no longer pre-filter broken packages for modular solver

The modular solver can now deal with broken packages in the
index. I have, however, discovered, that broken packages were
filtered before even passing the installed package index to
the solver. I have made that filtering conditional on using
the topdown solver for now.

Related is the issue that we should not warn about possible
breakage of already broken packages. So we now exclude
already broken packages when considering whether an install
plan contains dangerous reinstalls.

cabal-install/Distribution/Client/Dependency.hs

@@ -367,7 +367,13 @@ resolveDependencies platform comp  solver params =
       noReinstalls
       shadowing
       maxBkjumps      = dontUpgradeBasePackage
-                      . hideBrokenInstalledPackages
+                      -- TODO:
+                      -- The modular solver can properly deal with broken packages
+                      -- and won't select them. So the 'hideBrokenInstalledPackages'
+                      -- function should be moved into a module that is specific
+                      -- to the Topdown solver.
+                      . (if solver /= Modular then hideBrokenInstalledPackages
+                                              else id)
                       $ params
 
     preferences = interpretPackagesPreference

cabal-install/Distribution/Client/Install.hs

@@ -358,13 +358,21 @@ checkPrintPlan verbosity installed installPlan installFlags pkgSpecifiers = do
   let lPlan = linearizeInstallPlan installed installPlan
   -- Are any packages classified as reinstalls?
   let reinstalledPkgs = concatMap (extractReinstalls . snd) lPlan
+  -- Packages that are already broken.
+  let oldBrokenPkgs =
+          map Installed.installedPackageId
+        . PackageIndex.reverseDependencyClosure installed
+        . map (Installed.installedPackageId . fst)
+        . PackageIndex.brokenPackages
+        $ installed
+  let excluded = reinstalledPkgs ++ oldBrokenPkgs
   -- Packages that are reverse dependencies of replaced packages are very
-  -- likely to be broken.
-  let brokenPkgs =
-        filter (\ p -> not (Installed.installedPackageId p `elem` reinstalledPkgs))
+  -- likely to be broken. We exclude packages that are already broken.
+  let newBrokenPkgs =
+        filter (\ p -> not (Installed.installedPackageId p `elem` excluded))
                (PackageIndex.reverseDependencyClosure installed reinstalledPkgs)
   let containsReinstalls = not (null reinstalledPkgs)
-  let breaksPkgs         = not (null brokenPkgs)
+  let breaksPkgs         = not (null newBrokenPkgs)
 
   let adaptedVerbosity
         | containsReinstalls && not overrideReinstall = verbosity `max` verbose
@@ -383,7 +391,7 @@ checkPrintPlan verbosity installed installPlan installFlags pkgSpecifiers = do
       then do
         (if dryRun || overrideReinstall then warn verbosity else die) $ unlines $
             "The following packages are likely to be broken by the reinstalls:"
-          : map (display . Installed.sourcePackageId) brokenPkgs
+          : map (display . Installed.sourcePackageId) newBrokenPkgs
           ++ if overrideReinstall
                then if dryRun then [] else
                  ["Continuing even though the plan contains dangerous reinstalls."]