[GHC] #16076: Internal compiler error when instance uses FFI function and defining other instance of the same class through Template Haskell

[GHC]

#16076: Internal compiler error when instance uses FFI function and defining other
instance of the same class through Template Haskell
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        Reporter:  radekchannable    |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.4.4
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by radekchannable):

 But there is no reason for it to call the showFoo function; its result is
 not used in any Template Haskell code.

 This would be a rather arbitrary limitation; not being able to define
 instances using Template Haskell just because an FFI function was declared
 in some module somewhere.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16076#comment:2>
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