[GHC] #15359: Quantified constraints do not work with equality constraints

[GHC]

#15359: Quantified constraints do not work with equality constraints
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 All the examples in comment:7 involve superclass constraints, mostly on
 classes without any methods. This is interesting for type-level
 programming, but it's not clear where (even in type-level programming) the
 rubber is hitting the road here.

 Can you give a concrete example of a function that usefully uses an
 equality constraint in the head of an implication? This should be a
 function that I can call (that is, the constraints are satisfiable).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15359#comment:8>
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