[jay23606]

Bug

mypy crashes with INTERNAL ERROR (RecursionError) when a class is checked against a contravariant generic whose base refers back to the class:

from typing import Generic, TypeVar

Z = TypeVar("Z", contravariant=True)

class N(Generic[Z]): ...

class C(N[N["C"]]): ...

def f(c: C) -> N[C]:
    return c

Checking C <: N[C] needs C <: N[C] again for the contravariant argument, so the recursion never ends. Covariant and invariant parameters do not loop here, which is why only contravariance crashes.

Change

While checking a contravariant type argument in SubtypeVisitor.visit_instance, push the (left, right) pair onto the existing subtype assumption stack. A nested check of the same pair then hits the existing is_assumed_subtype check at the top of is_subtype and returns True (coinductive assumption). The assumption is popped on exit, so a failed check still reports the error.

Only contravariant arguments push the pair, so other variances keep their current behaviour.

Testing

• Added testContravariantSelfReferentialBaseDoesNotCrash to test-data/unit/check-generics.test, covering the issue's C example and the generic variant D[X]. With the fix it passes (PYTHONPATH=.venv/Lib/site-packages python -m pytest -o addopts="" -q -k testContravariantSelfReferentialBaseDoesNotCrash mypy/test/testcheck.py). I have not confirmed that it fails on the unfixed code: the run on the base commit stopped at collection with ModuleNotFoundError: No module named 'librt' before reaching the test.
• PYTHONPATH=.venv/Lib/site-packages python -m pytest -o addopts="" -q -k "Variance or Generic or Protocol or Subtype or Callable or Tuple or TypeVar or Recursive or Alias or Overload or Typeddict or Namedtuple" mypy/test/testcheck.py: 3161 passed, 0 failed.

Not run here: the full runtests.py suite and the self-check. This machine's .venv has no pytest, so the commands above use the global Python with the venv's site-packages on PYTHONPATH. CI will run the full suite.

Regression test: untested here: CI verifies this change

Fixes #22360

Disclosure: this change was prepared with AI assistance.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