[GHC] #7137: Unnecessary -XRank2Types requirement involving type alias containing "forall" from another module

[GHC]

#7137: Unnecessary -XRank2Types requirement involving type alias containing
"forall" from another module
----------------------------------------+-----------------------------------
  Reporter:  joeyadams                  |          Owner:  simonpj         
      Type:  bug                        |         Status:  closed          
  Priority:  highest                    |      Milestone:  7.6.1           
 Component:  Compiler                   |        Version:  7.7             
Resolution:  wontfix                    |       Keywords:                  
        Os:  Unknown/Multiple           |   Architecture:  Unknown/Multiple
   Failure:  GHC rejects valid program  |     Difficulty:  Unknown         
  Testcase:                             |      Blockedby:                  
  Blocking:                             |        Related:                  
----------------------------------------+-----------------------------------
Changes (by simonpj):

  * status:  new => closed
  * resolution:  => wontfix


Comment:

 Joey is right that `Int -> forall a. a` is technically a rank-1 type.

 But #5957 complained about a rank-1 type of this sort, in fact more like
 `Int -> Ord a => a -> a`, that was accepted without any flags.  And that
 does seem wrong.

 So I did the technically-incorrect thing of requiring `Rank2Types` or
 `RankNTypes` if you want a forall after an arrow.  It just doesn't seem
 worth the flaggery to allow only rank-1 types, but with foralls after
 arrows.

 I propose not to fix this unless it's very painful for some reason.  Re-
 open if so.  I will improve the documentation though.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7137#comment:4>
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