[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
-------------------------------------+-------------------------------------
           Reporter:                 |             Owner:  (none)
  radekchannable                     |
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.4.4
           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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 When one module defines an FFI function and uses it in an instance, and a
 Template Haskell function to derive an instance of the same class, using
 the function in another module causes GHC to crash, complaining it cannot
 find the FFI function.

 See the attached modules for an example. Use the following command to
 trigger the bug (ensure template-haskell is available):

 {{{
 $ ghc Bar.hs
 [1 of 2] Compiling Foo              ( Foo.hs, Foo.o )
 [2 of 2] Compiling Bar              ( Bar.hs, Bar.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.4.4 for x86_64-unknown-linux):
         Loading temp shared object failed: /tmp/ghc13955_0/libghc_6.so:
 undefined symbol: showFoo

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16076>
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