Fix predicate which determines whether to build a shared library

Consider the which determines whether we should build shared libraries.

      pkgsUseSharedLibrary :: Set PackageId
      pkgsUseSharedLibrary =
        packagesWithLibDepsDownwardClosedProperty needsSharedLib
        where
          needsSharedLib pkg =
            fromMaybe
              compilerShouldUseSharedLibByDefault
              (liftM2 (||) pkgSharedLib pkgDynExe)
            where
              pkgid = packageId pkg
              pkgSharedLib = perPkgOptionMaybe pkgid packageConfigSharedLib
              pkgDynExe = perPkgOptionMaybe pkgid packageConfigDynExe

In English the intended logic is:

If we have enabled shared libraries or dynamic executables then we need
to build a shared libary.

but, a common mistake:

(liftM2 (||) pkgSharedLib pkgDynExe)

instead says, if we explicitly request shared libraries and also explicitly configure whether we want dynamic executables then build a shared library.

It should instead use the monoid instance:

getMax <$> ((Max <$> pkgSharedLib) <> (Max <$> pkgDynExe))

which captures the original logic.

This failure is currently manifested in the way that if you write
--disable-shared then --enable-shared is still passed to ./Setup.

If you pass both --disable-shared and --disable-executable-dynamic then
you don't build a shared object.

Fixes #10050

Author: mpickering

cabal-install/src/Distribution/Client/ProjectPlanning.hs

@@ -219,6 +219,7 @@ import Distribution.Solver.Types.ProjectConfigPath
 import System.FilePath
 import Text.PrettyPrint (colon, comma, fsep, hang, punctuate, quotes, text, vcat, ($$))
 import qualified Text.PrettyPrint as Disp
+import Data.Semigroup
 
 -- | Check that an 'ElaboratedConfiguredPackage' actually makes
 -- sense under some 'ElaboratedSharedConfig'.
@@ -2365,7 +2366,7 @@ elaborateInstallPlan
           needsSharedLib pkg =
             fromMaybe
               compilerShouldUseSharedLibByDefault
-              (liftM2 (||) pkgSharedLib pkgDynExe)
+              (getMax <$> ((Max <$> pkgSharedLib) <> (Max <$> pkgDynExe)))
             where
               pkgid = packageId pkg
               pkgSharedLib = perPkgOptionMaybe pkgid packageConfigSharedLib