Crash when TypeVar of Generic dataclass has undefined bound

[jamiesandenr]

Crash Report

MyPy crashes when run against a Generic dataclass using a TypeVar with an ill-defined bound which is passed in quotes.

Traceback

crash.py:7: error: Name "NotDefined" is not defined
crash.py:11: 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: 0.942
Traceback (most recent call last):
  File "C:\Users\<user>\AppData\Local\Programs\Python\Python38\lib\runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "C:\Users\<user>\AppData\Local\Programs\Python\Python38\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "mypy\semanal.py", line 5220, in accept
  File "mypy\nodes.py", line 739, in accept
  File "mypy\semanal.py", line 630, in visit_func_def
  File "mypy\semanal.py", line 662, in analyze_func_def
  File "mypy\semanal.py", line 4920, in defer
AssertionError: Must not defer during final iteration
crash.py:11: : note: use --pdb to drop into pdb

To Reproduce

Run mypy against the following Python module (crash.py):

import dataclasses
from typing import Generic, TypeVar

T = TypeVar("T", bound="NotDefined")

@dataclasses.dataclass
class C(Generic[T]):
    x: float

N.B.:
The crash does not happen (and mypy reports crash.py:4: error: Name "NotDefined" is not defined as expected) if:

• NotDefined is passed without quotes
• The line x: float is replaced with ...
• T is not defined at all

Your Environment

• Mypy version used: 0.942
• Mypy command-line flags: none
• Python version used: 3.8.6
• Operating system and version: Windows 10 Pro
• Mypy configuration options from mypy.ini (and other config files):

[mypy]
python_version = 3.8
warn_return_any = True
warn_unused_configs = True
mypy_path = <path to mypy>
show_error_codes = True
ignore_missing_imports  = False
follow_imports = silent

[AlexWaygood]

Looks like it's dataclasses-specific. I can reproduce with dataclasses, but not without.

[AlexWaygood]

Looks like this crash was fixed in #12762. @JukkaL, do you think we should add another test case, or just close this out?

[AlexWaygood]

Looks like this crash was fixed in #12762. @JukkaL, do you think we should add another test case, or just close this out?

Cc. @JelleZijlstra as well :)

[JelleZijlstra]

I always like more test cases when we fix something.