Pattern match on LHS of pattern synonym declaration

Changing ZipList to a newtype

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

type ZipList = Fix (Tannen Maybe (,))

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

infixr 5 :::
pattern (:::) :: a -> ZipList a -> ZipList a
pattern a:::as = In (Tannen (Just (as, a)))

works fine for the Nil pattern synonym (it remains implicitly bidirectional)

newtype ZipList a = ZL (Fix (Tannen Maybe (,)) a)

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

but (:::) needs to be explicitly bidirectional

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

---

I would like to write it as

pattern (:::) :: a -> ZipList a -> ZipList a
pattern a:::ZL as = ZL (In (Tannen (Just (as, a))))

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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mentioned in issue #12203

changed weight to 5

added Tfeature request Trac import labels

closed

I'm closing this as a duplicate of #12203, since that ticket asks for the same thing (albeit with a different motivating example). I'll add this example to #12203 for posterity's sake.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedDuplicate
Related	- → #12203

added PatternSynonyms label

added Pnormal label