Illegal polymorphic or qualified type with generics

A cut-down tcrun007:

{-# LANGUAGE Generics, TypeOperators #-}

import Data.Generics

class Tag a where
  nCons :: a -> Int
  nCons {| a :*: b |} _ = 1

is failing with:

tcrun007.hs:5:0:
    Illegal polymorphic or qualified type: forall a b. a :*: b
    In the type pattern of a generic definition:
      forall a b. a :*: b
    In the class declaration for `Tag'

Trac metadata

Trac field	Value
Version	6.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case	tcrun007
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown