[GHC] #11753: Type hole(?) causes compiler failure

[GHC]

#11753: Type hole(?) causes compiler failure
-------------------------------------+-------------------------------------
        Reporter:  akfp              |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:  duplicate         |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11059            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mpickering):

 I have managed to test it now and it is indeed fixed.

 {{{
 [1 of 1] Compiling Servant.Files    ( test.hs, test.o )

 test.hs:62:20: error:
     • Found type wildcard ‘_’
         standing for ‘Network.Wai.Internal.Request
                       -> IO
                            (RouteResult ((([Param], [File (Storage b)]) ->
 IO r) -> IO r))’
       Where: ‘r’ is a rigid type variable bound by
                the inferred type of
                bodyCheck :: Network.Wai.Internal.Request
                             -> IO
                                  (RouteResult ((([Param], [File (Storage
 b)]) -> IO r) -> IO r))
                at test.hs:63:7
              ‘b’ is a rigid type variable bound by
                the instance declaration at test.hs:54:10
       To use the inferred type, enable PartialTypeSignatures
     • In the type signature:
         bodyCheck :: _
       In an equation for ‘route’:
           route Proxy config subserver
             = WithRequest
               $ \ request
                   -> route
                        (Proxy :: Proxy sublayout)
                        config
                        (addBodyCheck subserver (bodyCheck request))
             where
                 bodyCheck :: _
                 bodyCheck request
                   = return
                     $ Route (\ f -> withBackend (Proxy :: Proxy b) $ \ pb
 -> ...)
       In the instance declaration for
         ‘HasServer (Files b :> sublayout) config’
     • Relevant bindings include
         bodyCheck :: Network.Wai.Internal.Request
                      -> IO
                           (RouteResult ((([Param], [File (Storage b)]) ->
 IO r) -> IO r))
           (bound at test.hs:63:7)
         subserver :: Delayed (Server (Files b :> sublayout))
           (bound at test.hs:59:22)
         config :: Config config (bound at test.hs:59:15)
         route :: Proxy (Files b :> sublayout)
                  -> Config config
                  -> Delayed (Server (Files b :> sublayout))
                  -> Router
           (bound at test.hs:59:3)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11753#comment:5>
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