[GHC] #15639: Surprising failure combining QuantifiedConstraints with Coercible

[GHC]

#15639: Surprising failure combining QuantifiedConstraints with Coercible
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Replying to [comment:10 simonpj]:
 > The best thing is to avoid overlap.

 This is a very good general principle (and the reason I was concerned
 about bundling implication constraints with quantified constraints). But I
 think the special case of `Coercible` deserves some special consideration.

 Overlap is very often (perhaps usually) unavoidable when using quantified
 constraints with `Coercible`. There's just no way to hide the global
 mechanism. So I think it pays to consider how we can refine the overlap
 rules to make these constraints as useful as possible. I ''conjecture''
 that we will get strictly or almost strictly better results by applying
 quantified constraints before and between built-in rules (other than
 symmetry, which I imagine does some global canonicalization). We seem to
 already do something special to allow things to work as expected when
 pattern matching on `Coercion`s to locally reveal `Coercible` instances.
 Can we do something similar for quantified constraints?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15639#comment:11>
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