ghc: panic! occurred with use of isInstance, newName and a type splice

Compiling the below code:

{-# LANGUAGE TemplateHaskell #-}
module Test where
import Language.Haskell.TH
import Control.Monad.Instances ()    

instanceVar = $(do
                 xName <- newName "x"
                 instanceType <- [t| (Int -> $(varT xName)) |]
                 instanceOfFunctor <- isInstance ''Functor [instanceType]
                 stringE $ show instanceOfFunctor
               )

Results in the following error:

[1 of 1] Compiling Test             ( test.hs, interpreted )

test.hs:6:17:
    Exception when trying to run compile-time code:
      ghc: panic! (the 'impossible' happened)
  (GHC version 7.4.1 for x86_64-apple-darwin):
	nameModule x_a1ws{tv}

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

      Code: do { xName <- newName "x";
                 instanceType <- [t| Int -> $(varT xName) |]
                                 pending [(splice, varT xName)];
                 instanceOfFunctor <- isInstance ''Functor [instanceType];
                 .... }
    In the expression:
      $(do { xName <- newName "x";
             instanceType <- [t| Int -> $(varT xName) |];
             instanceOfFunctor <- isInstance ''Functor [instanceType];
             .... })
    In an equation for `instanceVar':
        instanceVar
          = $(do { xName <- newName "x";
                   instanceType <- [t| Int -> $(varT xName) |];
                   .... })
Failed, modules loaded: none.

While the code is not correct (replacing the instanceType line with

instanceType <- [t| (->) Int |]

fixes the problem), a better error message would be desirable.

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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