[GHC] #16394: GHC internal error while typechecking of instance definition

[GHC]

#16394: GHC internal error while typechecking of instance definition
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        Reporter:  Day1721           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.6.3
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Happily, HEAD gives
 {{{
 T16394.hs:9:10: error:
     Illegal class instance: `C a => C b => C (a, b)'
       Class instances must be of the form
         context => C ty_1 ... ty_n
       where `C' is a class
   |
 9 | instance C a => C b => C (a, b) where
   |          ^^^^^^^^^^^^^^^^^^^^^^
 }}}
 Would someone like to add a regression test and close this?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16394#comment:1>
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