[bug] Assigning a builtin to a Callable changes its type to '<nothing>'

$ python --version                                                                                                                                 
Python 3.7.2

$ mypy --version
mypy 0.730+dev.8782ae7f789ad5b8cab97d3c8419f9f98b6eb285

Assigning a builtin to a Callable on the same line as the type declaration works, but doing so on separate lines does not:

$ mypy --strict -c "from typing import Callable, Any; cast: Callable[[Any], Any] = str; cast(5)"
Success: no issues found in 1 source file
$ mypy --strict -c "from typing import Callable, Any; cast: Callable[[Any], Any]; cast = str; cast(5)"
<string>:1: error: <nothing> not callable
Found 1 error in 1 file (checked 1 source file)

This becomes a problem when trying to assign builtins conditionally. Assigning lambda x: str(x) is a workaround, but is rather a hack.

Issue にある 2 つの最小限の mypy --strict -c 再現例から始め、callable の型推論が同じ行での代入と別々の代入をどのように扱うかを比較します。mypy における代入と builtin の処理を追跡し、条件付きの builtin の代入を含む両方の例が、<nothing> not callable を報告せずに型チェックを通過することを確認します。