[GHC] #8745: GeneralizedNewtypeDeriving is still not Safe

[GHC]

#8745: GeneralizedNewtypeDeriving is still not Safe
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        Reporter:  goldfire          |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.1-rc1
      Resolution:                    |         Keywords:  Safe
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):

 We have some special handling in Safe mode in place where we do the
 Coerible thing, but it is neither well tested nor well discussed.
 (`getCoercibleInst` in `TcInteract`)

 Currently, in Safe Mode, the following additional restrictions exist when
 checking whether something is  `Coercible`:

 > To coerce under a type constructor, ''all'' data constructors of ''all''
 type constructors involved in the definition of that type constructors
 need to be in scope.

 No additional constraint is put on unfolding a newtype, but there we
 already require the constructor to be in scope.

 I believe that this condition is sufficient for “the use can implement
 coerce himself (ignoring the run-time cost)”. If anything, it might turn
 out too strong.

 If that works as intended, `Coercible` and GND should be safe. There might
 possibly be ways to break it, maybe with different modules, some safe and
 some not... but that would simply be bugs then, and I’m not claiming bug
 freeness.

 A trustworthy `GHC.Coerce` might be nice. Or maybe even `Data.Coerce`?
 After all the interface could be provided by other compilers as well...
 Not only for Safe, but to avoid people having to import the mingle-mangle
 of `GHC.Exts`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8745#comment:3>
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