[GHC] #13859: Bad error message when compacting a Compact#

[GHC]

#13859: Bad error message when compacting a Compact#
-------------------------------------+-------------------------------------
        Reporter:  andrewthad        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by andrewthad):

 The `Compactable` class would be nice for most users, and provided that
 the instances are generated by GHC (like Typeable and Coercible). Also in
 my situation, it would still be nice to get a better runtime error since
 I'm doing dangerous stuff by thawing arrays on the compact heap.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13859#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler