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When I say

{-# LANGUAGE ScopedTypeVariables, TypeInType, TypeOperators, TypeFamilies,
             AllowAmbiguousTypes #-}

import Data.Proxy
import Data.Type.Equality
import Data.Type.Bool
import Data.Kind

data SameKind :: forall k. k -> k -> Type
type family IfK (e :: Proxy (j :: Bool)) (f :: m) (g :: n) :: If j m n where
   IfK (_ :: Proxy True)  f _ = f
   IfK (_ :: Proxy False) _ g = g

y :: forall ck (c :: ck). ck :~: Proxy True -> ()
y Refl = let x :: forall a b (d :: a). SameKind (IfK c b d) d
             x = undefined
         in ()

HEAD says

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.7.20180827 for x86_64-apple-darwin):
	ASSERT failed!
  Bad coercion hole co_a3iZ: If
                               j_a3j0[tau:2] m_a3j1[tau:2] a_a3gV[sk:3]
                             a_a3gV[sk:3]
                             nominal
                             If j_a3j0[tau:2] m_a3j1[tau:2] a_a3jj[sk:3] ~# a_a3jj[sk:3]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable
        pprPanic, called at compiler/utils/Outputable.hs:1219:5 in ghc:Outputable
        assertPprPanic, called at compiler/typecheck/TcMType.hs:316:25 in ghc:TcMType

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

It's as yet unclear whether the program should be accepted. My best guess is that it should, but that (even with this panic fixed) GHC isn't up to the task.

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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