[GHC] #8826: Allow more coercions in Safe Haskell

[GHC]

#8826: Allow more coercions in Safe Haskell
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        Reporter:  goldfire          |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.9
      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 simonpj):

 > Why do we need this? In order to forestall any possible abstraction
 breaking.

 I don't buy this.  Why are inferred '''roles''' different from inferred
 '''kinds'''?  What bad things would happen if we simply abandoned the
 restriction that the constructors of the type must be in scope to do allow
 coercion from `(T Int)` to `(T Age)`?

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8826#comment:1>
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