Internal error using redundant forall with default signature

{-# LANGUAGE ScopedTypeVariables, DefaultSignatures #-}

class Foo x where
  foo :: forall a . x a -> x a
  default foo :: forall a . x a -> x a
  foo x = go
    where go :: x a
          go = undefined

GHC (7.8.3 and 8.0.1) chokes on the above with the message

Buggy.hs:7:19: error:
    • GHC internal error: ‘a’ is not in scope during type checking, but it passed the renamer
      tcl_env of environment: [aoY :-> Type variable ‘x’ = x,
                               ap0 :-> Type variable ‘a’ = a,
                               ap1 :-> Identifier[x::x a, <NotTopLevel>]]
    • In the first argument of ‘x’, namely ‘a’
      In the type signature:
        go :: x a
      In an equation for ‘foo’:
          foo x
            = go
            where
                go :: x a
                go = undefined

If I remove the redundant forall it works fine:

class Foo x where
  foo :: x a -> x a
  default foo :: forall a . x a -> x a
  foo x = go
    where go :: x a
          go = undefined

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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