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False positive Case pattern can never match when matching union against protocol #5059
Description
Describe the Bug
When matching a union type against a Protocol, pyrefly incorrectly thinks matching is impossible.
The warning doesn't get triggered when a non-union type is used.
from typing import Protocol, runtime_checkable
@runtime_checkable
class SupportsMethod(Protocol):
def method(self) -> None: ...
class Foo: ...
def check(arg: Foo) -> None:
match arg:
case SupportsMethod(): # OK
pass
def check_union(arg: Foo | None) -> None:
match arg:
case SupportsMethod(): # WARNING: case pattern can never match
pass WARN sandbox.py:17:9-18:17: Case pattern can never match subject of type `Foo | None` [unreachable-match-case]
Sandbox Link
(Only applicable for extension issues) IDE Information
No response
Activity
shahhamdihassan commentedon Oct 23, 2026
Thanks for the report! This reproduces on 1.4.0, and None isn't needed to trigger it (Foo | Bar does too): https://pyrefly.org/sandbox/?project=v2.xVVLboMwEL3KSNmURTgAq6qVssuqB0AuGGw1sRHYaqVevvM8DlAaVVlECuxseWae38fft0fmEmmdZOQYXbDMWGN084EwATls0yvr4ry3OKDSdBSHXAoW1VXzLOKhsiylttQ5eP9n7UWNq7XnNOLcl_NRt6ttdGL59yf9lAzKBbdSPavQmGTfRfCcvHoLoqiIdqS_BliR3Oyxyycqw7-jg2ID0eAnCzYqUjTFdxnRdxiCOzhcg2Wj55soShk34l2oP20wNZrMc-dcuM_40bFJGgPO9qnCHmduBcRp51pJKQ4hVjE5rVvdUvAURtv3Gr79hcd3dX5X1oCYy8fgefUjLElwGcujysoBnCSoBNAogNvyk_YFjEDJRx9Dz7-e_wE
We'll follow up here once we have a fix.
github-actions commentedon Nov 7, 2026
This issue has someone assigned, but has not had recent activity for more than 2 weeks.
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shahhamdihassan commentedon Nov 9, 2026
Still on this one. The fix is written and waiting for review on our side. I'll post here once it lands.