[GHC] #11793: Confusing type error from constrained class method

[GHC]

#11793: Confusing type error from constrained class method
-------------------------------------+-------------------------------------
        Reporter:  gridaphobe        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  8.0.1-rc2
      Resolution:                    |             Keywords:  error-message
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Iceland_jack):

 Is this intended?

 {{{
 $ ghci -XImplicitParams -XConstrainedClassMethods -ignore-dot-ghci
 GHCi, version 8.1.20160117: http://www.haskell.org/ghc/  :? for help
 Prelude> class C a where foo :: (?x::Int) => a -> a
 Prelude> class C a where foo :: ?x::Int => a -> a

 <interactive>:2:32: error: parse error on input ‘=>’
 Prelude>
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11793#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler