Use of `Annotated[T, ...]` in a recursive type results in a hang or crash
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crash
topic-recursive-types
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- Python
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Description
The following code causes mypy to never terminate.
from typing import Annotated, TypeVar
T = TypeVar("T")
R = Annotated[T, ...]
A = R["A"]
It can be turned into a sample that causes a segmentation fault.
from typing import Annotated, TypeVar
T = TypeVar("T")
R = Annotated[T, ...]
A = list[R["A"]]
def f() -> A:
return []
- Mypy version used: 1.10.0
- Mypy command-line flags: none
- Mypy configuration options from
mypy.ini(and other config files): none - Python version used: 3.12
- Operating system and version: Linux, Windows (on Windows it immediately exits with code 1)
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by running the two Python reproductions with mypy 1.10.0 and confirm the nontermination or crash described in the issue. Trace how the recursive Annotated type alias is analyzed, then add regression coverage for both examples; done means mypy terminates without crashing or hanging.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- devtools
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100