fix Sequence pattern in match/case does not narrow tuple out of a union #3147

[asukaminato0721]

Summary

Fixes #3147

Sequence patterns like (_, _) were picking up Placeholder narrows from wildcard and other irrefutable subpatterns, and those placeholders intentionally block negative narrowing.

In a later case, that prevented the fallback arm from collapsing to Never.

now strip those placeholders when every sequence subpattern is irrefutable, so the carried negation stays precise.

Test Plan

add test

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[Copilot]

Pull request overview

Fixes pyrefly’s match/case narrowing so that an irrefutable sequence pattern (e.g. (_, _)) properly participates in negative narrowing across subsequent case arms, allowing union exhaustiveness to be recognized (Issue #3147).

Changes:

• Add a regression test covering float | tuple[float, float] exhaustiveness with a tuple-shaped sequence pattern plus a float() class pattern.
• In sequence-pattern binding, detect when all subpatterns are irrefutable/wildcards and strip Placeholder narrow ops so they don’t block negated narrowing across later cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pyrefly/lib/test/pattern_match.rs	Adds a regression test for match exhaustiveness/narrowing in the reported tuple-in-union scenario.
pyrefly/lib/binding/pattern.rs	Adjusts sequence-pattern narrowing to remove spurious placeholders when the sequence subpatterns are all irrefutable, improving cross-case narrowing.

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