Crash while hashing a callable type

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Issue 类型
缺陷
描述清晰度
需要澄清
活跃度
停滞
技术栈
python
领域
compilers

调研方向

Start with the traceback locations in mypy/build.py, especially finish_passes and _patch_indirect_dependencies, then inspect the hash and is_type_obj implementations in mypy/types.py. Reproduce the crash if possible and determine how a callable type reaches hashing with a missing fallback. Done means the crash is prevented and the regression is covered by a test.

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描述

needs discussion refactoring

Some time ago there appeared a crash ending in:

...
File "[somewhere]/mypy/build.py", line 1954, in finish_passes
    self.type_map())
File "[somewhere]/mypy/build.py", line 1964, in _patch_indirect_dependencies
    types = set(type_map.values())
File "[somewhere]/mypy/types.py", line 938, in __hash__
    return hash(tuple(self.items()))
File "[somewhere]/mypy/types.py", line 839, in __hash__
    return hash((self.ret_type, self.is_type_obj(),
File "[somewhere]/mypy/types.py", line 723, in is_type_obj
    return self.fallback.type.is_metaclass()
AttributeError: 'NoneType' object has no attribute 'type'

There is no repro yet. There however may be a point for a wider discussion: currently most types in mypy are hashable (they have both __hash__ and __eq__) while they are also mutable. Recently there was a discussion on Python-Dev list and some people said this is a bad pattern. I think this is OK in our case (we use hashes to cache is_subtype results).

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