diff --git a/Cabal-syntax/src/Distribution/Types/InstalledPackageInfo.hs b/Cabal-syntax/src/Distribution/Types/InstalledPackageInfo.hs
index b510a34d540..b1758312611 100644
--- a/Cabal-syntax/src/Distribution/Types/InstalledPackageInfo.hs
+++ b/Cabal-syntax/src/Distribution/Types/InstalledPackageInfo.hs
@@ -42,8 +42,10 @@ data InstalledPackageInfo = InstalledPackageInfo
     sourcePackageId :: PackageId
   , sourceLibName :: LibraryName
   , installedComponentId_ :: ComponentId
+  , installedSublibs :: [InstalledPackageInfo]
   , libVisibility :: LibraryVisibility
   , installedUnitId :: UnitId
+  , installedInstanceUnitId :: InstanceUnitId
   , -- INVARIANT: if this package is definite, OpenModule's
     -- OpenUnitId directly records UnitId.  If it is
     -- indefinite, OpenModule is always an OpenModuleVar
@@ -135,6 +137,8 @@ emptyInstalledPackageInfo =
     , sourceLibName = LMainLibName
     , installedComponentId_ = mkComponentId ""
     , installedUnitId = mkUnitId ""
+    , installedInstanceUnitId = mkInstanceUnitId $ mkUnitId ""
+    , installedSublibs = mempty
     , instantiatedWith = []
     , compatPackageKey = ""
     , license = Left SPDX.NONE
diff --git a/Cabal-syntax/src/Distribution/Types/InstalledPackageInfo/FieldGrammar.hs b/Cabal-syntax/src/Distribution/Types/InstalledPackageInfo/FieldGrammar.hs
index 6d2849c5142..78283c238fc 100644
--- a/Cabal-syntax/src/Distribution/Types/InstalledPackageInfo/FieldGrammar.hs
+++ b/Cabal-syntax/src/Distribution/Types/InstalledPackageInfo/FieldGrammar.hs
@@ -61,6 +61,7 @@ ipiFieldGrammar
      , c (Identity LibraryVisibility)
      , c (Identity PackageName)
      , c (Identity UnitId)
+     , c (Identity InstanceUnitId)
      , c (Identity UnqualComponentName)
      , c (List FSep (Identity AbiDependency) AbiDependency)
      , c (List FSep (Identity UnitId) UnitId)
@@ -85,6 +86,7 @@ ipiFieldGrammar =
     <@> blurFieldGrammar basic basicFieldGrammar
     -- Basic fields
     <@> optionalFieldDef "id" L.installedUnitId (mkUnitId "")
+    <@> optionalFieldDef "instance-id" L.installedInstanceUnitId (mkInstanceUnitId $ mkUnitId "")
     <@> optionalFieldDefAla "instantiated-with" InstWith L.instantiatedWith []
     <@> optionalFieldDefAla "key" CompatPackageKey L.compatPackageKey ""
     <@> optionalFieldDefAla "license" SpecLicenseLenient L.license (Left SPDX.NONE)
@@ -134,6 +136,7 @@ ipiFieldGrammar =
         (PackageIdentifier pn _basicVersion)
         (combineLibraryName ln _basicLibName)
         (mkComponentId "") -- installedComponentId_, not in use
+        mempty -- installedSublibs
         _basicLibVisibility
       where
         MungedPackageName pn ln = _basicName
diff --git a/Cabal-syntax/src/Distribution/Types/InstalledPackageInfo/Lens.hs b/Cabal-syntax/src/Distribution/Types/InstalledPackageInfo/Lens.hs
index 5ced273f460..cbd53817bee 100644
--- a/Cabal-syntax/src/Distribution/Types/InstalledPackageInfo/Lens.hs
+++ b/Cabal-syntax/src/Distribution/Types/InstalledPackageInfo/Lens.hs
@@ -10,7 +10,7 @@ import Prelude ()
 import Distribution.Backpack (OpenModule)
 import Distribution.License (License)
 import Distribution.ModuleName (ModuleName)
-import Distribution.Package (AbiHash, ComponentId, PackageIdentifier, UnitId)
+import Distribution.Package (AbiHash, ComponentId, InstanceUnitId, PackageIdentifier, UnitId)
 import Distribution.Types.InstalledPackageInfo (AbiDependency, ExposedModule, InstalledPackageInfo)
 import Distribution.Types.LibraryName (LibraryName)
 import Distribution.Types.LibraryVisibility (LibraryVisibility)
@@ -27,6 +27,10 @@ installedUnitId :: Lens' InstalledPackageInfo UnitId
 installedUnitId f s = fmap (\x -> s{T.installedUnitId = x}) (f (T.installedUnitId s))
 {-# INLINE installedUnitId #-}
 
+installedInstanceUnitId :: Lens' InstalledPackageInfo InstanceUnitId
+installedInstanceUnitId f s = fmap (\x -> s{T.installedInstanceUnitId = x}) (f (T.installedInstanceUnitId s))
+{-# INLINE installedInstanceUnitId #-}
+
 installedComponentId_ :: Lens' InstalledPackageInfo ComponentId
 installedComponentId_ f s = fmap (\x -> s{T.installedComponentId_ = x}) (f (T.installedComponentId_ s))
 {-# INLINE installedComponentId_ #-}
diff --git a/Cabal-syntax/src/Distribution/Types/UnitId.hs b/Cabal-syntax/src/Distribution/Types/UnitId.hs
index 0b5ca4bdf7b..7f5538493c1 100644
--- a/Cabal-syntax/src/Distribution/Types/UnitId.hs
+++ b/Cabal-syntax/src/Distribution/Types/UnitId.hs
@@ -13,6 +13,9 @@ module Distribution.Types.UnitId
   , newSimpleUnitId
   , mkLegacyUnitId
   , getHSLibraryName
+  , InstanceUnitId
+  , mkInstanceUnitId
+  , unInstanceUnitId
   ) where
 
