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I would expect test1 and test2 below to typecheck.

This is a reduced test case from trying to use DeriveAnyClass on the Generic class of the generics-sop package, which unfortunately fails due to this bug.

{-# LANGUAGE DeriveAnyClass, StandaloneDeriving, TypeFamilies #-}
module Test where

class C1 a where
  type T1 a
  type instance T1 a = Char

class C2 a where -- equivalent to C1
  type T2 a
  type instance T2 a = Char

class C3 a where -- equivalent to C1, C2
  type T3 a
  type instance T3 a = Char

data A = B
  deriving C1

deriving instance C2 A

instance C3 A

-- fails
-- test1 :: T1 A
-- test1 = 'x'

-- fails
-- test2 :: T2 A
-- test2 = 'x'

-- succeeds
test3 :: T3 A
test3 = 'x'

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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