[GHC] #2935: "A lazy (~) pattern cannot bind existential type variables" happens for non-existential GADTs

[GHC]

#2935: "A lazy (~) pattern cannot bind existential type variables" happens for
non-existential GADTs
-----------------------------+----------------------------------------------
Reporter:  ganesh            |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  6.10.1            |       Severity:  normal          
Keywords:                    |       Testcase:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
-----------------------------+----------------------------------------------
 This program:
 {{{
 {-# LANGUAGE GADTs #-}
 module Foo where

 data Foo a where
    Foo :: a -> Foo (a, Int)

 foo :: Foo (a, Int) -> a
 foo ~(Foo a) = a
 }}}
 causes this error:
 {{{
 Foo.hs:8:4:
     A lazy (~) pattern cannot bind existential type variables
       `a' is a rigid type variable bound by
           the constructor `Foo' at Foo.hs:8:6
     In the pattern: ~(Foo a)
     In the definition of `foo': foo ~(Foo a) = a
 }}}

 This doesn't seem like an existential, as there aren't any type variables
 in the arguments to Foo that aren't in the result type.

 If easy it would be nice if the restriction were relaxed to allow for this
 case, otherwise I think the error message should be improved.

 Tested with ghc 6.10.1.20081209 and ghc 6.11.20090107.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2935>
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