 import Distribution.Compat.Prelude
@@ -131,3 +134,16 @@ instance Parsec DefUnitId where
 -- is to ensure that the 'DefUnitId' invariant holds.
 unsafeMkDefUnitId :: UnitId -> DefUnitId
 unsafeMkDefUnitId = DefUnitId
+
+-- | A 'UnitId' for an instance (group of components). Typically
+-- this matches a main library 'UnitId'
+newtype InstanceUnitId = InstanceUnitId {unInstanceUnitId :: UnitId}
+  deriving (Generic, Read, Show, Eq, Ord, Data, Binary, NFData, Pretty)
+
+instance Structured InstanceUnitId
+
+instance Parsec InstanceUnitId where
+  parsec = InstanceUnitId <$> parsec
+
+mkInstanceUnitId :: UnitId -> InstanceUnitId
+mkInstanceUnitId = InstanceUnitId
diff --git a/Cabal-tests/tests/ParserTests/ipi/Includes2.cabal b/Cabal-tests/tests/ParserTests/ipi/Includes2.cabal
index efaad1e6540..6c6ccbaca0c 100644
--- a/Cabal-tests/tests/ParserTests/ipi/Includes2.cabal
+++ b/Cabal-tests/tests/ParserTests/ipi/Includes2.cabal
@@ -1,6 +1,7 @@
 name: z-Includes2-z-mylib
 version: 0.1.0.0
 id: Includes2-0.1.0.0-inplace-mylib+3gY9SyjX86dBypHcOaev1n
+instance-id: Includes2-0.1.0.0-inplace-mylib+3gY9SyjX86dBypHcOaev1n
 instantiated-with: Database=Includes2-0.1.0.0-inplace-mysql:Database.MySQL
 package-name: Includes2
 lib-name: mylib
diff --git a/Cabal-tests/tests/ParserTests/ipi/Includes2.expr b/Cabal-tests/tests/ParserTests/ipi/Includes2.expr
index cc002958c05..f0ed3e0d738 100644
--- a/Cabal-tests/tests/ParserTests/ipi/Includes2.expr
+++ b/Cabal-tests/tests/ParserTests/ipi/Includes2.expr
@@ -9,10 +9,13 @@ InstalledPackageInfo {
     (UnqualComponentName "mylib"),
   installedComponentId_ =
   ComponentId "",
+  installedSublibs = [],
   libVisibility =
   LibraryVisibilityPrivate,
   installedUnitId = UnitId
     "Includes2-0.1.0.0-inplace-mylib+3gY9SyjX86dBypHcOaev1n",
+  installedInstanceUnitId = InstanceUnitId
+    (UnitId "Includes2-0.1.0.0-inplace-mylib+3gY9SyjX86dBypHcOaev1n"),
   instantiatedWith = [
     _×_
       (ModuleName "Database")
diff --git a/Cabal-tests/tests/ParserTests/ipi/Includes2.format b/Cabal-tests/tests/ParserTests/ipi/Includes2.format
index ab6445d811d..3eece63182c 100644
--- a/Cabal-tests/tests/ParserTests/ipi/Includes2.format
+++ b/Cabal-tests/tests/ParserTests/ipi/Includes2.format
@@ -3,6 +3,7 @@ version:              0.1.0.0
 package-name:         Includes2
 lib-name:             mylib
 id:                   Includes2-0.1.0.0-inplace-mylib+3gY9SyjX86dBypHcOaev1n
+instance-id:          Includes2-0.1.0.0-inplace-mylib+3gY9SyjX86dBypHcOaev1n
 instantiated-with:    Database=Includes2-0.1.0.0-inplace-mysql:Database.MySQL
 key:                  Includes2-0.1.0.0-inplace-mylib+3gY9SyjX86dBypHcOaev1n
 license:              BSD3
diff --git a/Cabal-tests/tests/ParserTests/ipi/internal-preprocessor-test.expr b/Cabal-tests/tests/ParserTests/ipi/internal-preprocessor-test.expr
index 3c3be058152..350a782ee81 100644
--- a/Cabal-tests/tests/ParserTests/ipi/internal-preprocessor-test.expr
+++ b/Cabal-tests/tests/ParserTests/ipi/internal-preprocessor-test.expr
@@ -8,10 +8,13 @@ InstalledPackageInfo {
   sourceLibName = LMainLibName,
   installedComponentId_ =
   ComponentId "",
+  installedSublibs = [],
   libVisibility =
   LibraryVisibilityPublic,
   installedUnitId = UnitId
     "internal-preprocessor-test-0.1.0.0",
+  installedInstanceUnitId = InstanceUnitId
+    (UnitId ""),
   instantiatedWith = [],
   compatPackageKey =
   "internal-preprocessor-test-0.1.0.0",
diff --git a/Cabal-tests/tests/ParserTests/ipi/issue-2276-ghc-9885.expr b/Cabal-tests/tests/ParserTests/ipi/issue-2276-ghc-9885.expr
index d5da47695e5..9e57e7896ca 100644
--- a/Cabal-tests/tests/ParserTests/ipi/issue-2276-ghc-9885.expr
+++ b/Cabal-tests/tests/ParserTests/ipi/issue-2276-ghc-9885.expr
@@ -8,10 +8,13 @@ InstalledPackageInfo {
   sourceLibName = LMainLibName,
   installedComponentId_ =
   ComponentId "",
+  installedSublibs = [],
   libVisibility =
   LibraryVisibilityPublic,
   installedUnitId = UnitId
     "transformers-0.5.2.0",
+  installedInstanceUnitId = InstanceUnitId
+    (UnitId ""),
   instantiatedWith = [],
   compatPackageKey =
   "transformers-0.5.2.0",
diff --git a/Cabal-tests/tests/ParserTests/ipi/transformers.cabal b/Cabal-tests/tests/ParserTests/ipi/transformers.cabal
index 2c6387c2206..5329fd959ea 100644
--- a/Cabal-tests/tests/ParserTests/ipi/transformers.cabal
+++ b/Cabal-tests/tests/ParserTests/ipi/transformers.cabal
@@ -1,6 +1,7 @@
 name: transformers
 version: 0.5.2.0
 id: transformers-0.5.2.0
+instance-id: transformers-0.5.2.0
 key: transformers-0.5.2.0
 license: BSD3
 maintainer: Ross Paterson <R.Paterson@city.ac.uk>
diff --git a/Cabal-tests/tests/ParserTests/ipi/transformers.expr b/Cabal-tests/tests/ParserTests/ipi/transformers.expr
index bcb40470a87..cd2a369d90e 100644
--- a/Cabal-tests/tests/ParserTests/ipi/transformers.expr
+++ b/Cabal-tests/tests/ParserTests/ipi/transformers.expr
@@ -8,10 +8,13 @@ InstalledPackageInfo {
   sourceLibName = LMainLibName,
   installedComponentId_ =
   ComponentId "",
+  installedSublibs = [],
   libVisibility =
   LibraryVisibilityPublic,
   installedUnitId = UnitId
     "transformers-0.5.2.0",
+  installedInstanceUnitId = InstanceUnitId
+    (UnitId "transformers-0.5.2.0"),
   instantiatedWith = [],
   compatPackageKey =
   "transformers-0.5.2.0",
diff --git a/Cabal-tests/tests/ParserTests/ipi/transformers.format b/Cabal-tests/tests/ParserTests/ipi/transformers.format
index 66a0e14e5b5..7de14b06258 100644
--- a/Cabal-tests/tests/ParserTests/ipi/transformers.format
+++ b/Cabal-tests/tests/ParserTests/ipi/transformers.format
@@ -2,6 +2,7 @@ name:                 transformers
 version:              0.5.2.0
 visibility:           public
 id:                   transformers-0.5.2.0
+instance-id:          transformers-0.5.2.0
 key:                  transformers-0.5.2.0
 license:              BSD3
 maintainer:           Ross Paterson <R.Paterson@city.ac.uk>
diff --git a/Cabal-tests/tests/UnitTests/Distribution/Utils/Structured.hs b/Cabal-tests/tests/UnitTests/Distribution/Utils/Structured.hs
index 93ec21c5b9f..734efdd45d6 100644
--- a/Cabal-tests/tests/UnitTests/Distribution/Utils/Structured.hs
+++ b/Cabal-tests/tests/UnitTests/Distribution/Utils/Structured.hs
@@ -33,4 +33,4 @@ md5CheckGenericPackageDescription proxy = md5Check proxy
 
