[GHC] #12135: Failure to recompile when #include file is created earlier on include path

[GHC]

#12135: Failure to recompile when #include file is created earlier on include path
-------------------------------------+-------------------------------------
           Reporter:  NeilMitchell   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Originally discovered when trying to improve Hadrian, for context see:
 https://github.com/snowleopard/hadrian/issues/247#issuecomment-222317602

 If you create a file {{{Test.hs}}}

 {{{#!hs
 #include "a/Test.hs"
 main = print message
 }}}

 And a file {{{a/Test.h}}}

 {{{#!hs
 message = 1
 }}}

 Then run and compile with {{{ghc --make -XCPP -Ib;a Main.hs}}} it compiles
 and runs properly, printing 1. Changing {{{a/Test.h}}} to {{{message =
 2}}} then running {{{ghc}}} rebuilds and prints 2. Creating a fresh
 {{{b/Test.h}}} with {{{message = 3}}} does not cause {{{ghc}}} to
 recompile, and thus incorrectly still prints 2. Touching {{{Main.hs}}}
 causes recompilation, and then the correct header file is picked up,
 resulting in 3 being printed.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12135>
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