[GHC] #12557: Regression in type inference with RankNTypes

31 Aug 2016


      #12557: Regression in type inference with RankNTypes
-------------------------------------+-------------------------------------
           Reporter:  slindley       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider the following code:

 {{{#!hs
 foo :: ((forall a.f a) -> f r) -> f r
 foo g = undefined
 }}}

 In GHC 7.10.3:
 {{{
 *Main> :t \g -> foo g
 \g -> foo g :: ((forall a. f a) -> f r) -> f r
 }}}

 In GHC 8.0.1:
 {{{
 *Main> :t \g -> foo g

 <interactive>:1:11: error:
     • Couldn't match expected type ‘(forall a. f a) -> f r’
                   with actual type ‘t’
       ‘t’ is a rigid type variable bound by
         the inferred type of it :: t -> f r at <interactive>:1:1
     • In the first argument of ‘foo’, namely ‘g’
       In the expression: foo g
       In the expression: \ g -> foo g
     • Relevant bindings include g :: t (bound at <interactive>:1:2)
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12557
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