[GHC] #15974: QuantifiedConstraints: Spurious error involving superclass constraints

[GHC]

#15974: QuantifiedConstraints: Spurious error involving superclass constraints
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        Reporter:  lexi.lambda       |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.6.3
       Component:  Compiler          |              Version:  8.6.2
      Resolution:  invalid           |             Keywords:
                                     |  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 simonpj):

 Great explanation lexi.lambda.  I too claim this is not a but.  Reason: as
 you say:

 >  `forall a. B a => B (f a)` does not actually entail `forall a. A a => A
 (f a)`

 So it's not a shortcoming in the implementation (although there are plenty
 of those).  It's just that there is no way to construct evidence for
 `forall a. A a => A (f a)` from evidence for `forall a. B a => B (f a)`

 If you agree, would you like to close as invalid?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15974#comment:2>
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