[GHC] #8503: New GeneralizedNewtypeDeriving check still isn't permissive enough

[GHC]

#8503: New GeneralizedNewtypeDeriving check still isn't permissive enough
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        Reporter:  goldfire          |            Owner:  goldfire
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #8541
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Comment (by nomeata):

 Have you checked that it fails? I believe it would work:

 {{{
 Coercible (Maybe (Fix Maybe)) (Fix Maybe)
 }}}
 is only matched by
 {{{
 instance Coercible a (Maybe (Fix Maybe) => Coercible a (Fix Maybe)
 }}}
 so we are left with
 {{{
 Coercible (Maybe (Fix Maybe)) (Maybe (Fix Maybe))
 }}}
 which is solved by the reflexive instance
 {{{
 Coercible a a
 }}}

 (I’m not claiming that all cases work as smoothly, but they need to be a
 bit more constructed :-))

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8503#comment:22>
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