Type issues with Callables

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

• Please insert below the code you are checking with mypy

from typing import Optional, Iterable, TypeVar, Callable

T = TypeVar('T')


def is_even(n: int) -> bool:
    return n % 2 == 0


# equivalent to https://ruby-doc.org/core-2.5.0/Enumerable.html#method-i-detect
def detect(function: Callable[[T], bool],
           iterable: Iterable[T]) -> Optional[T]:
    for element in iterable:
        if function(element):
            return element
    return None


numbers = [1, 2, 3, 4, 5]

# using a lambda
print(detect(lambda n: n % 2 == 0, numbers))

# using a function
print(detect(is_even, numbers))

• What is the actual behavior/output?

$ mypy detect.py 
detect.py:22: error: Unsupported operand types for % ("object" and "int")
detect.py:25: error: Argument 1 to "detect" has incompatible type "Callable[[int], bool]"; expected "Callable[[object], bool]"
Found 2 errors in 1 file (checked 1 source file)

• What is the behavior/output you expect?
  I was expecting the types to be inferred and the above code to typecheck.

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

$ mypy --version
mypy 0.770
$ python --version
Python 3.7.7

• Do you see the same issue after installing mypy from Git master?
  yes

$ mypy detect.py
detect.py:22: error: Unsupported operand types for % ("object" and "int")
detect.py:25: error: Argument 1 to "detect" has incompatible type "Callable[[int], bool]"; expected "Callable[[object], bool]"
Found 2 errors in 1 file (checked 1 source file)
$ mypy --version
mypy 0.770+dev.22c67daac7d2b28f33d4e9c7bee4f1a06e61a3e5

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

• If mypy crashed with a traceback, please paste the full traceback below.
  It didn't crash

まず、提供された detect.py の再現コードを報告された mypy のバージョンで実行し、Callable と TypeVar の推論が lambda および is_even 引数をどのように処理するかを調べます。関連する型チェックの動作を追跡し、適切な場合はリグレッションテストのカバレッジを追加します。例が報告された 2 つのエラーなしで型チェックを通過すれば完了です。