Prevent dependency on private library

Cabal/Distribution/Simple/Configure.hs

@@ -477,6 +477,7 @@ configure (pkg_descr0, pbi) cfg = do
                 (dependencySatisfiable
                     use_external_internal_deps
                     (fromFlagOrDefault False (configExactConfiguration cfg))
+                    (fromFlagOrDefault False (configAllowDependingOnPrivateLibs cfg))
                     (packageName pkg_descr0)
                     installedPackageSet
                     internalPackageSet
@@ -882,6 +883,7 @@ getInternalPackages pkg_descr0 =
 dependencySatisfiable
     :: Bool -- ^ use external internal deps?
     -> Bool -- ^ exact configuration?
+    -> Bool -- ^ allow depending on private libs?
     -> PackageName
     -> InstalledPackageIndex -- ^ installed set
     -> Map PackageName (Maybe UnqualComponentName) -- ^ internal set
@@ -890,7 +892,9 @@ dependencySatisfiable
     -> (Dependency -> Bool)
 dependencySatisfiable
   use_external_internal_deps
-  exact_config pn installedPackageSet internalPackageSet requiredDepsMap
+  exact_config
+  allow_private_deps
+  pn installedPackageSet internalPackageSet requiredDepsMap
   (Dependency depName vr sublibs)
 
     | exact_config
@@ -951,6 +955,9 @@ dependencySatisfiable
     visible lib = maybe
                     False -- Does not even exist (wasn't in the depsMap)
                     (\ipi -> Installed.libVisibility ipi == LibraryVisibilityPublic
+                          -- If the override is enabled, the visibility does
+                          -- not matter (it's handled externally)
+                          || allow_private_deps
                           -- If it's a library of the same package then it's
                           -- always visible.
                           -- This is only triggered when passing a component

Cabal/Distribution/Simple/Setup.hs

@@ -273,9 +273,13 @@ data ConfigFlags = ConfigFlags {
       -- ^Halt and show an error message indicating an error in flag assignment
     configRelocatable :: Flag Bool, -- ^ Enable relocatable package built
     configDebugInfo :: Flag DebugInfoLevel,  -- ^ Emit debug info.
-    configUseResponseFiles :: Flag Bool
+    configUseResponseFiles :: Flag Bool,
       -- ^ Whether to use response files at all. They're used for such tools
       -- as haddock, or or ld.
+    configAllowDependingOnPrivateLibs :: Flag Bool
+      -- ^ Allow depending on private sublibraries. This is used by external
+      -- tools (like cabal-install) so they can add multiple-public-libraries
+      -- compatibility to older ghcs by checking visibility externally.
   }
   deriving (Generic, Read, Show)
 
@@ -703,6 +707,13 @@ configureOptions showOrParseArgs =
          configUseResponseFiles
          (\v flags -> flags { configUseResponseFiles = v })
          (boolOpt' ([], ["disable-response-files"]) ([], []))
+
+      ,option "" ["allow-depending-on-private-libs"]
+         (  "Allow depending on private libraries. "
+         ++ "If set, the library visibility check MUST be done externally." )
+         configAllowDependingOnPrivateLibs
+         (\v flags -> flags { configAllowDependingOnPrivateLibs = v })
+         trueArg
       ]
   where
     liftInstallDirs =

cabal-install/Distribution/Client/Config.hs

@@ -413,7 +413,8 @@ instance Semigroup SavedConfig where
         configExactConfiguration  = combine configExactConfiguration,
         configFlagError           = combine configFlagError,
         configRelocatable         = combine configRelocatable,
-        configUseResponseFiles    = combine configUseResponseFiles
+        configUseResponseFiles    = combine configUseResponseFiles,
+        configAllowDependingOnPrivateLibs = combine configAllowDependingOnPrivateLibs
         }
         where
           combine        = combine'        savedConfigureFlags

cabal-install/Distribution/Client/ProjectConfig/Legacy.hs

@@ -687,7 +687,8 @@ convertToLegacyAllPackageConfig
       configFlagError           = mempty,                --TODO: ???
       configRelocatable         = mempty,
       configDebugInfo           = mempty,
-      configUseResponseFiles    = mempty
+      configUseResponseFiles    = mempty,
+      configAllowDependingOnPrivateLibs = mempty
     }
 
     haddockFlags = mempty {
@@ -757,7 +758,8 @@ convertToLegacyPerPackageConfig PackageConfig {..} =
       configFlagError           = mempty,                --TODO: ???
       configRelocatable         = packageConfigRelocatable,
       configDebugInfo           = packageConfigDebugInfo,
-      configUseResponseFiles    = mempty
+      configUseResponseFiles    = mempty,
+      configAllowDependingOnPrivateLibs = mempty
     }
 
     installFlags = mempty {

cabal-install/Distribution/Client/ProjectPlanning.hs

@@ -3390,6 +3390,9 @@ setupHsConfigureFlags (ReadyPackage elab@ElaboratedConfiguredPackage{..})
     configUserInstall         = mempty -- don't rely on defaults
     configPrograms_           = mempty -- never use, shouldn't exist
     configUseResponseFiles    = mempty
+    -- TODO set to true when the solver can prevent private-library-deps by itself
+    -- (issue #6039)
+    configAllowDependingOnPrivateLibs = mempty
 
 setupHsConfigureArgs :: ElaboratedConfiguredPackage
                      -> [String]