[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):

 As you point out, the last two instance always overlap looking only at the
 instance heads. So I think you'd still need `{-# OVERLAPPING #-}`.

 Previously, Simon was worried that the equality constraints would always
 be redundant. I think that's still true in your examples, but with a key
 twist: the equality constraints can be used for improvement during type
 checking. That may indeed be correct.

 As a practical matter though, I can't imagine how to implement them. And,
 given the fact that we have many ways of expressing these ideas already,
 without quantified equality constraints, (for example, your `ElimElem`
 could be implemented as a closed type family), I'm not yet motivated to
 start thinking about how to write a solver than can deal with these.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15359#comment:11>
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