[GHC] #16204: GHC HEAD-only Core Lint error (Argument value doesn't match argument type)

[GHC]

#16204: GHC HEAD-only Core Lint error (Argument value doesn't match argument type)
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.7
  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:  #16188            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by monoidal):

 I simplified to:

 {{{
 #!haskell
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 module T16204 where

 import Data.Kind

 data family Sing :: forall k. k -> Type
 type family Rep (a :: Type) :: Type
 type family PTo (x :: Rep a) :: a

 sTo :: forall (k :: Type) (a :: Rep k). (Sing a, Sing (PTo a :: k))
 sTo = sTo

 x :: forall (a :: Type). Sing a
 x = fst sTo
 }}}

 This program is rejected (IMO correctly) by 8.6, but gives Core Lint error
 in HEAD.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16204#comment:2>
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