GeneralizedNewtypeDeriving and polymorphic arguments don't play nicely together

I'm not sure if this is a bug per se, but ghci says to report it so here it is:

test.hs:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

class NameOf a where
  nameOf :: proxy a -> String

instance NameOf Int where
  nameOf _ = "Int"

newtype MyInt = MyInt Int
  deriving (NameOf)

$ ghci test.hs
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( test.hs, interpreted )

test.hs:10:13: warning: [-Wdeferred-type-errors]
    • Couldn't match representation of type ‘proxy1 Int’
                               with that of ‘proxy1 MyInt’
        arising from a use of ‘GHC.Prim.coerce’
      NB: We cannot know what roles the parameters to ‘proxy1’ have;
        we must assume that the role is nominal
    • In the expression:
        GHC.Prim.coerce
          @(forall (proxy :: TYPE GHC.Types.PtrRepLifted
                             -> TYPE GHC.Types.PtrRepLifted).
            proxy Int -> String)
          @(forall (proxy :: TYPE GHC.Types.PtrRepLifted
                             -> TYPE GHC.Types.PtrRepLifted).
            proxy MyInt -> String)
          nameOf
      In an equation for ‘nameOf’:
          nameOf
            = GHC.Prim.coerce
                @(forall (proxy :: TYPE GHC.Types.PtrRepLifted
                                   -> TYPE GHC.Types.PtrRepLifted).
                  proxy Int -> String)
                @(forall (proxy :: TYPE GHC.Types.PtrRepLifted
                                   -> TYPE GHC.Types.PtrRepLifted).
                  proxy MyInt -> String)
                nameOf
      When typechecking the code for ‘nameOf’
        in a derived instance for ‘NameOf MyInt’:
        To see the code I am typechecking, use -ddump-deriv
      In the instance declaration for ‘NameOf MyInt’
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.2 for x86_64-apple-darwin):
	opt_univ fell into a hole {a15j}

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

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture