[GHC] #14759: ListSetOps WARNING causes tests to fail

[GHC]

#14759: ListSetOps WARNING causes tests to fail
-------------------------------------+-------------------------------------
        Reporter:  ezyang            |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4628
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by alpmestan):

 > See my note on Phab.

 Thanks for being so responsive. I just left 2 comments on the differential
 as well, trying to find a solution that makes everyone happy. It should
 not be too hard given the small number of users of that `unionLists`
 function.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14759#comment:8>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler