[GHC] #15346: Core Lint error in GHC 8.6.1: From-type of Cast differs from type of enclosed expression

[GHC]

#15346: Core Lint error in GHC 8.6.1: From-type of Cast differs from type of
enclosed expression
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            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:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by monoidal):

 Here's a smaller version:

 {{{
 #!hs
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeApplications #-}
 module SGenerics where

 import Data.Kind
 import Data.Proxy

 -----

 type family Rep (a :: Type) :: Type
 type instance Rep () = ()

 type family PFrom (x :: a) :: Rep a

 -----

 class SDecide k where
   test :: forall (a :: k). Proxy a

 instance SDecide () where
   test = undefined

 test1 :: forall (a :: k). SDecide (Rep k) => Proxy a
 test1 = seq (test @_ @(PFrom a)) Proxy

 test2 :: forall (a :: ()). Proxy a
 test2 = test1
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15346#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler