[GHC] #15286: "Can't use Natural in base" when compiling GHC.Natural with -O0

[GHC]

#15286: "Can't use Natural in base" when compiling GHC.Natural with -O0
-------------------------------------+-------------------------------------
           Reporter:  alpmestan      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:                 |           Version:  8.5
  libraries/base                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Building GHC
  Unknown/Multiple                   |  failed
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{
 _build/stage0/bin/ghc -Wall -hisuf hi -osuf o -hcsuf hc -static -hide-all-
 packages -no-user-package-db '-package-db
 _build/stage1/lib/package.conf.d' '-this-unit-id base-4.12.0.0' '-package-
 id ghc-prim-0.5.3' '-package-id integer-simple-0.1.1.1' '-package-id
 rts-1.0' -i -i_build/stage1/libraries/base/build
 -i_build/stage1/libraries/base/build/autogen -ilibraries/base/. -Iincludes
 -I_build/generated -I_build/stage1/libraries/base/build
 -I_build/stage1/libraries/base/build/include -Ilibraries/base/include
 -I/home/travis/build/snowleopard/hadrian/ghc/_build/stage1/lib/x86_64
 -linux-ghc-8.5.20180617/rts-1.0/include -I_build/generated -optc-
 I_build/generated -optP-include
 -optP_build/stage1/libraries/base/build/autogen/cabal_macros.h -optc-
 std=gnu99 -optc-fno-stack-protector -optP-std=gnu99 -odir
 _build/stage1/libraries/base/build -hidir
 _build/stage1/libraries/base/build -stubdir
 _build/stage1/libraries/base/build -Wnoncanonical-monad-instances -optc-
 Werror=unused-but-set-variable -optc-Wno-error=inline -c
 libraries/base/GHC/Num.hs -o _build/stage1/libraries/base/build/GHC/Num.o
 -O0 -H64m -this-unit-id base -Wcompat -Wnoncanonical-monad-instances
 -XHaskell2010 -ghcversion-
 file=/home/travis/build/snowleopard/hadrian/ghc/_build/generated/ghcversion.h
 -Wno-deprecated-flags -Wno-trustworthy-safe
 Exit code: 1
 Stderr:
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.5.20180617 for x86_64-unknown-linux):
         Can't use Natural in base
 }}}

 We noticed this in hadrian land (in
 [https://github.com/snowleopard/hadrian/issues/499 this PR] and
 [https://github.com/snowleopard/hadrian/issues/629]), and noticed that
 this happens:

 - with 8.4.2 as the boot compiler but not 8.2.2
 - with integer-simple as the integer library
 - only with the quickest flavour (or more generally when we build
 GHC.Natural with -O0)

 Some optimisation seems critical to making any trace of `Natural`
 disappear, but this seems rather fragile. The code that throws the error
 got introduced in fe770c211631e7b4c9b0b1e88ef9b6046c6585ef.

 The full build log is available [https://travis-
 ci.org/snowleopard/hadrian/jobs/393259151#L4209 here], anchored to where
 the error appears.

 An example of hadrian command to reproduce the problem, assuming a
 configured build tree: `hadrian/build.sh --flavour=quickest --integer-
 simple -j4`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15286>
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