GADTs and type synonyms

Another GADT `bug report': type synonyms are not unfolded
when checking the signature of the data constructors: given

> type Hello a = a -> a
> data World :: * where
>   Msg :: Hello World

GHC reports

GADT.lhs:2:1:
    Data constructor `Msg' returns type `Hello'
      instead of its parent type
    When checking the data constructor: Msg
    In the data type declaration for `World'

Applies to both version 6.4.1 and 6.5.

Cheers, Ralf