[GHC] #12533: Internal error using redundant forall with default signature

[GHC]

#12533: Internal error using redundant forall with default signature
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        Reporter:  dfeuer            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by dfeuer):

 * component:  Compiler => Compiler (Type checker)


Comment:

 mniip says this is a renamer problem, with `-ddump-rn` producing

 {{{#!hs
 class Main.Foo x_apt where
   Main.foo :: forall a_apu. x_apt a_apu -> x_apt a_apu
   default Main.foo :: forall a_apv. x_apt a_apv -> x_apt a_apv
   Main.foo x_apw
     = go_apx
     where
         go_apx :: x_apt a_apu
         go_apx = undefined
 }}}

 That is, GHC is using `a_apu` when it should be using `a_apv`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12533#comment:1>
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