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import statements under a parametrised protocol results in a crash #14889

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@XM-Helico

Description

@XM-Helico

Crash Report & Reproduction

The following snippet will cause a crash:

import typing as tp

T = tp.TypeVar("T")

class Proto(tp.Protocol[T]):
    import urllib
**Traceback**
<file>: 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: 1.1.1
Traceback (most recent call last):
  File "mypy/checker.py", line 582, in accept
  File "mypy/nodes.py", line 1123, in accept
  File "mypy/checker.py", line 2219, in visit_class_def
  File "mypy/checker.py", line 2403, in check_protocol_variance
  File "mypy/subtypes.py", line 177, in is_subtype
  File "mypy/subtypes.py", line 336, in _is_subtype
  File "mypy/types.py", line 1355, in accept
  File "mypy/subtypes.py", line 603, in visit_instance
  File "mypy/subtypes.py", line 1041, in is_protocol_implementation
AssertionError: 

No issues happen when the protocol isn't parameterised.

Your Environment

  • Mypy version used: 1.1.1 and master (mypy-play.net)
  • Mypy command-line flags: None
  • Mypy configuration options from mypy.ini (and other config files): None
  • Python version used: 3.10 and 3.11

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