[GHC] #9749: ghc -M 7.8 does not add underscores when -dep-suffix is used

[GHC]

#9749: ghc -M 7.8 does not add underscores when -dep-suffix is used
-------------------------------------+-------------------------------------
       Reporter:  nh2                |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:  7.8.4
      Component:  Build System       |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Easy (less than 1  |         Type of failure:
  hour)                              |  None/Unknown
     Blocked By:                     |               Test Case:
Related Tickets:  7381               |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 https://www.haskell.org/ghc/docs/7.8.3/html/users_guide/separate-
 compilation.html says:

 ''Make extra dependencies that declare that files with suffix
 `.<suf>_<osuf>` depend on interface files with suffix `.<suf>_hi`''

 but ghc 7.8 doesn't do that any more, it drops the `_` underscore.

 This undocumented (neither in User Guide nor in Changelog) change was
 introduced for #7381 in
 https://git.haskell.org/ghc.git/commitdiff/af072fc35d8dbe7962e62700da052593e999c0ef.

 Is this a documentation bug or a `ghc -M` bug?

 How should one write `ghc -M` invocations that work on both 7.6 and 7.8?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9749>
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