stand-alone deriving sometimes fails for GADTs

Consider the following module

{-# LANGUAGE GADTs, StandaloneDeriving #-}
module Foo where

data Foo a where
  A, B :: Foo Int

deriving instance Enum (Foo a)

Loading it into GHCi causes the following message:

ghc: panic! (the 'impossible' happened)
  (GHC version 7.0.1 for x86_64-unknown-linux):
        maybe_is_tagToEnum_call.extract_constr_Ids

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

In the case of Bounded instead of Enum in the original code, it does not work either but the error message is more "even-tempered":

Foo.hs:7:1:
    Couldn't match type `a' with `Int'
      `a' is a rigid type variable bound by
          the instance declaration at Foo.hs:7:32
    In the expression: A
    In an equation for `minBound': minBound = A
    When typechecking the code for  `minBound'
      in a standalone derived instance for `Bounded (Foo a)':
      To see the code I am typechecking, use -ddump-deriv
    In the instance declaration for `Bounded (Foo a)'

Foo.hs:7:1:
    Couldn't match type `a' with `Int'
      `a' is a rigid type variable bound by
          the instance declaration at Foo.hs:7:32
    In the expression: B
    In an equation for `maxBound': maxBound = B
    When typechecking the code for  `maxBound'
      in a standalone derived instance for `Bounded (Foo a)':
      To see the code I am typechecking, use -ddump-deriv
    In the instance declaration for `Bounded (Foo a)'
Failed, modules loaded: none.

I suspect that this is also a bug, as it would be perfectly reasonable to have minBound and maxBound for a type such as Foo Int.

A similar deriving fails also for Read class, among those I tried.

Trac metadata

Trac field	Value
Version	7.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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