New "Must not defer during final iteration" crash

[hauntsaninja]

Crash Report

It looks like mypy has started crashing on one of the mypy_primer projects, https://github.com/FasterSpeeding/Tanjun (after the latest commit to that project). I noticed this in #14238 (comment)

mypy 0.991 does not crash on the project. I've bisected the crash to #14159 cc @ilevkivskyi

The crash is caused by the latest commit to the project: Cursed-Solutions/Tanjun@942b8b4

Traceback

...
	  File "/private/tmp/mypy_primer/bisect_mypy/mypy/mypy/build.py", line 3403, in process_stale_scc
	    mypy.semanal_main.semantic_analysis_for_scc(graph, scc, manager.errors)
	  File "/private/tmp/mypy_primer/bisect_mypy/mypy/mypy/semanal_main.py", line 85, in semantic_analysis_for_scc
	    process_functions(graph, scc, patches)
	  File "/private/tmp/mypy_primer/bisect_mypy/mypy/mypy/semanal_main.py", line 243, in process_functions
	    process_top_level_function(
	  File "/private/tmp/mypy_primer/bisect_mypy/mypy/mypy/semanal_main.py", line 282, in process_top_level_function
	    deferred, incomplete, progress = semantic_analyze_target(
	  File "/private/tmp/mypy_primer/bisect_mypy/mypy/mypy/semanal_main.py", line 339, in semantic_analyze_target
	    analyzer.refresh_partial(
	  File "/private/tmp/mypy_primer/bisect_mypy/mypy/mypy/semanal.py", line 602, in refresh_partial
	    self.accept(node)
	  File "/private/tmp/mypy_primer/bisect_mypy/mypy/mypy/semanal.py", line 6175, in accept
	    node.accept(self)
	  File "/private/tmp/mypy_primer/bisect_mypy/mypy/mypy/nodes.py", line 790, in accept
	    return visitor.visit_func_def(self)
	  File "/private/tmp/mypy_primer/bisect_mypy/mypy/mypy/semanal.py", line 829, in visit_func_def
	    self.analyze_func_def(defn)
	  File "/private/tmp/mypy_primer/bisect_mypy/mypy/mypy/semanal.py", line 864, in analyze_func_def
	    self.defer(defn)
	  File "/private/tmp/mypy_primer/bisect_mypy/mypy/mypy/semanal.py", line 5852, in defer
	    assert not self.final_iteration, "Must not defer during final iteration"
	AssertionError: Must not defer during final iteration

To Reproduce

git clone https://github.com/FasterSpeeding/Tanjun
cd Tanjun
python -m venv env
env/bin/python -m pip install hikari alluka

mypy tanjun --show-traceback --python-executable=env/bin/python

[hauntsaninja]

Okay, here's a minimised repro:

from typing import Any, Callable, Coroutine, Generic, Optional, TypeVar, Union
from typing_extensions import Concatenate, ParamSpec, Self

_P = ParamSpec("_P")
_T = TypeVar("_T")
_ContextT_contra = TypeVar("_ContextT_contra", bound="Context", contravariant=True)
_CoroT = Coroutine[Any, Any, _T]

_MaybeAwaitable = Callable[_P, Union[_CoroT[_T], _T]]

_ErrorHookSig = _MaybeAwaitable[Concatenate[_ContextT_contra, Exception, _P], Optional[bool]]
ErrorHookSig = _ErrorHookSig[_ContextT_contra, ...]

class Hooks(Generic[_ContextT_contra]):
    def add_on_error(self, callback: ErrorHookSig[_ContextT_contra], /) -> Self:
        raise NotImplementedError

class Context: ...

[hauntsaninja]

And slightly simpler:

from typing import Callable, TypeVar
from typing_extensions import Concatenate, ParamSpec

_P = ParamSpec("_P")
_ContextT = TypeVar("_ContextT", bound="Context")

_MaybeAwaitable = Callable[_P, bool]
_ErrorHookSig = _MaybeAwaitable[Concatenate[_ContextT, _P], bool]

def add_on_error(callback: _ErrorHookSig[_ContextT, ...]) -> None:
    raise NotImplementedError

class Context: ...