[GHC] #8821: Functional dependency coverage check does not use type equalities

[GHC]

#8821: Functional dependency coverage check does not use type equalities
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       Reporter:  glguy                    |             Owner:
           Type:  bug                      |            Status:  new
       Priority:  normal                   |         Milestone:
      Component:  Compiler (Type checker)  |           Version:  7.6.3
       Keywords:                           |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Unknown                  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:                           |         Test Case:
                                           |          Blocking:
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 When checking the coverage condition to validate an instance, we should
 also consider equality constraints because they have a dependency in both
 directions.

 {{{
 class C a b | a -> b
 instance (a ~ b) => C a b

 --  Illegal instance declaration for ‛C a b’
 --    The coverage condition fails in class ‛C’
 --      for functional dependency: ‛a -> b’
 --    Reason: lhs type ‛a’ does not determine rhs type ‛b’
 --    Using UndecidableInstances might help
 --  In the instance declaration for ‛C a b’
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8821>
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