[GHC] #14769: The RecompBecause [TH] check is not resume-build-safe

[GHC]

#14769: The RecompBecause [TH] check is not resume-build-safe
-------------------------------------+-------------------------------------
           Reporter:  nh2            |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #481
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 `ghc --make` aims to make compilation behave correctly and produce up-to-
 date, no matter whether it completes or is interrupted (e.g. with Ctrl+C)
 and later resumed

 At no point should interrupting a build and running it again produce "less
 correct" outputs than running it to the end; specifically interrupting
 shouldn't result in necessary build steps to be "forgotten".

 However, this seems to be the case with the `[TH]` check introduced in
 #481.

 Minimal test case with 3 files: https://github.com/nh2/th-recomp-test

 Good behaviour when running without interrupting:

 {{{
 % touch A.hs && ~/.stack/programs/x86_64-linux/ghc-8.0.1/bin/ghc --make
 Main.hs
 [1 of 3] Compiling A                ( A.hs, A.o )
 [2 of 3] Compiling B                ( B.hs, B.o ) [TH]
 [3 of 3] Compiling Main             ( Main.hs, Main.o ) [TH]
 Linking Main ...
 }}}

 Bad behaviour when interrupting and running again:

 {{{
 % touch A.hs; timeout 0.1 ~/.stack/programs/x86_64-linux/ghc-8.0.1/bin/ghc
 --make Main.hs; ~/.stack/programs/x86_64-linux/ghc-8.0.1/bin/ghc --make
 Main.hs
 [1 of 3] Compiling A                ( A.hs, A.o )
 [2 of 3] Compiling B                ( B.hs, B.o ) [TH]
 Linking Main ...
 }}}

 As you can see, when interrupting, the step `Compiling Main  ( Main.hs,
 Main.o ) [TH]` went missing.

 This suggests to me that either:

 * the `[TH]`s are unnecessary in the first place (unlikely and I buy into
 the reason explained by #481 that changed values may be picked up by TH in
 higher-level modules)
 * the resumed `ghc --make` invocation may potentially produce wrong / not-
 up-to-date outputs (most likely)
 * re-running somehow retrieves the information that in fact recompiling
 `Main` is not necessary for a correct build (if this is the case, then I'd
 really like to know how that works and we should document it, likely also
 try to use that way to avoid recompiling `Main` in the uninterrupted run)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14769>
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