Core lint error with PatternSynonyms and undefined

I've submitted quite a few Core Lint errors lately, I don't have access to GHC HEAD so ignore this ticket if this has already been resolved

{-# Options_GHC -dcore-lint #-}

{-# Language ViewPatterns, PatternSynonyms #-}

import Data.Bifunctor.Fix
import Data.Bifunctor.Tannen

newtype ZipList a = ZL { getZipList :: Fix (Tannen Maybe (,)) a }

pattern Nil :: ZipList a
pattern Nil = ZL (In (Tannen Nothing))

infixr 5 :::
pattern (:::) :: a -> ZipList a -> ZipList a
pattern a:::as <- ZL (In (Tannen (undefined -> (a, as))))
  where a:::as = undefined 

↑ works fine without -dcore-lint but fails with it.

Works fine if undefined -> (a, as) is replaced with let x = x in x -> (a, as).

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture