[GHC] #10872: More informative assertion for non-unique TH names

[GHC]

#10872: More informative assertion for non-unique TH names
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        Reporter:  jstolarek         |                Owner:  goldfire
            Type:  task              |               Status:  closed
        Priority:  low               |            Milestone:  8.0.1
       Component:  Template Haskell  |              Version:  7.11
      Resolution:  fixed             |             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:                    |
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Comment (by bgamari):

 Cherry-picked to `ghc-8.0` as a1a054b1c91a4dc234d879064a0612eefdcd3fcf.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10872#comment:10>
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