[GHC] #9664: Add identity functor to base

[GHC]

#9664: Add identity functor to base
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              Reporter:  hvr         |            Owner:  ekmett
                  Type:  task        |           Status:  closed
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Core        |          Version:
  Libraries                          |         Keywords:
            Resolution:  fixed       |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Easy (less than 1
  Unknown/Multiple                   |  hour)
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:  Phab:D313   |
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Comment (by Herbert Valerio Riedel <hvr@…>):

 In [changeset:"8cbd25a49051171da7c73db57ebd87bb0296c2f7/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="8cbd25a49051171da7c73db57ebd87bb0296c2f7"
 Make Data.Functor.Identity trustworthy again

 Alas `{-# LANGUAGE Safe #-}` can't be used since `Data.Coerce` isn't
 "safe".
 However, we use `coerce` just as an optimisation
 (see also 4ba884bdd3a9521ea92fcda8f601a7d0f3537bc1 which broke the
 safe-inferred status of `Data.Functor.Identity`), so this module at least
 deserves `{-# LANGUAGE Trustworthy #-}`.

 NOTE: `Data.Functor.Identity` was added to `base` in the context of #9664

 Reviewed By: luite

 Differential Revision: https://phabricator.haskell.org/D507
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9664#comment:12>
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