[GHC] #14469: Rebuilding profiled stage2 after building stage3 is broken

[GHC]

#14469: Rebuilding profiled stage2 after building stage3 is broken
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.4.1
          Component:  Build System   |           Version:  8.2.1
           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:
-------------------------------------+-------------------------------------
 The following appears to break,

 {{{
 $ cat <<EOF >mk/build.mk
 BuildFlavour = prof
 ifneq "$(BuildFlavour)" ""
 include mk/flavours/$(BuildFlavour).mk
 endif
 EOF
 $ make stage=2
 $ make stage=3
 $ touch compiler/stage2/build/Simplify.*
 $ make stage=2
 ...
 ghc/GHCi/UI/Info.hs:43:1: error:
     Could not find module ‘TcHsSyn’
     Perhaps you haven't installed the profiling libraries for package
 ‘ghc-8.3’?
     Locations searched:
       ghc/./TcHsSyn.p_hi
       ghc/./TcHsSyn.p_hi-boot
       ghc/stage2/build/TcHsSyn.p_hi
       ghc/stage2/build/TcHsSyn.p_hi-boot
       ghc/stage2/build/ghc/autogen/TcHsSyn.p_hi
       ghc/stage2/build/ghc/autogen/TcHsSyn.p_hi-boot
       /mnt/work/ghc/ghc/compiler/stage3/build/TcHsSyn.p_hi
    |
 43 | import           TcHsSyn
    | ^^^^^^^^^^^^^^^^^^^^^^^^

 }}}

 The problem appears to be that the build system registers the stage3
 `ghc`, which we didn't build in the profiled way, overwriting the stage2
 registration. Hopefully Hadrian will do better here.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14469>
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