[GHC] #8390: regression in handling of type variables in constraints on instances which do not appear in the instance head

[GHC]

#8390: regression in handling of type variables in constraints on instances which
do not appear in the instance head
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        Reporter:  aavogt                     |            Owner:
            Type:  bug                        |           Status:  new
        Priority:  normal                     |        Milestone:
       Component:  Compiler                   |          Version:  7.7
      Resolution:                             |         Keywords:
Operating System:  Unknown/Multiple           |     Architecture:
 Type of failure:  GHC rejects valid program  |  Unknown/Multiple
       Test Case:                             |       Difficulty:  Unknown
        Blocking:                             |       Blocked By:
                                              |  Related Tickets:
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Comment (by aavogt):

 goldfire, you're asking for a message `suggested fix:
 -XAllowAmbiguousTypes'? The reasoning for rejecting the Compose f g
 instance by default makes sense here.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8390#comment:5>
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