[GHC] #12845: -XPartialTypeSignatures results in unbound variables

[GHC]

#12845: -XPartialTypeSignatures results in unbound variables
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        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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 RyanGlScott):

 If I'm reading this right, this appears to work in GHC HEAD:

 {{{
 $ /opt/ghc/head/bin/ghci Bug.hs
 GHCi, version 8.1.20161010: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/ryanglscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )

 Bug.hs:16:70: warning: [-Wpartial-type-signatures]
     • Found type wildcard ‘_’ standing for ‘() :: Constraint’
     • In the type signature:
         broken :: forall r r' rngs.
                   ('(r, r') ~ Head rngs, Bar r r' ~ True, _) =>
                   Foo r -> Proxy rngs -> ()
 Ok, modules loaded: Main.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12845#comment:1>
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