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The following code compiles on GHC 8.0.2 through 8.6.2:

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
module Bug where

import Data.Kind

class C k where
  type S :: k -> Type

data D :: Type -> Type
data SD :: forall a. D a -> Type

instance C (D a) where
  type S = SD

But fails to compile on GHC HEAD (commit 73cce63f):

$ ~/Software/ghc2/inplace/bin/ghc-stage2 Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:15:3: error:
    • Type indexes must match class instance head
      Expected: S @(D a)
        Actual: S @(D a1)
    • In the type instance declaration for ‘S’
      In the instance declaration for ‘C (D a)’
   |
15 |   type S = SD
   |   ^^^^^^^^^^^

This regression prevents the stitch library from compiling.

Trac metadata

Trac field	Value
Version	8.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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Architecture