Crash: Must not defer during final iteration

[cbornet]

Crash Report

Mypy crashed when changing the import path of the parent class.
The crash happens on checking

class BaseLanguageModel(
    RunnableSerializable[LanguageModelInput, LanguageModelOutputVar], ABC
)

when changing the RunnableSerializable import from

from langchain_core.runnables import Runnable, RunnableSerializable

to

from langchain_core.runnables.base import Runnable, RunnableSerializable

./langchain_core/language_models/base.py:116: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.19.1

Traceback

Traceback (most recent call last):
  File "mypy/semanal.py", line 7568, in accept
  File "mypy/nodes.py", line 995, in accept
  File "mypy/semanal.py", line 978, in visit_func_def
  File "mypy/semanal.py", line 1023, in analyze_func_def
  File "mypy/semanal.py", line 7214, in defer
AssertionError: Must not defer during final iteration

To Reproduce

See langchain-ai/langchain#34520

• clone the project

git clone https://github.com/cbornet/langchain.git

• checkout PR branch

git checkout fix-tc

• go to core sub-project

cd core

• run mypy (using uv)

uv run --all-groups mypy . --show-traceback

Your Environment

• Mypy version used: 1.18.2, 1.19.1
• Mypy command-line flags:
• Mypy configuration options from pyproject.toml:

plugins = ["pydantic.mypy"]
strict = true
enable_error_code = "deprecated"
disallow_any_generics = false

• Python version used: 3.10, 3.11, 3.12, 3.13, 3.14
• Operating system and version: macOS 15.1, Ubuntu 24.04