No error on new union syntax with union of fake types (`TypeAlias` and `None`)
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bug
topic-pep-604
- Dominant language
- Python
- Stars
- 20.6k
- Forks
- 3.3k
- PR merge metrics
- PR metrics pending
Description
from typing import TypeAlias
AMONGUS: TypeAlias = "int"
IMPOSTER: TypeAlias = "str"
a: AMONGUS | IMPOSTER = 0 # SUS ALERT
b: AMONGUS | None = None # SUS ALERT
Any types in a new union syntax that are fake(looking at you None) will be runtime errors.
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 provided Python reproducer and confirm how mypy handles the TypeAlias references and None in the new union syntax. Trace the relevant type-checking path for these union members, then add regression coverage demonstrating the intended diagnostic or behavior; done means the reported fake-type cases no longer pass without the expected error.
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
- 35/100