 md5CheckLocalBuildInfo :: Proxy LocalBuildInfo -> Assertion
 md5CheckLocalBuildInfo proxy = md5Check proxy
-    0x78979713e08179ab070d6ab10cd5ef6c
+    0xe394537933f4b964768db669ff128aa7
diff --git a/Cabal-tree-diff/src/Data/TreeDiff/Instances/Cabal.hs b/Cabal-tree-diff/src/Data/TreeDiff/Instances/Cabal.hs
index f8eaad4bf88..f6e5fc093e1 100644
--- a/Cabal-tree-diff/src/Data/TreeDiff/Instances/Cabal.hs
+++ b/Cabal-tree-diff/src/Data/TreeDiff/Instances/Cabal.hs
@@ -27,7 +27,7 @@ import Distribution.Types.AbiHash                  (AbiHash)
 import Distribution.Types.ComponentId              (ComponentId)
 import Distribution.Types.DumpBuildInfo            (DumpBuildInfo)
 import Distribution.Types.PackageVersionConstraint
-import Distribution.Types.UnitId                   (DefUnitId, UnitId)
+import Distribution.Types.UnitId                   (DefUnitId, InstanceUnitId, UnitId)
 import Distribution.Utils.NubList                  (NubList)
 import Distribution.Utils.Path                     (SymbolicPathX)
 import Distribution.Utils.ShortText                (ShortText, fromShortText)
@@ -113,6 +113,7 @@ instance ToExpr ForeignLibOption
 instance ToExpr ForeignLibType
 instance ToExpr HaddockTarget
 instance ToExpr IncludeRenaming
+instance ToExpr InstanceUnitId
 instance ToExpr InstalledPackageInfo
 instance ToExpr KnownRepoType
 instance ToExpr LegacyExeDependency
diff --git a/Cabal/src/Distribution/Backpack/Configure.hs b/Cabal/src/Distribution/Backpack/Configure.hs
index 55d1ae03254..fb6f91e69af 100644
--- a/Cabal/src/Distribution/Backpack/Configure.hs
+++ b/Cabal/src/Distribution/Backpack/Configure.hs
@@ -67,6 +67,7 @@ configureComponentLocalBuildInfos
   -> Bool -- deterministic
   -> Flag String -- configIPID
   -> Flag ComponentId -- configCID
+  -> Flag InstanceUnitId -- configIUID
   -> PackageDescription
   -> ([PreExistingComponent], [ConfiguredPromisedComponent])
   -> FlagAssignment -- configConfigurationsFlags
@@ -81,6 +82,7 @@ configureComponentLocalBuildInfos
   deterministic
   ipid_flag
   cid_flag
+  iuid_flag
   pkg_descr
   (prePkgDeps, promisedPkgDeps)
   flags
@@ -127,6 +129,7 @@ configureComponentLocalBuildInfos
         deterministic
         ipid_flag
         cid_flag
+        iuid_flag
         pkg_descr
         conf_pkg_map
         (map fst graph0)
@@ -391,6 +394,7 @@ mkLinkedComponentsLocalBuildInfo comp rcs = map go rcs
            in LibComponentLocalBuildInfo
                 { componentPackageDeps = cpds
                 , componentUnitId = this_uid
+                , componentInstanceUnitId = this_instance_uid
                 , componentComponentId = this_cid
                 , componentInstantiatedWith = insts
                 , componentIsIndefinite_ = is_indefinite
@@ -445,6 +449,7 @@ mkLinkedComponentsLocalBuildInfo comp rcs = map go rcs
             }
       where
         this_uid = rc_uid rc
+        this_instance_uid = rc_instance_id rc
         this_open_uid = rc_open_uid rc
         this_cid = rc_cid rc
         cname = componentName (rc_component rc)
diff --git a/Cabal/src/Distribution/Backpack/ConfiguredComponent.hs b/Cabal/src/Distribution/Backpack/ConfiguredComponent.hs
index 9fd78352b7c..569de9cf988 100644
--- a/Cabal/src/Distribution/Backpack/ConfiguredComponent.hs
+++ b/Cabal/src/Distribution/Backpack/ConfiguredComponent.hs
@@ -1,3 +1,5 @@
+{-# LANGUAGE PatternSynonyms #-}
+
 -- | See <https://github.com/ezyang/ghc-proposals/blob/backpack/proposals/0000-backpack.rst>
 module Distribution.Backpack.ConfiguredComponent
   ( ConfiguredComponent (..)
@@ -22,7 +24,7 @@ import Distribution.CabalSpecVersion
 import Distribution.Package
 import Distribution.PackageDescription
 import Distribution.Simple.BuildToolDepends
-import Distribution.Simple.Flag (Flag)
+import Distribution.Simple.Flag (Flag, pattern Flag, pattern NoFlag)
 import Distribution.Simple.LocalBuildInfo
 import Distribution.Types.AnnotatedId
 import Distribution.Types.ComponentInclude
@@ -43,6 +45,7 @@ import qualified Text.PrettyPrint as PP
 data ConfiguredComponent = ConfiguredComponent
   { cc_ann_id :: AnnotatedId ComponentId
   -- ^ Unique identifier of component, plus extra useful info.
+  , cc_instance_unit_id :: InstanceUnitId
   , cc_component :: Component
   -- ^ The fragment of syntax from the Cabal file describing this
   -- component.
@@ -98,11 +101,12 @@ dispConfiguredComponent cc =
 mkConfiguredComponent
   :: PackageDescription
   -> ComponentId
+  -> InstanceUnitId
   -> [AnnotatedId ComponentId] -- lib deps
   -> [AnnotatedId ComponentId] -- exe deps
   -> Component
   -> LogProgress ConfiguredComponent
-mkConfiguredComponent pkg_descr this_cid lib_deps exe_deps component = do
+mkConfiguredComponent pkg_descr this_cid this_iuid lib_deps exe_deps component = do
   -- Resolve each @mixins@ into the actual dependency
   -- from @lib_deps@.
   explicit_includes <- forM (mixins bi) $ \(Mixin pn ln rns) -> do
@@ -142,6 +146,7 @@ mkConfiguredComponent pkg_descr this_cid lib_deps exe_deps component = do
             , ann_pid = package pkg_descr
             , ann_cname = componentName component
             }
+      , cc_instance_unit_id = this_iuid
       , cc_component = component
       , cc_public = is_public
       , cc_exe_deps = exe_deps
@@ -170,11 +175,12 @@ type ConfiguredComponentMap =
 toConfiguredComponent
   :: PackageDescription
   -> ComponentId
+  -> InstanceUnitId
   -> ConfiguredComponentMap
   -> ConfiguredComponentMap
   -> Component
   -> LogProgress ConfiguredComponent
-toConfiguredComponent pkg_descr this_cid lib_dep_map exe_dep_map component = do
+toConfiguredComponent pkg_descr this_cid this_iuid lib_dep_map exe_dep_map component = do
   lib_deps <-
     if newPackageDepsBehaviour pkg_descr
       then fmap concat $
@@ -202,6 +208,7 @@ toConfiguredComponent pkg_descr this_cid lib_dep_map exe_dep_map component = do
   mkConfiguredComponent
     pkg_descr
     this_cid
+    this_iuid
     lib_deps
     exe_deps
     component
@@ -246,6 +253,7 @@ toConfiguredComponent'
   -> Bool -- deterministic
   -> Flag String -- configIPID (todo: remove me)
   -> Flag ComponentId -- configCID
+  -> Flag InstanceUnitId -- configIUID
   -> ConfiguredComponentMap
   -> Component
   -> LogProgress ConfiguredComponent
@@ -256,12 +264,14 @@ toConfiguredComponent'
   deterministic
   ipid_flag
   cid_flag
+  iuid_flag
   dep_map
   component = do
     cc <-
       toConfiguredComponent
         pkg_descr
         this_cid
+        this_iuid
         dep_map
         dep_map
         component
@@ -279,6 +289,10 @@ toConfiguredComponent'
           (package pkg_descr)
           (componentName component)
           (Just (deps, flags))
+      this_iuid =
+        case iuid_flag of
+          Flag iuid -> iuid
+          NoFlag -> mkInstanceUnitId $ mkUnitId $ unComponentId this_cid
       deps =
         [ ann_id aid | m <- Map.elems dep_map, aid <- Map.elems m
         ]
@@ -308,6 +322,7 @@ toConfiguredComponents
   -> Bool -- deterministic
   -> Flag String -- configIPID
   -> Flag ComponentId -- configCID
+  -> Flag InstanceUnitId -- configIUID
   -> PackageDescription
   -> ConfiguredComponentMap
   -> [Component]
@@ -318,6 +333,7 @@ toConfiguredComponents
   deterministic
   ipid_flag
   cid_flag
+  iuid_flag
   pkg_descr
   dep_map
   comps =
@@ -332,6 +348,7 @@ toConfiguredComponents
             deterministic
             ipid_flag
             cid_flag
+            iuid_flag
             m
             component
         return (extendConfiguredComponentMap cc m, cc)
diff --git a/Cabal/src/Distribution/Backpack/LinkedComponent.hs b/Cabal/src/Distribution/Backpack/LinkedComponent.hs
index d041c715aac..961a604ec74 100644
--- a/Cabal/src/Distribution/Backpack/LinkedComponent.hs
+++ b/Cabal/src/Distribution/Backpack/LinkedComponent.hs
@@ -50,6 +50,8 @@ import Text.PrettyPrint (Doc, hang, hsep, quotes, text, vcat, ($+$))
 data LinkedComponent = LinkedComponent
   { lc_ann_id :: AnnotatedId ComponentId
   -- ^ Uniquely identifies linked component
+  , lc_instance_id :: InstanceUnitId
+  -- ^ Uniquely identifies instance (group of components)
   , lc_component :: Component
   -- ^ Corresponds to 'cc_component'.
   , lc_exe_deps :: [AnnotatedId OpenUnitId]
@@ -128,6 +130,7 @@ toLinkedComponent
   pkg_map
   ConfiguredComponent
     { cc_ann_id = aid@AnnotatedId{ann_id = this_cid}
+    , cc_instance_unit_id = iuid
     , cc_component = component
     , cc_exe_deps = exe_deps
     , cc_public = is_public
@@ -378,6 +381,7 @@ toLinkedComponent
     return $
       LinkedComponent
         { lc_ann_id = aid
+        , lc_instance_id = iuid
         , lc_component = component
         , lc_public = is_public
         , -- These must be executables
diff --git a/Cabal/src/Distribution/Backpack/ReadyComponent.hs b/Cabal/src/Distribution/Backpack/ReadyComponent.hs
index 3eef45fadbb..143a49435a5 100644
--- a/Cabal/src/Distribution/Backpack/ReadyComponent.hs
+++ b/Cabal/src/Distribution/Backpack/ReadyComponent.hs
@@ -55,6 +55,8 @@ import Distribution.Version
 -- for every way these packages can be fully instantiated.
 data ReadyComponent = ReadyComponent
   { rc_ann_id :: AnnotatedId UnitId
+  , rc_instance_id :: InstanceUnitId
+  -- ^ Uniquely identifies instance (group of components)
   , rc_open_uid :: OpenUnitId
   -- ^ The 'OpenUnitId' for this package.  At the moment, this
   -- is used in only one case, which is to determine if an
@@ -319,6 +321,7 @@ toReadyComponents pid_map subst0 comps =
             Just
               ReadyComponent
                 { rc_ann_id = (lc_ann_id lc){ann_id = uid}
+                , rc_instance_id = lc_instance_id lc
                 , rc_open_uid = DefiniteUnitId (unsafeMkDefUnitId uid)
                 , rc_cid = lc_cid lc
                 , rc_component = lc_component lc
@@ -387,6 +390,7 @@ toReadyComponents pid_map subst0 comps =
             Just
               ReadyComponent
                 { rc_ann_id = (lc_ann_id lc){ann_id = uid}
+                , rc_instance_id = lc_instance_id lc
                 , rc_cid = lc_cid lc
                 , rc_open_uid = lc_uid lc
                 , rc_component = lc_component lc
diff --git a/Cabal/src/Distribution/Simple/Build.hs b/Cabal/src/Distribution/Simple/Build.hs
index e79a1924eb9..5954b139367 100644
--- a/Cabal/src/Distribution/Simple/Build.hs
+++ b/Cabal/src/Distribution/Simple/Build.hs
@@ -851,6 +851,7 @@ testSuiteLibV09AsLibAndExe
           , componentIsPublic = False
           , componentIncludes = componentIncludes clbi
           , componentUnitId = componentUnitId clbi
+          , componentInstanceUnitId = componentInstanceUnitId clbi
           , componentComponentId = componentComponentId clbi
           , componentInstantiatedWith = []
           , componentCompatPackageName = compat_name
diff --git a/Cabal/src/Distribution/Simple/Configure.hs b/Cabal/src/Distribution/Simple/Configure.hs
index df1acb72658..64cfc629f88 100644
--- a/Cabal/src/Distribution/Simple/Configure.hs
+++ b/Cabal/src/Distribution/Simple/Configure.hs
@@ -721,6 +721,10 @@ preConfigurePackage verbHandles cfg g_pkg_descr = do
   when (isJust (flagToMaybe (configCID cfg)) && isNothing mb_cname) $
     dieWithException verbosity ConfigCIDValidForPreComponent
 
