@christiaanb, thanks for your PR! By analyzing the history of the files in this pull request, we identified @dcoutts, @hvr and @ezyang to be potential reviewers.

Is this ready for review or still WIP?

This is ready for review

Looks OK to me. Just add a changelog note and this will be good to go.

Looks like there are failing tests.

Yes, the failing test was due to https://ci.appveyor.com/project/ezyang/cabal/build/1.0.1740, however, that seems to be fixed in the latest master. I've rebased, everything should go through.

I'm not sure your analysis is correct, but let's see.

Yep, still fails. Looks like the problem is that we're not filtering out --hidir when invoking Setup scripts compiled with versions of Cabal that don't support it.

@christiaanb Look at filterConfigureFlags to get an idea of how we usually deal with such problems.

Seems to be still not fully fixed.

OK, I need help... I thought that 3a3b99c ensures that the --hidir flag is not passed to older Cabal's. It seems that it is not enough. Can anyone see what I'm doing wrong?

I'll look into this later today or tomorrow.

OK, I figured it out, all tests are green now.

Nice. Are we OK with old Setups putting all .hi files in $libdir on macOS?

@23Skidoo No... that shouldn't happen. Although I see how my patch does that sigh

Although I have no idea how to fix that properly.

• The problem is that I have no idea how and where to determine that the --libsubdir that we are passing to the Setup executable is accidentally equal to the default $libsubdir, or whether it is purposely set to a filepath that just happens to be equal to the $libsubdir specified by the user.

I think that you have to refactor the Flag type (or at least InstallDirs) to preserve that information. Then during config file/command line parsing you would remember whether it's set by user or uses the default value.

Ugg.

@23Skidoo do you recall how we used to make this work with ELF .so libs? I'm pretty sure they used to go into the $libdir not $libdir/$libsubdir. How did that work without us having a separate hidir?

I want to note that I'm not too happy with c0ab0e8 either. But I'm feeling a bit under the weather, and I only have time to work on this during the evening, and there's the pending GHC 8.0.2 release. I tried to refector Flag into

data Flag a = UserFlag a | DefaultFlag a | NoFlag

But that just turned into a big mess.

I guess I could revert c0ab0e8 and then we'll just end up putting .hi files in the same directory as the .dylib when we use a Setup build against an older version of Cabal

@dcoutts Not sure about that, but I've noticed that we have a dynlibdir field in InstallDirs. Maybe we can reuse that instead of adding hidir?

Also there's this comment by @ezyang in D.C.Install:

-- TODO: decide if we need the user to be able to control the libdir
-- for shared libs independently of the one for static libs. If so
-- it should also have a flag in the command line UI
-- For the moment use dynlibdir = libdir

@dcoutts Turns out that this comment was originally added by you in 0e5cf59 and 51d621e. So --dynlibdir is ignored by versions of Cabal released after 2009. It probably still makes sense to revive it instead of adding hidir, butt this won't help with backwards compat.

I tried to refector Flag into

Yeah, bad idea. If you want to do something like that, just do it for the particular field. (Have not been following convo, don't know the global picture.)

Given the problem of copying .hi files when using a Setup build again an older Cabal, I'm abandoning this PR. Instead, I'm reviving --dynlibdir: I'm building and testing it now and will submit a new PR soon.

The new PR is #3979