[GHC] #13056: Deriving Foldable causes GHC to take a long time (GHC 8.0 ONLY)

[GHC]

#13056: Deriving Foldable causes GHC to take a long time (GHC 8.0 ONLY)
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        Reporter:  ezyang            |                Owner:
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.0.3
       Component:  Compiler          |              Version:  8.0.2-rc2
      Resolution:                    |             Keywords:  deriving-perf
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Compile-time      |            Test Case:
  performance bug                    |  perf/compiler/T13056
      Blocked By:                    |             Blocking:
 Related Tickets:  #12234            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by EyalLotem):

 Much smaller example that reproduces the issue:

 {{{
 data A a = A (B a) (B a) deriving (Functor)
 data B a = B (A a) deriving (Functor)
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13056#comment:10>
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