+  -- configIUID is only valid for per-component configure
+  when (isJust (flagToMaybe (configIUID cfg)) && isNothing mb_cname) $
+    dieWithException verbosity ConfigIUIDValidForPreComponent
+
   -- Make a data structure describing what components are enabled.
   let enabled :: ComponentRequestedSpec
       enabled =
@@ -1427,7 +1431,7 @@ configureComponents
       -- components (which may build-depends on each other) and form a graph.
       -- From there, we build a ComponentLocalBuildInfo for each of the
       -- components, which lets us actually build each component.
-      ( buildComponents :: [ComponentLocalBuildInfo]
+      ( buildComponents0 :: [ComponentLocalBuildInfo]
         , packageDependsIndex :: InstalledPackageIndex
         ) <-
         runLogProgress verbosity $
@@ -1438,6 +1442,7 @@ configureComponents
             (fromFlagOrDefault False (configDeterministic cfg))
             (configIPID cfg)
             (configCID cfg)
+            (configIUID cfg)
             pkg_descr
             externalPkgDeps
             (configConfigurationsFlags cfg)
@@ -1445,6 +1450,47 @@ configureComponents
             installedPackageSet
             comp
 
+      -- Set a common 'InstanceUnitId' for all components so they can be
+      -- identified as belonging to the same group.
+      --
+      -- If the package has a main library, its 'InstanceUnitId' is used.
+      -- Otherwise, an arbitrary component's 'InstanceUnitId' is used.
+      --
+      -- Only for 'ComponentRequestedSpec'.
+      -- For 'OneComponentRequestedSpec', 'InstanceUnitId' is passed via 'configIUID' flag.
+      let maybeSetCommonInstanceUnitId =
+            case enabled of
+              -- Whole package configure, set InstanceUnitIDs
+              ComponentRequestedSpec{} ->
+                let
+                  setCommonInstanceUnitId
+                    :: [ComponentLocalBuildInfo]
+                    -> [ComponentLocalBuildInfo]
+                  setCommonInstanceUnitId [] = []
+                  setCommonInstanceUnitId [single] = [single]
+                  setCommonInstanceUnitId comps@(fstComp : _rest) =
+                    let
+                      assignOne newId LibComponentLocalBuildInfo{..} = LibComponentLocalBuildInfo{componentInstanceUnitId = newId, ..}
+                      assignOne _newId other = other
+
+                      assignAll newId = map (assignOne newId) comps
+
+                      matchMainLib lclbi@(LibComponentLocalBuildInfo{}) = componentLocalName lclbi == CLibName LMainLibName
+                      matchMainLib _ = False
+                     in
+                      case find matchMainLib comps of
+                        Nothing ->
+                          -- no main library, use arbitrary (first one) for componentInstanceUnitId assignment
+                          assignAll (mkInstanceUnitId $ componentUnitId fstComp)
+                        Just mainLib ->
+                          assignAll (componentInstanceUnitId mainLib)
+                 in
+                  setCommonInstanceUnitId
+              -- Component configure, InstanceUnitID passed via flag
+              OneComponentRequestedSpec{} -> id
+
+      let buildComponents = maybeSetCommonInstanceUnitId buildComponents0
+
       let buildComponentsMap =
             foldl'
               ( \m clbi ->
diff --git a/Cabal/src/Distribution/Simple/Errors.hs b/Cabal/src/Distribution/Simple/Errors.hs
index 4dc21def00d..6830af76aa7 100644
--- a/Cabal/src/Distribution/Simple/Errors.hs
+++ b/Cabal/src/Distribution/Simple/Errors.hs
@@ -115,6 +115,7 @@ data CabalException
   | NoValidComponent
   | ConfigureEitherSingleOrAll
   | ConfigCIDValidForPreComponent
+  | ConfigIUIDValidForPreComponent
   | SanityCheckForEnableComponents
   | SanityCheckForDynamicStaticLinking
   | UnsupportedLanguages PackageIdentifier CompilerId [String]
@@ -249,6 +250,7 @@ exceptionCode e = case e of
   NoValidComponent{} -> 5680
   ConfigureEitherSingleOrAll{} -> 2001
   ConfigCIDValidForPreComponent{} -> 7006
+  ConfigIUIDValidForPreComponent{} -> 7007
   SanityCheckForEnableComponents{} -> 5004
   SanityCheckForDynamicStaticLinking{} -> 4007
   UnsupportedLanguages{} -> 8074
@@ -510,6 +512,7 @@ exceptionMessage e = case e of
   NoValidComponent -> "No valid component targets found"
   ConfigureEitherSingleOrAll -> "Can only configure either a single component or all of them"
   ConfigCIDValidForPreComponent -> "--cid is only supported for per-component configure"
+  ConfigIUIDValidForPreComponent -> "--iuid is only supported for per-component configure"
   SanityCheckForEnableComponents ->
     "--enable-tests/--enable-benchmarks are incompatible with"
       ++ " explicitly specifying a component to configure."
diff --git a/Cabal/src/Distribution/Simple/Register.hs b/Cabal/src/Distribution/Simple/Register.hs
index 3e603a14d6a..22ba700cf15 100644
--- a/Cabal/src/Distribution/Simple/Register.hs
+++ b/Cabal/src/Distribution/Simple/Register.hs
@@ -501,7 +501,9 @@ generalInstalledPackageInfo adjustRelIncDirs pkg abi_hash lib lbi clbi installDi
   IPI.InstalledPackageInfo
     { IPI.sourcePackageId = packageId pkg
     , IPI.installedUnitId = componentUnitId clbi
+    , IPI.installedInstanceUnitId = componentInstanceUnitId clbi
     , IPI.installedComponentId_ = componentComponentId clbi
+    , IPI.installedSublibs = mempty
     , IPI.instantiatedWith = expectLibraryComponent (maybeComponentInstantiatedWith clbi)
     , IPI.sourceLibName = libName lib
     , IPI.compatPackageKey = expectLibraryComponent (maybeComponentCompatPackageKey clbi)
diff --git a/Cabal/src/Distribution/Simple/Setup/Config.hs b/Cabal/src/Distribution/Simple/Setup/Config.hs
index 92fb879df9a..4652ac15c0b 100644
--- a/Cabal/src/Distribution/Simple/Setup/Config.hs
+++ b/Cabal/src/Distribution/Simple/Setup/Config.hs
@@ -164,6 +164,8 @@ data ConfigFlags = ConfigFlags
   -- ^ explicit IPID to be used
   , configCID :: Flag ComponentId
   -- ^ explicit CID to be used
+  , configIUID :: Flag InstanceUnitId
+  -- ^ explicit InstanceUnitID to be used
   , configDeterministic :: Flag Bool
   -- ^ be as deterministic as possible
   -- (e.g., invariant over GHC, database,
diff --git a/Cabal/src/Distribution/Types/ComponentLocalBuildInfo.hs b/Cabal/src/Distribution/Types/ComponentLocalBuildInfo.hs
index 32f4aab473b..a7880664745 100644
--- a/Cabal/src/Distribution/Types/ComponentLocalBuildInfo.hs
+++ b/Cabal/src/Distribution/Types/ComponentLocalBuildInfo.hs
@@ -24,7 +24,6 @@ import Distribution.Types.UnitId
 
 import qualified Distribution.InstalledPackageInfo as Installed
 
--- | The first five fields are common across all algebraic variants.
 data ComponentLocalBuildInfo
   = LibComponentLocalBuildInfo
       { componentLocalName :: ComponentName
@@ -39,6 +38,9 @@ data ComponentLocalBuildInfo
       , componentUnitId :: UnitId
       -- ^ The computed 'UnitId' which uniquely identifies this
       -- component.  Might be hashed.
+      , componentInstanceUnitId :: InstanceUnitId
+      -- ^ The 'InstanceUnitId' which uniquely identifies package instance
+      -- (equal to componentUnitId for main component, sub components inherit it from main)
       , componentIsIndefinite_ :: Bool
       -- ^ Is this an indefinite component (i.e. has unfilled holes)?
       , componentInstantiatedWith :: [(ModuleName, OpenModule)]
diff --git a/cabal-install-solver/src/Distribution/Solver/Modular/ConfiguredConversion.hs b/cabal-install-solver/src/Distribution/Solver/Modular/ConfiguredConversion.hs
index 0e2e8ad5baa..e306631aa5a 100644
--- a/cabal-install-solver/src/Distribution/Solver/Modular/ConfiguredConversion.hs
+++ b/cabal-install-solver/src/Distribution/Solver/Modular/ConfiguredConversion.hs
@@ -5,8 +5,11 @@ module Distribution.Solver.Modular.ConfiguredConversion
 import Data.Maybe
 import Prelude hiding (pi)
 import Data.Either (partitionEithers)
+import Data.Set (Set)
+import qualified Data.Set as S
 
 import Distribution.Package (UnitId, packageId)
+import Distribution.Types.InstalledPackageInfo (InstalledPackageInfo(installedSublibs))
 
 import qualified Distribution.Simple.PackageIndex as SI
 
@@ -30,9 +33,10 @@ convCP :: SI.InstalledPackageIndex ->
           CP QPN -> ResolverPackage loc
 convCP iidx sidx (CP qpi fa es ds) =
   case convPI qpi of
-    Left  pi -> PreExisting $
+    Left (pi, subPis) ->
+                PreExisting $
                   InstSolverPackage {
-                    instSolverPkgIPI = fromJust $ SI.lookupUnitId iidx pi,
+                    instSolverPkgIPI = addSublibs iidx subPis $ fromJust $ SI.lookupUnitId iidx pi,
                     instSolverPkgLibDeps = fmap fst ds',
                     instSolverPkgExeDeps = fmap snd ds'
                   }
@@ -50,15 +54,29 @@ convCP iidx sidx (CP qpi fa es ds) =
     ds' :: ComponentDeps ([SolverId] {- lib -}, [SolverId] {- exe -})
     ds' = fmap (partitionEithers . map convConfId) ds
 
-convPI :: PI QPN -> Either UnitId PackageId
-convPI (PI _ (I _ (Inst pi))) = Left pi
-convPI pi                     = Right (packageId (either id id (convConfId pi)))
+    addSublibs
+      :: SI.InstalledPackageIndex
+      -> Set UnitId
+      -> InstalledPackageInfo
+      -> InstalledPackageInfo
+    addSublibs idx subPis info =
+      info { installedSublibs =
+               mapMaybe
+                 (SI.lookupUnitId idx)
+                 (S.toList subPis)
+           }
+
+convPI :: PI QPN -> Either (UnitId, Set UnitId) PackageId
+convPI (PI _ (I _ (Inst pi)))             = Left (pi, mempty)
+convPI (PI _ (I _ (InstGroup pi subPis))) = Left (pi, subPis)
+convPI pi                                 = Right (packageId (either id id (convConfId pi)))
 
 convConfId :: PI QPN -> Either SolverId {- is lib -} SolverId {- is exe -}
 convConfId (PI (Q (PackagePath _ q) pn) (I v loc)) =
     case loc of
         Inst pi -> Left (PreExistingId sourceId pi)
-        _otherwise
+        InstGroup pi _subPis -> Left (PreExistingId sourceId pi)
+        InRepo
           | QualExe _ pn' <- q
           -- NB: the dependencies of the executable are also
           -- qualified.  So the way to tell if this is an executable
diff --git a/cabal-install-solver/src/Distribution/Solver/Modular/Dependency.hs b/cabal-install-solver/src/Distribution/Solver/Modular/Dependency.hs
index fff2dacde4e..3b0fe5a95eb 100644
--- a/cabal-install-solver/src/Distribution/Solver/Modular/Dependency.hs
+++ b/cabal-install-solver/src/Distribution/Solver/Modular/Dependency.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE RecordWildCards #-}
+{-# LANGUAGE StandaloneDeriving #-}
 module Distribution.Solver.Modular.Dependency (
     -- * Variables
     Var(..)
@@ -94,6 +95,8 @@ data FlaggedDep qpn =
     -- | Dependencies which are always enabled, for the component 'comp'.
   | Simple (LDep qpn) Component
 
+deriving instance Show qpn => Show (FlaggedDep qpn)
+
 -- | Conservatively flatten out flagged dependencies
 --
 -- NOTE: We do not filter out duplicates.
@@ -115,6 +118,7 @@ type FalseFlaggedDeps qpn = FlaggedDeps qpn
 -- depending; having a 'Functor' instance makes bugs where we don't distinguish
 -- these two far too likely. (By rights 'LDep' ought to have two type variables.)
 data LDep qpn = LDep (DependencyReason qpn) (Dep qpn)
+  deriving Show
 
 -- | A dependency (constraint) associates a package name with a constrained
 -- instance. It can also represent other types of dependencies, such as
@@ -123,7 +127,7 @@ data Dep qpn = Dep (PkgComponent qpn) CI  -- ^ dependency on a package component
              | Ext Extension              -- ^ dependency on a language extension
              | Lang Language              -- ^ dependency on a language version
              | Pkg PkgconfigName PkgconfigVersionRange  -- ^ dependency on a pkg-config package
-  deriving Functor
+  deriving (Functor, Show)
 
 -- | An exposed component within a package. This type is used to represent
 -- build-depends and build-tool-depends dependencies.
diff --git a/cabal-install-solver/src/Distribution/Solver/Modular/Index.hs b/cabal-install-solver/src/Distribution/Solver/Modular/Index.hs
index 2f28d12de85..e04b62dd0c7 100644
--- a/cabal-install-solver/src/Distribution/Solver/Modular/Index.hs
+++ b/cabal-install-solver/src/Distribution/Solver/Modular/Index.hs
@@ -36,6 +36,7 @@ data PInfo = PInfo (FlaggedDeps PN)
                    (Map ExposedComponent ComponentInfo)
                    FlagInfo
                    (Maybe FailReason)
+  deriving Show
 
