Packages are only shadowed if the InstalledPackageId is *different*

If the two InstalledPackageIds are the same, then the packages are
interchangeable and we don't need to explicitly remove one of them.

diff --git a/compiler/main/Packages.lhs b/compiler/main/Packages.lhs
index 4b14462..4c22b34 100644 (file)
--- a/compiler/main/Packages.lhs
+++ b/compiler/main/Packages.lhs
@@ -556,12 +556,13 @@ shadowPackages pkgs preferred
    in  listToFM shadowed
  where
  check (shadowed,pkgmap) pkg
-      | Just oldpkg <- lookupUFM pkgmap (packageConfigId pkg)
-      = let
-          ipid_new = installedPackageId pkg
-          ipid_old = installedPackageId oldpkg
-        in
-        if ipid_old `elem` preferred
+      | Just oldpkg <- lookupUFM pkgmap (packageConfigId pkg),
+        let
+            ipid_new = installedPackageId pkg
+            ipid_old = installedPackageId oldpkg,
+        --
+        ipid_old /= ipid_new
+      = if ipid_old `elem` preferred
            then ( (ipid_new, ShadowedBy ipid_old) : shadowed, pkgmap )
            else ( (ipid_old, ShadowedBy ipid_new) : shadowed, pkgmap' )
       | otherwise