[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:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by rwbarton):

 The fact that the binding of `fromScalar'` is a local one is not actually
 important here: the program with
 {{{
 instance (Functor t, Num r) => T t r

 fromScalar' a = fromScalar a    -- or turn on NoMonomorphismRestriction

 convert :: (T t (F r))
   => Foo t r -> Foo t r
 convert (Scalar c) =
   Foo $ fromScalar' $ toF c
 }}}
 also gives the error about "Could not deduce (Num (F r)) arising from a
 use of ‘fromScalar'’".

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10338#comment:2>
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