cabal syntax errors with ghc-8.0

[bfrk]

I am using cabal-3.2. Everything works fine with ghc versions from 8.8 down to 8.2. However, when I use ghc-8.0 something very strange happens: cabal no longer recognizes the cabal file syntax:

Warning: darcs.cabal: Unknown fields: autogen-modules (line 356)

It then proceeds to compile everything just fine. But in the end it fails with

setup: Error: Could not find module: Version with any suffix:
["gc","chs","hsc","x","y","ly","cpphs","hs","lhs"]

even though there is a generated Version.hs file at

dist-newstyle/build/x86_64-linux/ghc-8.0.2/darcs-2.14.2/build/autogen/Version.hs

Why does cabal-3.2 switch to a different cabal file parser depending on the ghc version?

How can I fix this?

[phadej]

darcs uses build-type: Custom and has

custom-setup
    setup-depends: base      >= 4.9 && < 4.13,
                   Cabal     >= 1.24,

So with GHC-8.0.2 it actually uses Cabal-1.24 based ./Setup.hs. So probably caused by some mismatch between cabal-install-3.2 and Cabal-1.24 based ./Setup interactions. I suspect the issues is fixed by sdist changes I made recently (cabal-install used ./Setup sdist --list-sources to get a list of files to watch for rebuild tracking).

Yet, I cannot be sure as I fail to reproduce your issue. The released 2.14.2 doesn't have autogen-modules, and the darcs get http://darcs.net/ version doesn't build with GHC-8.0

[__1] rejecting: base-4.9.1.0/installed-4.9.1.0 (conflict: darcs => base>=4.10
&& <4.13)

[bfrk]

Thanks for your input! I actually found a solution, see below.

First of all, http://darcs.net/ is our current development version where we indeed require ghc>=8.2. I am currently working with the release-2.14 branch at http://darcs.net/releases/branch-2.14 in order to support building with ghc-8.8 and 8.10.

I could not reproduced the problem with branch-2.14 either, though. This prompted me to closely inspect my current changes. I tracked down the patch that breaks the build for ghc-8.0:

  * add autogen-modules: Version and other-modules: Version
--- old-branch-2.14/darcs.cabal 2020-04-21 11:28:15.411239259 +0200
+++ new-branch-2.14/darcs.cabal 2020-04-21 11:28:15.411239259 +0200
@@ -353,6 +353,7 @@
                       Darcs.Util.URL
                       Darcs.Util.Workaround
 
+    autogen-modules:  Version
     other-modules:    Version
                       Darcs.Util.Download.Curl
 
@@ -477,6 +478,9 @@
   main-is:          darcs.hs
   hs-source-dirs:   darcs
 
+  autogen-modules:  Version
+  other-modules:    Version
+
   if flag(optimize)
     ghc-options:    -O2
   else

Without this patch I can build with ghc-8.0.2, 8.2,2, 8.4.4, and 8.6.5. But then building with ghc >= 8.8 fails:

setup: Error: Could not find module: Version with any suffix:
["gc","chs","hsc","x","y","ly","cpphs","hs","lhs","hsig","lhsig"]. If the
module is autogenerated it should be added to 'autogen-modules'.

I now found that this failure is caused by adding the autogen-modules and other-modules to the darcs executable section. I did this because of the warning:

<no location info>: warning: [-Wmissing-home-modules]
    These modules are needed for compilation but not listed in your .cabal file's other-modules: 
        Version

It turns out that when I ignore this warning and add the autogen-modules: Version /only/ to the library section, then despite lots of cabal warnings everything seems to work with all ghc versions starting with ghc-8.0 up to ghc-8.10.1.

[phadej]

Yes. This issue will be solved in next cabal major release (3.4), and in fact doesn't affect the installation, only local development.