(🐞) Incorrect narrowing unions of intersections to `Never`
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bug
- Dominant language
- Python
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- 20.6k
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Description
class A: pass
class B: pass
class C: pass
x: A
reveal_type(x) # N: Revealed type is "A"
assert isinstance(x, B | C)
reveal_type(x) # N: Revealed type is "A & B | A & C"
assert isinstance(x, C)
reveal_type(x) # N: Revealed type is "Never"
# Expected "A & B & C | A & C" simplifies to "A & C"
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
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- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the linked mypy-play reproduction and compare the revealed types after each isinstance assertion. Trace the narrowing path that produces Never, then verify that the final revealed type matches the expected A & C result.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 42/100