[GHC] #8767: Add rules involving `coerce` to the libraries

[GHC]

#8767: Add rules involving `coerce` to the libraries
-------------------------------------+-------------------------------------
              Reporter:  nomeata     |            Owner:  ekmett
                  Type:  task        |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Core        |          Version:  7.9
  Libraries                          |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:  8718
       Type of failure:              |  Related Tickets:  #2110
  None/Unknown                       |
             Test Case:              |
  tests/simplCore/should_run/T2110.hs|
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------

Comment (by dfeuer):

 Replying to [comment:21 dmcclean]:

 > One possible thing would be to have an {-# UNLAWFUL #-} pragma, and when
 you are magically generating instances you branch three ways instead of 2.
 (Possibly such a pragma could allow more aggressive optimizations in other
 cases too, with an opt out for 'criminals'?)

 I think it would make more sense to track `Functor` instances believed to
 be (sufficiently) lawful. A functor could be labeled `{-# LAWFUL #-}` in
 either `Unsafe` or `Trustworthy` modules, and any ''derived'' `Functor`
 instance without a `Functor` context could be treated as lawful as well. A
 functor labeled as `{-# LAWFUL #-}` could of course be lawful only up to
 some isomorphism; the pragma would declare that the instance won't break
 in any important way if the compiler relies on the functor laws. A similar
 mechanism could presumably be applied to other classes as well.

 Interaction with extreme polymorphism: to really take advantage, you'd
 presumably need to be able to express lawfulness in a context. So you'd
 need to be able to write something like

 {{{#!hs
 g :: ({-# LAWFUL #-} Functor f) => ...
 }}}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8767#comment:25>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler