fix Match case on type don't exaust on types #598 #1994
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Pull request overview
This PR fixes issue #598 where match case patterns with type guards and aliases (using as) were not properly narrowing the original match subject. The fix ensures that when a case introduces an alias, type narrowing operations are mirrored back onto the original subject, allowing subsequent branches and exhaustiveness checks to work correctly.
Key Changes:
- Added logic to rebase narrowing operations from alias names back to original match subjects
- Implemented facet subject merging for proper handling of nested property access patterns
- Updated type narrowing to properly track and apply constraints across aliased patterns
Reviewed changes
Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
pyrefly/lib/test/pattern_match.rs |
Added test case demonstrating that match aliases now correctly narrow the original subject variable |
pyrefly/lib/test/flow_branching.rs |
Updated existing test assertions to reflect correct narrowing behavior (e.g., x now narrows to Literal[2] and B instead of broader types) |
pyrefly/lib/binding/pattern.rs |
Modified Pattern::MatchAs handling to rebase alias narrowing operations onto the original subject when they differ |
pyrefly/lib/binding/narrow.rs |
Added rebase_subject, rebase_facet_subject, and merge_facet_subjects functions plus and_for_subject method to support narrowing operation rebasing |
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According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
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Summary
Fixes #598
Updated Pattern::MatchAs so that, when a case introduces an alias, the resultant narrows are mirrored back onto the original match subject (without rebinding names twice), ensuring later branches see the refined type.
Introduced NarrowOp::rebase_subject plus facet-merging helpers and reworked NarrowOps::and_for_subject in to re-target alias narrows, compose facet chains/origins, and insert the rebased operations without placeholder pollution so negated flows correctly eliminate matched variants.
Test Plan
Added test_match_alias_narrows_subject