Pattern Synonyms in Typeclasses

data A t where
   AX :: A X
   AY :: A Y
   AZ :: A Z

class HasX a where
     data pattern X :: a X

instance HasX A where
    data pattern X = AX

d :: A t -> String
d = \case
    X  -> ""
    AY -> ""
    AZ -> "" 

The above example should compile and emit no warnings with -fwarn-incomplete-patterns. Would require #8779 (closed)

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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