Bound TypeVar doesn't unify with its bound

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#3,946 3 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
devtools

Research direction

Start by reproducing the reported example in foo.py with mypy and confirm the invalid index error for the bound TypeVar. Trace the type-checking entry point that handles Type[T] against Dict[Type[U], int], then add a regression test for this example. Done means the reported code type-checks without the invalid-index error.

Written by the indexing model from the issue text.

Description

bug priority-1-normal topic-type-variables topic-union-types 
from typing import *
U = Union[int, str]
d: Dict[Type[U], int] = {int: 1, str: 2}
T = TypeVar('T', bound=U)
def get(t: Type[T]) -> int:
    return d[t]

foo.py:6: error: Invalid index type Type[T] for Dict[Union[Type[int], Type[str]], int]; expected type "Union[Type[int], Type[str]]"

Using mypy 0.521

Dominant language
Python
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