Cabal disagrees on the datadir in the Path_ module, and where it ends up installing data files.

[angerman]

The issue seems to be the handling of components. If we explicitly ask for the component, we end up with a fully qualified $pkgid, if we don't we get the plain pkg name. This can be illustrated on the alex packaged quite neatly:

$ runghc Setup.hs configure exe:alex 
$ runghc Setup.hs build exe:alex 
$ cat dist/build/alex/autogen/Paths_alex.hs |grep datadir
bindir, libdir, dynlibdir, datadir, libexecdir, sysconfdir :: FilePath
datadir    = "/usr/local/share/x86_64-osx-ghc-8.6.3/alex-3.2.4-BnUEMKTG1CcF5xN1sTDkoW-alex"
getDataDir = catchIO (getEnv "alex_datadir") (\_ -> return datadir)

vs.

$ runghc Setup.hs configure
$ runghc Setup.hs build
$ cat dist/build/alex/autogen/Paths_alex.hs |grep datadir
bindir, libdir, dynlibdir, datadir, libexecdir, sysconfdir :: FilePath
datadir    = "/usr/local/share/x86_64-osx-ghc-8.6.3/alex-3.2.4"
getDataDir = catchIO (getEnv "alex_datadir") (\_ -> return datadir)

Now this wouldn't be an issue, if runghc Setup.hs install installed the data files into /usr/local/share/x86_64-osx-ghc-8.6.3/alex-3.2.4-BnUEMKTG1CcF5xN1sTDkoW-alex for the component configured case, but instead it will install them in /usr/local/share/x86_64-osx-ghc-8.6.3/alex-3.2.4, which then gives rise to amazing errors at runtime :(

This seems mildly related to #5543, but I'm not sure it's the identical situation.

[angerman]

Could this be the patch I'm looking for?

diff --git a/Cabal/Distribution/Simple/Build/PathsModule.hs b/Cabal/Distribution/Simple/Build/PathsModule.hs
index f6a203874..2aa406f74 100644
--- a/Cabal/Distribution/Simple/Build/PathsModule.hs
+++ b/Cabal/Distribution/Simple/Build/PathsModule.hs
@@ -192,10 +192,14 @@ generate pkg_descr lbi clbi =
           bindir     = flat_bindir,
           libdir     = flat_libdir,
           dynlibdir  = flat_dynlibdir,
-          datadir    = flat_datadir,
           libexecdir = flat_libexecdir,
           sysconfdir = flat_sysconfdir
         } = absoluteComponentInstallDirs pkg_descr lbi cid NoCopyDest
+
+        InstallDirs {
+          datadir    = flat_datadir
+        } = absoluteInstallDirs pkg_descr lbi NoCopyDest
+
         InstallDirs {
           bindir     = flat_bindirrel,
           libdir     = flat_libdirrel,

[angerman]

There is a note in C/D/S/Install.hs

-- | Copy package global files.
copyPackage :: Verbosity -> PackageDescription
            -> LocalBuildInfo -> FilePath -> CopyDest -> IO ()
copyPackage verbosity pkg_descr lbi distPref copydest = do
  let -- This is a bit of a hack, to handle files which are not
      -- per-component (data files and Haddock files.)
      InstallDirs {
         datadir    = dataPref,
         -- NB: The situation with Haddock is a bit delicate.  On the
         -- one hand, the easiest to understand Haddock documentation
         -- path is pkgname-0.1, which means it's per-package (not
         -- per-component).  But this means that it's impossible to
         -- install Haddock documentation for internal libraries.  We'll
         -- keep this constraint for now; this means you can't use
         -- Cabal to Haddock internal libraries.  This does not seem
         -- like a big problem.
         docdir     = docPref,
         htmldir    = htmlPref,
         haddockdir = interfacePref}
             -- Notice use of 'absoluteInstallDirs' (not the
             -- per-component variant).  This means for non-library
             -- packages we'll just pick a nondescriptive foo-0.1
             = absoluteInstallDirs pkg_descr lbi copydest

[23Skidoo]

Is this fixed by #6055?