[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:
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Comment (by nomeata):

 I was conservative when disallowing recursive newtypes for `Coercible`,
 but quite possibly too conservative. I also wanted to keep the code simple
 (Note that it just keeps generating constraints and passing them back to
 the constraint solver; it does *not* build the whole coercion in one go).

 I wasn’t aware of `TyCon.checkRecTc` and I’ll look into it. Thanks for
 pointing that out.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8503#comment:14>
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