Daemon crashes with a decorator returning `Generic/Protocol`, decorated method returning `Self`, and an undefined symbol

[bzoracler]

Crash Report & To Reproduce

With a relatively simple reproducer and small example, this causes a crash. Anything larger than a small example will cause dmypy to hang.

1. Set up the following dummy project
2. cd project
3. dmypy run
4. Change project/src/pkg/utils.py (e.g. add a = 1234 at the end of this file)
5. dmypy run again (crashes)

Dummy project:

project/
    src/
        pkg/
            __init__.py (blank)
            module.py
            utils.py
    mypy.ini

• mypy.ini

  [mypy]
  files = src/
  

• module.py

  from typing_extensions import Self
  from pkg.utils import deco
  
  var = UNDEFINED_SYMBOL
  
  class A:
      @deco
      def mymethod(self) -> Self: ...

• utils.py

  from collections.abc import Callable
  from typing import Protocol, TypeVar
  from typing_extensions import ParamSpec
  
  _P = ParamSpec("_P")
  _R_co = TypeVar("_R_co", covariant=True)
  _T = TypeVar("_T")
  
  def deco(f: Callable[_P, _T], /) -> _Method[_P, _T]: ...
  
  class _Method(Protocol[_P, _R_co]): ...

Traceback

Runtime error from non-compiled mypy is below on master (commit 842a8fd)

Daemon crashed!
Traceback (most recent call last):
  File ".../python3.13/site-packages/mypy/dmypy_server.py", line 237, in serve
    resp = self.run_command(command, data)
  File ".../python3.13/site-packages/mypy/dmypy_server.py", line 286, in run_command
    ret = method(self, **data)
  File ".../python3.13/site-packages/mypy/dmypy_server.py", line 354, in cmd_run
    return self.check(sources, export_types, is_tty, terminal_width)
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../python3.13/site-packages/mypy/dmypy_server.py", line 433, in check
    messages = self.fine_grained_increment_follow_imports(
        sources, explicit_export_types=export_types
    )
  File ".../python3.13/site-packages/mypy/dmypy_server.py", line 640, in fine_grained_increment_follow_imports
    messages = fine_grained_manager.update(changed, [], followed=True)
  File ".../python3.13/site-packages/mypy/server/update.py", line 286, in update
    changed_modules = propagate_changes_using_dependencies(
        self.manager,
    ...<5 lines>...
        self.processed_targets,
    )
  File ".../lib/python3.13/site-packages/mypy/server/update.py", line 857, in propagate_changes_using_dependencies
    raise RuntimeError("Max number of iterations (%d) reached (endless loop?)" % MAX_ITER)
RuntimeError: Max number of iterations (1000) reached (endless loop?)

Your Environment

• Mypy version used: 1.18.2, master
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.13
• Operating system and version: Pop OS! 22.04 LTS

[bzoracler]

It looks like the undefined symbol is not related; several errors could also trigger the crash. Here's another reproducer, this time without an undefined symbol.

Replace module.py with module.pyi and use the following contents:

from collections.abc import MutableMapping
from typing_extensions import Self

from pkg.utils import deco

class MyDict(MutableMapping[object, object]): ...

class A:
    @deco
    def mymethod(self) -> Self: ...

This has the same reproduction steps as the top comment.

There's an error which is emitted on the first dmypy run, which is likely part of the crash reproducer.