TYPE_CHECKING block leads to "Reached allegedly unreachable code!"

[kasium]

If a module contains a TYPE_CHECKING block, it can lead to a runtime error.

from __future__ import annotations

from typing import TYPE_CHECKING, Any

if TYPE_CHECKING:
    BaseDict = dict[Any, Any]
else:
    BaseDict = dict


class MyDict(BaseDict):
    pass

To reproduce

mypyc foo.py
python -c "import foo"

Error

Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import foo
  File "foo.py", line 8, in <module>
    BaseDict = dict
RuntimeError: Reached allegedly unreachable code!

[ilevkivskyi]

Hm, this is pretty bad, if TYPE_CHECKING: ... is a quite common pattern. I am surprised no one encountered this before. cc @JukkaL

[JukkaL]

We should probably treat TYPE_CHECKING as False when running the code. However, the else block in the example hasn't been fully processed by mypy, so compiling it may be tricky.