[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 AntC):

 Replying to [comment:11 goldfire]:
 >  the last two instance always overlap looking only at the instance
 heads. So I think you'd still need `{-# OVERLAPPING #-}`.
 >

 Yeah kinda; but no. GHC's overlap is a marvel of engineering compromise.
 So you're not squinting at the right angle.

 * The two instances are in no substitution order, so
 `OVERLAPPING`/`OVERLAPPABLE` would be rejected.
 * "It is fine for there to be a ''potential'' of overlap ...; an error is
 only reported if a particular constraint matches more than one." [Users
 guide]
   So I'm relying on type improvement to apply before instance selection:
 then GHC should never be looking for a Wanted that matches more than one
 instance.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15359#comment:13>
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