[GHC] #10347: Spurious "unused constructor" warning with Coercible

[GHC]

#10347: Spurious "unused constructor" warning with Coercible
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        Reporter:  goldfire          |                   Owner:  goldfire
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by goldfire):

 Turns out this is harder to fix than I thought.

 The problem is that usage information for locally defined things is in
 `tcg_dus :: [DefUse]` in the `TcGblEnv`. Deep within the solver, there is
 no way to change this field, and threading the information back up to the
 top seems horrendous. It seems the only way forward is to change `tcg_dus`
 to be a `TcRef [DefUse]`, but that makes me a bit sad.

 Any ideas?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10347#comment:1>
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