"Type variable is invalid as target for type alias" error on new union syntax when not using `TypeAlias` type
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bug
- Dominant language
- Python
- Stars
- 20.6k
- Forks
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- PR merge metrics
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Description
from typing import TypeVar, TypeAlias
T = TypeVar("T")
MaybeList = T | list[T] # error: Type variable "__main__.T" is invalid as target for type alias
MaybeList2: TypeAlias = T | list[T] # no error
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Research direction
Reproduce the reported snippet in the linked mypy playground and trace the type-alias handling for an unannotated union assignment versus an explicit TypeAlias. Done means the new union syntax no longer reports the invalid target error while the explicit TypeAlias example continues to behave correctly.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- devtools
- Issue type
- Bug
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Stale
- Clarity
- Clearly specified
- Newbie friendliness
- 42/100