[GHC] #8799: Orientation of given `Coercible` constraints

[GHC]

#8799: Orientation of given `Coercible` constraints
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        Reporter:  nomeata                  |            Owner:
            Type:  feature request          |           Status:  new
        Priority:  normal                   |        Milestone:
       Component:  Compiler (Type checker)  |          Version:  7.6.3
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:  #8555
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Comment (by rwbarton):

 FWIW this works (with !ScopedTypeVariables):
 {{{
 let f :: forall a b. Coercible a b => b -> a; f = coerce (id :: a -> a)
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8799#comment:2>
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