Crash in version 1.11 lambda processing (regression since version 1.10) #17595

Crash Report

We maintain a large (1M+ LoC), proprietary Python code base, currently using mypy version 1.10. During an upgrade to version 1.11, I noticed some INTERNAL ERROR output so decided to investigate.

Traceback

bug.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: 1.11.0
Traceback (most recent call last):
  File "mypy/checkexpr.py", line 5830, in accept
  File "mypy/nodes.py", line 2278, in accept
  File "mypy/checkexpr.py", line 5294, in visit_lambda_expr
  File "/opt/homebrew/Cellar/python@3.12/3.12.4/Frameworks/Python.framework/Versions/3.12/lib/python3.12/contextlib.py", line 137, in __enter__
    return next(self.gen)
           ^^^^^^^^^^^^^^
  File "mypy/binder.py", line 436, in frame_context
AssertionError:

To Reproduce

Since I am unable to provide our full source code, I created the following small stand-alone example:

def foo(x):
    pass


class Bar:
    def baz(self, x):
        pass


class Qux(Bar):
    @foo(lambda x: None)
    def baz(self, x) -> None:
        pass

The crash seems to occur when a method decorator defined with a lambda overrides another method of the same name in a base class and there's an annotation mismatch.

That is, for the above example, the crash can be avoided by adding a return value annotation to baz() in the base class:

 class Bar:
-    def baz(self, x):
+    def baz(self, x) -> None:
         pass

Investigation

I narrowed the regression down to the following commit (the crash is avoided if I revert this change):

5059ffd

The crash can also be avoided if I adjust the assertion at the crash site:

--- binder.py.bak	2024-07-26 10:54:02
+++ binder.py	2024-07-26 10:54:07
@@ -433,7 +433,7 @@
         whether any types changed in the newly-topmost frame as a result
         of popping this frame.
         """
-        assert len(self.frames) > 1
+        assert len(self.frames) > 0

         if break_frame:
             self.break_frames.append(len(self.frames) - break_frame)

pytest -q mypy passes after this change, and a full run of mypy 1.11 on our large code base also succeeds without any tracebacks or obvious ill-effects, but I'm otherwise not familiar enough with the innards of mypy to have confidence in its correctness (hence I felt more comfortable providing the FYI here instead of submitting a PR).

Your Environment

• Mypy version used: 1.11
• Mypy command-line flags: None (defaults)
• Mypy configuration options from mypy.ini (and other config files): Reproduced with defaults
• Python version used: 3.11.9
• Operating system and version: macOS Sonoma 14.5