[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):

 If it helps, the error with ghc-7.7 is:

 {{{
 inf.hs:26:10: Warning:
     Could not deduce (Fun g a b0)
       arising from the ambiguity check for an instance declaration
     from the context (Fun f b c, Fun g a b)
       bound by an instance declaration:
                  (Fun f b c, Fun g a b) => Fun (Compose f g) a c
       at inf.hs:26:10-56
     The type variable ?b0? is ambiguous
     In the ambiguity check for:
       forall f g a c b. (Fun f b c, Fun g a b) => Fun (Compose f g) a c
     In the instance declaration for ?Fun (Compose f g) a c?
 Ok, modules loaded: Main.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8390#comment:1>
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