Maybe indicative of deeper issues:

{-# Language RankNTypes #-}
{-# Language DataKinds  #-}
{-# Language PolyKinds  #-}
{-# Language GADTs      #-}

import Data.Kind

data WHICH = OP | OPOP

data Fun :: forall (a :: WHICH). a ~ OP => Type -> Type -> Type where
  MkFun :: (a -> b) -> Fun a b

There are some artifacts Fun ('GHC.Type.Eq# <>) in the type of MkFun that shouldn't be there

$ ~/code/unmodifiedghc/inplace/bin/ghc-stage2 --interactive -ignore-dot-ghci ~/hs/655_bug.hs
GHCi, version 8.7.20181029: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( /home/baldur/hs/655_bug.hs, interpreted )
Ok, one module loaded.
*Main> :t MkFun
MkFun :: (a -> b) -> Fun ('GHC.Types.Eq# <>) a b
*Main> :k Fun
Fun :: (a ~ 'OP) => * -> * -> *
*Main>

Tangent: Omitting {-# Language GADTs #-} we get the term "equational constraint" which is not the term I have seen in the wild

$ latestbug 655_bug.hs
GHCi, version 8.7.20181017: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( 655_bug.hs, interpreted )

655_bug.hs:9:1: error:
    Illegal equational constraint a ~ 'OP
    (Use GADTs or TypeFamilies to permit this)
  |
9 | data Fun :: forall (a :: WHICH). a ~ OP => Type -> Type -> Type where
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
Failed, no modules loaded.
Prelude>