[GHC] #13021: Inaccessible RHS warning is confusing for users

[GHC]

#13021: Inaccessible RHS warning is confusing for users
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        Reporter:  mpickering        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
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 rwbarton):

 I don't understand what is going on in this example. Suppose `B` was a
 pattern synonym and we couldn't see its definition. Then the `COMPLETE`
 pragma could be semantically correct. But if we match `A` before `B` then
 the match isn't redundant, the behavior of the function could be different
 in the `A` case.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13021#comment:7>
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