List gets silently converted into Iterable in itertools.chain.from_iterable

[ruuda]

• I am reporting a bug.
• Consider the following minimal example:

from typing import Callable, Iterable, List, TypeVar
import itertools

T = TypeVar('T')

def tabulate(f: Callable[[int], T]) -> Iterable[T]:
    return map(f, itertools.count(0))

def take_while(p: Callable[[T], bool], xs: Iterable[T]) -> Iterable[T]:
    for x in xs:
        if p(x):
            yield x
        else:
            break

def singleton(i: int) -> List[int]:
    return [i]

fine_1 = take_while(lambda x: len(x) > 0, tabulate(singleton))
fine_2 = itertools.chain.from_iterable(fine_1)

bad = itertools.chain.from_iterable(take_while(lambda x: len(x) > 0, tabulate(singleton)))

• What is the actual behavior/output?

22: error: Argument 1 to "len" has incompatible type "Iterable[int]"; expected "Sized"

• What is the behavior/output you expect? I would expect argument 1 to len to take List[int], because that is what T should be instantiated to. I would expect fine_2 and bad to both typecheck; inlining the expression should not change its type.

• What are the versions of mypy and Python you are using? Mypy 0.720, Python 3.7.4.

• Do you see the same issue after installing mypy from Git master? Yes, with mypy-0.730+dev.8782ae7f789ad5b8cab97d3c8419f9f98b6eb285.

• What are the mypy flags you are using? Just the defaults.

Some things I tried:

• Replace the type var for take_while with a fresh type var U = TypeVar('U'). This did not help.
• Change the lambdas into defs with type annotations. This did not help.

[ilevkivskyi]

This is a duplicate of #5738. As you can see from the discussion there, the fix might take some time.