AssertionError: Must not defer during final iteration on large codebase #15723

Crash Report

Running mypy . on a large codebase that has not been type-checked crashed with AssertionError: Must not defer during final iteration.

mypy outputs 481 lines of messages (mostly errors and notes) before crashing.
pyright on the same codebase results in 9756 errors.

Traceback

Traceback (most recent call last):
  File "mypy/semanal.py", line 6451, in accept
  File "mypy/nodes.py", line 786, in accept
  File "mypy/semanal.py", line 833, in visit_func_def
  File "mypy/semanal.py", line 868, in analyze_func_def
  File "mypy/semanal.py", line 6123, in defer
AssertionError: Must not defer during final iteration

To Reproduce

It can be reproduced on our codebase, but no attempt to make small example was successful.
Unfortunately only info we can share publicly is above traceback.
Running mypy only on the file that the INTERNAL ERROR was raised did not result in crash.

Note: rm -rf .mypy_cache/ did not have any effect.

Your Environment

• Mypy version used: 1.4.1
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used:
• Operating system and version: