build-type: Custom seems to break doctests when using haddock

[ezzieyguywuf]

Describe the bug
If the build-type: Custom is used, which is necessary for doctest, then running ./setup haddock ends up deleting some files that are necessary for the test-suite to run. Re-running ./setup build will re-generate the deleted files.

To Reproduce

1. Set up a project using doctest
2. Make sure that the test-suite target depends on the library
3. Run the following commands

ghc Setup.lhs -o setup
./setup configure --enable-tests
./setup build
./setup haddock
./setup test # tests fail
./setup build # the deleted files are re-built
./setup test # tests pass

Please use version-prefixed commands (e.g. `v2-build` or `v1-build`) to avoid ambiguity.

**Expected behavior**
I expect that regardless of the `build-type`, the `test-suite` will pass if indeed the code is sound. I do **not** expect for the `./setup haddock` call to delete anything.

**System information**
 - Gentoo Linux
 - ghc 8.10.2
 - cabal library version 3.2.0.0

**Additional context**
A full working, minimal project can be found [here](https://gitlab.com/ezzieyguywuf/testfailure) which reproduces the error.

[Mikolaj]

AFAIK, doctests don't require Custom setups any more. BTW, the most up to date cabal ticket about doctests (with links to other tickets), is here: #4500

[andreasabel]

@Mikolaj wrote:

AFAIK, doctests don't require Custom setups any more.

Ah, that is something I should learn. Do you have a link to a howto?

[Mikolaj]

Actually, I'm not sure about the unqualified version of my claim any more. However, if you are willing to run doctests with cabal build . && cabal exec cabal test doctests the following setup works for me (and documentation for the packages I use may have some further links: https://github.com/LambdaHack/LambdaHack/blob/master/LambdaHack.cabal#L458

Edit: also on topic: https://twitter.com/TechnoEmpress/status/1457398534525923328

[fgaz]

Another alternative: https://github.com/phadej/cabal-extras/tree/master/cabal-docspec

[andreasabel]

@fgaz: Thanks for the pointer. cabal-docspec seems to have the limitation that you can only doctest exported functions. (But it is likely faster than doctest.)

@Mikolaj: Ah, you are using https://github.com/Hexirp/doctest-driver-gen. I need to look into this!

[Mikolaj]

@Mikolaj: Ah, you are using https://github.com/Hexirp/doctest-driver-gen. I need to look into this!

I think there are other such tools, as well.