[GHC] #11386: Improve error message

[GHC]

#11386: Improve error message
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{
 Prelude> class (A, B)

 <interactive>:16:8: error:
     Unexpected type ‘A’
     In the class declaration for ‘GHC.Classes.(%,%)’
     A class declaration should have form
       class GHC.Classes.(%,%) a b where ...
 }}}

 in 7.10.2 it gave the more sensible

 {{{
 ghci> class (A, B)

 <interactive>:30:7:
     Malformed head of type or class declaration: (A, B)
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11386>
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