[GHC] #12820: Regression around pattern synonyms and higher-rank types

[GHC]

#12820: Regression around pattern synonyms and higher-rank types
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        Reporter:  goldfire          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             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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Comment (by simonpj):

 Hmm. Works ok if you put the forall after the arrow
 {{{
 pattern P x <- (((\ _ -> id) :: forall b. b -> forall c. c -> c) -> x)
 }}}
 Reason: we infer the type of `f`, and then try to decompose and arrow,
 instantiating if necessary.  But we don't re-generalise.

 I suppose we could but this is a pretty exotic corner and it's not keeping
 me awake at night.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12820#comment:3>
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