[GHC] #7888: Impredicativity flag needed more often

[GHC]

#7888: Impredicativity flag needed more often
------------------------------------------------+---------------------------
  Reporter:  monoidal                           |          Owner:                  
      Type:  bug                                |         Status:  closed          
  Priority:  normal                             |      Milestone:                  
 Component:  Compiler (Type checker)            |        Version:  7.6.3           
Resolution:  fixed                              |       Keywords:                  
        Os:  Unknown/Multiple                   |   Architecture:  Unknown/Multiple
   Failure:  Incorrect warning at compile-time  |     Difficulty:  Unknown         
  Testcase:  typecheck/should_compile/T7888     |      Blockedby:                  
  Blocking:                                     |        Related:                  
------------------------------------------------+---------------------------
Changes (by simonpj):

  * status:  new => closed
  * difficulty:  => Unknown
  * resolution:  => fixed
  * testcase:  => typecheck/should_compile/T7888


Comment:

 There really is a reason for this.  The elaborated definition would be
 {{{
 f :: forall b. (forall a. a) -> b
   = /\b. undefined @((forall a. a) -> b)
 }}}
 So we need to instantiate `undefined` at a polytype, which isn't usually
 allowed.  But it is for `error` and there's no reason not to allow
 `undefined` too.

 See the commit comments above.  Both for `error` and `undefined`, it's a
 bit of a hack.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7888#comment:2>
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