Pyrefly doesn't fully narrow tuple in exhaustible match-case #5206

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1. asukaminato0721

   added

   narrowingIssues with narrowing - root cause is usually narrowing, flow handling, or bothIssues with narrowing - root cause is usually narrowing, flow handling, or both

   on Oct 12, 2026
2. StefanBRas commented on Oct 13, 2026

   

   StefanBRas

   on Oct 13, 2026

   Author

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   I tried avoiding wildcards but got something similar, let me know if this should be a separate issue. It does seem related to how narrowing works for tuples though. Also it works differently in the sandbox versus my local install (v1.3.2), it differs in pick2.

   from typing import assert_never
   class A: ...
   
   def pick(left: A | None, right: A | None) -> A: # ERROR sandbox.py:4:46-47: Function declared to return `A`, but one or more paths are missing an explicit `return` [bad-return]
       match left, right:
           case None, None:
               return A()
           case None, A():
               return right
           case A(), None:
               return left
   
   # However it correctly asserts it to never (in sandbox, not in locally installed version v1.3.2)
   
   def pick2(left: A | None, right: A | None) -> A:
       match left, right:
           case None, None:
               return A()
           case None, A():
               return right
           case A(), None:
               return left
           case A(), A():
               return right
           case wildcard:
               assert_never(wildcard) # Works in sandbox, not in v1.3.2
   
   # Looks to be related to tuples, this works fine
   
   class B: ...
   def pick(left: A | B) -> A | B:
       match left:
           case A():
               return left
           case B():
               return left
           

   sandbox: https://pyrefly.org/sandbox/?project=v2.1VZPT4MwFP8qL-GwmUyIk2jCjSUaDyaaXTx4gUEFMrYu0G367f29lmYMkGxHb9D09bW_f-0VkdmOJOulMCDXda1yd0WynpbiSwUUNnpFPBVZ3hrQkkCZQ0_L5duSTl0DP_Afbv3HgJ4bB0BIaAMUSEkrxyiMZrTaIy9gBghhA9B0YNTwkKBNUUPXGcVbEt-7skigt8iURvS5ilMgyz9gwohKJTnxlu1GT_JGzormDB1DtdwRsiQHKjA-XKC7dEoweawHb48hduhFHhl9qB6hWrF6yp-GFwSzYpg0PTSFNRpkZwQ9sVVKmbCE8QnbI5FTsko93Ln37lx7y9I4v5DHfwdjv-KqHseiTJO4SjsVbW9M7ZwbiPxDVmtQ02fDYG5ofZUSk0DeSqB3GSMc-E_tcU3hflU5Mv2oF0IM6bvGuG_RuG_AewvDD3_1OOqS8ycAA4gtxub2I9GhdzNgzw_dIkuzPZ4cUB727uC1IShXalcHnmfLZZV5YuulMqm9swqPOP3jQ1zoh8z5ciR1hNdYdjiL_YuyeGKieDKWxBMOYswYDddf

   👍React with 👍1Reacted by Felix Puscasu
3. rchen152

   self-assigned this

   on Nov 5, 2026
4. rchen152

   marked Exhaustive tuple match reports a missing return #5621 as a duplicate of this issue on Nov 13, 2026
5. rchen152 commented on Nov 13, 2026

   

   rchen152

   on Nov 13, 2026

   Contributor

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   Related:

   def without_fallback(left: bool, right: bool) -> int:  # spurious bad-return
       match left, right:
           case True, True: return 0
           case True, False: return 1
           case False, True: return 2
           case False, False: return 3
   
6. rchen152 commented on Nov 13, 2026

   

   rchen152

   on Nov 13, 2026

   Contributor

   More actions

   Looks like the original two examples work in the sandbox now, but #5206 (comment) and the pick example are still broken:

   class A: ...
   
   def pick(left: A | None, right: A | None) -> A: # ERROR sandbox.py:4:46-47: Function declared to return `A`, but one or more paths are missing an explicit `return` [bad-return]
       match left, right:
           case None, None:
               return A()
           case None, A():
               return right
           case A(), None:
               return left
           case A(), A():
               return right