[GHC] #10531: modules that can be linked successfully when compiled with optimizations, fail to link with: multiple definition of `__stginit_ZCMain'

[GHC]

#10531: modules that can be linked successfully when compiled with optimizations,
fail to link with: multiple definition of `__stginit_ZCMain'
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        Reporter:  imz               |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.6.1
      Resolution:                    |                Keywords:
Operating System:  Linux             |            Architecture:  x86_64
 Type of failure:  GHC rejects       |  (amd64)
  valid program                      |               Test Case:  attached
      Blocked By:                    |                Blocking:
 Related Tickets:  #437              |  Differential Revisions:
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Comment (by imz):

 But somehow if `-O1` is added everywhere, it sees the need to recompile A
 when building `b`. Isn't that a disturbing inconsistency?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10531#comment:7>
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