doctests with ghc-9.12.2 can't parse

[philderbeast]

Describe the bug
We run quickjobs with one version of GHC:

cabal/.github/workflows/quick-jobs.yml

Line 18 in fb32f3d

GHC_FOR_QUICK_JOBS: 9.10.3

I tested doctests with later versions and running doctests on Cabal fails with:

Can't parse reexported module flag: Language.Haskell.Extension\ as\ Language.Haskell.Extension

To Reproduce

$ make doctest-install
cabal install doctest --overwrite-policy=always --ignore-project --flag cabal-doctest
...

$ doctest --version
doctest version 0.24.3
using version 9.12.2 of the GHC API

$ cd Cabal

$ cabal doctest --allow-newer=False
doctest version 0.24.3
using version 9.12.2 of the GHC API
...
Preprocessing library for Cabal-3.17.0.0...
doctest-0.24.3: Uncaught exception ghc-9.12.2-9b33:GHC.Utils.Panic.GhcException:

Can't parse reexported module flag: Language.Haskell.Extension\ as\ Language.Haskell.Extension

While handling Can't parse reexported module flag: Language.Haskell.Extension\ as\ Language.Haskell.Extension

HasCallStack backtrace:
  throwIO, called at src/Run.hs:152:12 in doctest-0.24.3-bfacc00613da87380f139bf815745504e2634b62b9154c5a8f287d9e3e0f1020:Run

Expected behavior
That we could run make doctest with ghc-9.12.2.

System information
Ubuntu 25.10, ghc-9.12.2, cabal-3.16.1.0

[philderbeast]

It seems to be choking with this reexport of a module from Cabal-syntax:

cabal/Cabal/src/Distribution/Simple.hs

Line 49 in fb32f3d

, module Language.Haskell.Extension

[philderbeast]

This may help if we can get doctest to ignore the module that doesn't parse:

-- $setup
-- >>> import Data.Version
-- >>> import Distribution.Version
-- >>> import qualified System.Info as Info
--
-- >>> :{
-- checkCanDoctest = do
--   let v = mkVersion $ versionBranch Info.compilerVersion
--   let r = earlierVersion $ mkVersion [9,12]
--   if withinRange v r
--     then "DOCTEST should work with this version of GHC"
--     else "DOCTEST fails with this version of GHC, please try a version < 9.12" 
-- :}
--
-- >>> checkCanDoctest
-- "DOCTEST should work with this version of GHC"

[ulysses4ever]

Should we ask @sol for his opinion? It looks more like a problem somewhere between doctest and GHC than cabal?

[philderbeast]

The relevant change in GHC for multirepl, the one with the "Can't parse reexported module" message, is Extend -reexported-module flag to support module renaming.

[philderbeast]

Following the doctest guide, I can get doctests to work with ghc >= 9.12:

- $ cabal doctest Cabal
+ $ cabal repl Cabal --with-compiler=doctest --build-depends=QuickCheck --repl-options='-w -Wdefault'

• Similar to the make doctest calls that use cabal-doctest doctest:exe:cabal-doctest (built with the cabal-doctest flag), as an cabal external command:

$ cabal doctest Cabal
...
Build profile: -w ghc-9.14.1 -O1
...
Preprocessing library for Cabal-3.17.0.0...
doctest-0.24.3: Uncaught exception ghc-9.14.1-c6c3:GHC.Utils.Panic.GhcException:

Can't parse reexported module flag: Language.Haskell.Extension\ as\ Language.Haskell.Extension

While handling Can't parse reexported module flag: Language.Haskell.Extension\ as\ Language.Haskell.Extension

HasCallStack backtrace:

• Suggested use in the doctest readme:

$ cabal repl Cabal --with-compiler=doctest --build-depends=QuickCheck --repl-options='-w -Wdefault'
...
Build profile: -w ghc-9.14.1 -O1
...
Examples: 25  Tried: 25  Errors: 0  Failures: 0

[ulysses4ever]

Oh, so it's a deficiency of the new external-command-based mode of doctest. I wonder what sol's take is on this.

Great sleuthing, @philderbeast , thanks!

[philderbeast]

I've just added a couple of doctests to an executable for #11634. The way we're currently running doctests doesn't find those but the cabal repl --with-compiler=doctest method does:

$ cd Cabal-tests && cabal doctest Cabal-tests:exe:lang-tests
...
doctest version 0.24.3
using version 9.10.3 of the GHC API
...
Examples: 0  Tried: 0  Errors: 0  Failures: 0

$ cabal repl Cabal-tests:exe:lang-tests --with-compiler=doctest --build-depends=QuickCheck --repl-options='-w -Wdefault'
...
Build profile: -w ghc-9.10.3 -O1
...
Preprocessing executable 'lang-tests' for Cabal-tests-3...
Examples: 2  Tried: 2  Errors: 0  Failures: 0

[philderbeast]

I see doctest-0.25.0 was released 2026-05-03 with "Full GHC 9.14 compatibility / -unit-support" from its changelog.

Trying this version with our makefile targets passes for ghc-9.12.4 but fails for ghc-9.14.1:

$ git rev-parse HEAD
e6ff0ccf6f2761f1d3d82b4383b053c21f816e27

$ cabal update --ignore-project

$ make doctest-install
...
Build profile: -w ghc-9.14.1 -O1
In order, the following will be built (use -v for more details):
 - doctest-0.25.0 (lib) (requires build)
 - doctest-0.25.0 (exe:doctest) (requires build)
 - doctest-0.25.0 (exe:cabal-doctest) (requires build)
...


$ make doctest
...
Preprocessing library for Cabal-3.17.0.0...
/home/.../cabal/Cabal/src/Distribution/Verbosity.hs:452: \
  failure in expression `import Test.QuickCheck (Arbitrary (..), arbitraryBoundedEnum)'
expected: 
 but got: <no location info>: error: [GHC-45102]
          ^
              Ambiguous module name ‘Test.QuickCheck’.
              it was found in multiple packages:
              QuickCheck-2.18.0.0 QuickCheck-2.18.0.0
          

Examples: 25  Tried: 17  Errors: 0  Failures: 1
make: *** [Makefile:156: doctest] Error 1