[GHC] #15419: GHC 8.6.1 regression (buildKindCoercion)

[GHC]

#15419: GHC 8.6.1 regression (buildKindCoercion)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15346            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by monoidal):

 I've tried to simplify the original bug report, but it's still long:

 {{{
 #!hs
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE UndecidableInstances #-}
 module Bug where

 import Data.Kind

 data Prod a b
 data Proxy p = Proxy

 -----

 data family Sing :: forall k. k -> Type
 data instance Sing x = STuple

 -----

 type family Rep1 (f :: k -> Type) :: k -> Type
 type instance Rep1 ((,) a) = Prod a

 type family From1 (f :: Type -> Type) a (z :: f a) :: Rep1 f a
 type family To1 (f :: Type -> Type) a (z :: Rep1 f a) :: f a

 class Generic1 (f :: Type -> Type) where
   sFrom1 :: forall (a :: Type) (z :: f a).      Proxy z -> Sing (From1 f a
 z)
   sTo1   :: forall (a :: Type) (r :: Rep1 f a). Proxy r -> Proxy (To1 f a
 r :: f a)

 instance Generic1 ((,) a) where
   sFrom1 Proxy = undefined
   sTo1   Proxy = undefined

 -----

 type family Fmap (g :: b) (x :: f a) :: f b
 type instance Fmap (g :: b) (x :: (u, a)) = To1 ((,) u) b (Fmap g (From1
 ((,) u) a x))

 class PFunctor (f :: Type -> Type) where
   sFmap         :: forall a b (g :: b) (x :: f a).
                    Proxy g -> Sing x -> Proxy (Fmap g x)

 instance PFunctor (Prod a) where
   sFmap _ STuple = undefined

 sFmap1 :: forall (f :: Type -> Type) (u :: Type) (b :: Type) (g :: b) (x
 :: f u).
                  (Generic1 f,
                   PFunctor (Rep1 f),
                   Fmap g x ~ To1 f b (Fmap g (From1 f u x)) )
               => Proxy g -> Proxy x -> Proxy (Fmap g x)
 sFmap1 sg sx = sTo1 (sFmap sg (sFrom1 sx))

 sFmap2  :: forall (p :: Type) (a :: Type) (b :: Type) (g :: b) (x :: (p,
 a)).
           Proxy g -> Proxy x -> Proxy (Fmap g x)
 sFmap2 = sFmap1
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15419#comment:4>
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