[GHC] #10338: GHC Forgets Constraints

[GHC]

#10338: GHC Forgets Constraints
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        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.3
      Resolution:                    |             Keywords:
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 dfeuer):

 If you're not using overlapping instances, then you can solve the problem
 by imposing

 {{{#!hs
 class (Functor t, Num r) => T t r where
 }}}

 In that context, the only (hypothetical) downside is that you don't have
 the option of hiding the constraint. I don't know if that's a real
 concern.

 If you are using overlapping instances, ~~you've dug yourself into a hole
 and I don't have much sympathy for you~~ the `ScopedTypeVariables` work-
 around doesn't seem too onerous.

 It would be interesting to see if this can be reproduced using code that
 makes sense ''without'' `OverlappingInstances`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10338#comment:7>
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