ghc7 does not equate type synonyms under RankNTypes

[kyagrd]

See the source at

 http://hpaste.org/43297/ghc7_problem_with_rankntypes_a

It works on ghc6 but not on ghc 7

The type error message is

ghc7rankn.hs:23:20:
    Couldn't match expected type `forall a'1.
                                  x a'1 -> Const (Const b a') a'1'
                with actual type `forall a'1. x1 a'1 -> Const b a'1'
    Expected type: (forall a'1. x a'1 -> Const (Const b a') a'1)
                   -> f x a
                   -> Const (Const b a') a
      Actual type: (forall a'1. x1 a'1 -> Const b a'1)
                   -> f1 x1 a1
                   -> Const b a1
    In the first argument of `mcata', namely `f'
    In the first argument of `f', namely `(mcata f)'

[simonpj]

Very good point, thank you. Fixed by

Tue Jan 25 11:01:12 GMT 2011  simonpj@microsoft.com
  * Fix Trac #4917: try a bit harder to unify on-the-fly
  
  This is generally a modest improvement but, more important,
  it fixes a "unify-under-forall" problem.  See Note [Avoid deferring].
  
  There's still a lurking unsatisfactory-ness in that we can't
  defer arbitrary constraints that are trapped under a forall.

    M ./compiler/typecheck/TcUnify.lhs -8 +28

Ian, please merge.

Simon

[igloo]

Merged