[GHC] #10608: Compile error regression from GHC 7.10 to 7.11

[GHC]

#10608: Compile error regression from GHC 7.10 to 7.11
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        Reporter:  hvr               |                   Owner:  simonpj
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Other             |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by simonpj):

 Harump.   This was deliberate:
 {{{
 commit b83160d07e626bee685f329a9a73e90a4a6074ae
 Author: Simon Peyton Jones <simonpj at microsoft.com>
 Date:   Thu Apr 30 09:18:49 2015 +0100

     Tidy up treatment of FlexibleContexts

     Previously (Trac #10351) we could get

         Non type-variable argument in the constraint: C [t]
         (Use FlexibleContexts to permit this)
         When checking that `f' has the inferred type
           f :: forall t. C [t] => t -> ()

     which is a bit stupid: we have *inferred* a type that we
     immediately *reject*.  This patch arranges that that the
     generalisation mechanism (TcSimplify.decideQuantification)
     doesn't pick a predicate that will be rejected by the
     subsequent validity check.
 }}}
 Moreover #10351 comment 3 makes essentially the same point as you do here.

 I'll just reverse the change, which is easy to do.  Two people have
 complained about the new behaviour and I don't think anyone complained
 about the old!

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10608#comment:1>
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