[GHC] #11139: Add Semigroup warnings

[GHC]

#11139: Add Semigroup warnings
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           Reporter:  quchen         |             Owner:
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.0.1
          Component:  Compiler       |           Version:
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Similar to #8004, do the following for Semigroup/Monoid:

   1. Warn when an instance of a class has been given, but the type does
 not
      have a certain superclass instance
   2. Warn when top-level definitions conflict with future Prelude names

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11139>
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