: Callable[[], _T]) -> _T:
    ...
def accept(arg: Optional[_T]) -> _T:
    ...
accept(make(list))
accept(None)
foo = accept(make(list))                   # This works
bar: List[str] = accept(make(list))    # Argument 1 to "accept" has incompatible type "List[_T]"; expected "Optional[List[str]]"

Note: The real code has more arguments and an overload that handles the None case but I cut it down to the shortest repro.

The attrs version of this code is:

import attr
x: List[str] = attr.ib(attr.Factory(list))

[ilevkivskyi]

I think this is another manifestation (if not a duplicate) of #1317