 -- | Info associated with each library and executable in a package instance.
 data ComponentInfo = ComponentInfo {
diff --git a/cabal-install-solver/src/Distribution/Solver/Modular/IndexConversion.hs b/cabal-install-solver/src/Distribution/Solver/Modular/IndexConversion.hs
index f150235631f..b3235d0dd69 100644
--- a/cabal-install-solver/src/Distribution/Solver/Modular/IndexConversion.hs
+++ b/cabal-install-solver/src/Distribution/Solver/Modular/IndexConversion.hs
@@ -6,6 +6,7 @@ import Distribution.Solver.Compat.Prelude
 import Prelude ()
 
 import qualified Data.List as L
+import qualified Data.Maybe
 import qualified Data.Map.Strict as M
 import qualified Distribution.Compat.NonEmptySet as NonEmptySet
 import qualified Data.Set as S
@@ -18,8 +19,6 @@ import Distribution.Types.ExeDependency              -- from Cabal
 import Distribution.Types.PkgconfigDependency        -- from Cabal
 import Distribution.Types.ComponentName              -- from Cabal
 import Distribution.Types.CondTree                   -- from Cabal
-import Distribution.Types.MungedPackageId            -- from Cabal
-import Distribution.Types.MungedPackageName          -- from Cabal
 import Distribution.PackageDescription               -- from Cabal
 import Distribution.PackageDescription.Configuration
 import qualified Distribution.Simple.PackageIndex as SI
@@ -62,43 +61,118 @@ convPIs :: OS -> Arch -> CompilerInfo -> Map PN [LabeledPackageConstraint]
         -> Index
 convPIs os arch comp constraints sip strfl solveExes iidx sidx =
   mkIndex $
-  convIPI' sip iidx ++ convSPI' os arch comp constraints strfl solveExes sidx
+       groupInstalledSublibs (convIPI' sip iidx)
+    ++ (convSPI' os arch comp constraints strfl solveExes sidx)
+
+-- | Group packages with the same package name and version,
+-- merge their sub-libraries and dependencies so we get
+-- a similar looking package as if it came from repository.
+groupInstalledSublibs
+  :: [(PN, I, InstanceUnitId, PInfo)]
+  -> [(PN, I, PInfo)]
+groupInstalledSublibs xs =
+    remapPInfoDepsToInstGroups
+  $ M.elems
+  $ M.map (\(pn, i, _instId, pInfo) -> (pn, i, pInfo))
+  $ foldl
+      (\acc x@(pn, I ver _, instId, _) ->
+        M.insertWith
+          (\(_, newI, newInstId, newInfo) (_, oldI, _oldInstId, oldInfo) ->
+            (pn, mergeIs oldI newI, newInstId, mergeInfos oldInfo newInfo)
+          )
+          (pn, ver, instId)
+          x
+          acc
+      )
+      M.empty
+      xs
+  where
+  -- flags are probably safe to ignore here as they are fixed for installed anyway
+  mergeInfos :: PInfo -> PInfo -> PInfo
+  mergeInfos (PInfo deps comps flagNfo fr) (PInfo deps' comps' _flagNfo _fr) =
+    PInfo
+      (deps <> deps')
+      (comps <> comps')
+      flagNfo
+      fr
+
+  -- this pass creates InstGroup(s)
+  mergeIs :: I -> I -> I
+  mergeIs (I ver (Inst pId)) (I _ver (Inst subPId)) =
+    I ver (InstGroup pId (S.singleton subPId))
+  mergeIs (I ver (InstGroup pId subPIds)) (I _ver (Inst subPId)) =
+    I ver (InstGroup pId (S.insert subPId subPIds))
+  -- XXX/srk, can't really happen as they are lexicographically ordered
+  mergeIs a b = error $ "Absurd mergeIs" <> show (a,b)
+
+  -- now some deps from convIP/convIPId pass refer to Inst when they should refer to InstGroup
+  remapPInfoDepsToInstGroups :: [(PN, I, PInfo)] -> [(PN, I, PInfo)]
+  remapPInfoDepsToInstGroups ps =
+    let
+      -- Inst -> InstGroup mapping, other Loc(s) are preserved
+      locMap :: Map Loc Loc
+      locMap =
+          M.fromList
+        $ concatMap
+            (\(_pn, I _ver loc, _pInfo) -> case loc of
+              ip@(Inst _pId) ->
+                pure (ip, ip)
+              g@(InstGroup pId subPIds) ->
+                (Inst pId, g):(map (\x -> (Inst x, g)) (S.toList subPIds))
+              InRepo ->
+                pure (InRepo, InRepo)
+            )
+            ps
+
+      remapDep :: FlaggedDep PN -> FlaggedDep PN
+      remapDep (D.Simple (LDep dr (Dep depComp (Fixed (I ver loc)))) comp) =
+        let newLoc = Data.Maybe.fromJust $ M.lookup loc locMap
+        in  (D.Simple (LDep dr (Dep depComp (Fixed (I ver newLoc)))) comp)
+      remapDep x = x
+
+    in map
+        (\(pn, i, PInfo deps comps flagNfo fr) ->
+          (pn, i, PInfo (map remapDep deps) comps flagNfo fr)
+        )
+        ps
+
 
 -- | Convert a Cabal installed package index to the simpler,
 -- more uniform index format of the solver.
-convIPI' :: ShadowPkgs -> SI.InstalledPackageIndex -> [(PN, I, PInfo)]
+convIPI' :: ShadowPkgs -> SI.InstalledPackageIndex -> [(PN, I, InstanceUnitId, PInfo)]
 convIPI' (ShadowPkgs sip) idx =
     -- apply shadowing whenever there are multiple installed packages with
     -- the same version
     [ maybeShadow (convIP idx pkg)
-    -- IMPORTANT to get internal libraries. See
-    -- Note [Index conversion with internal libraries]
+    -- IMPORTANT to use @allPackagesBySourcePackageIdAndLibName@ to get internal libraries
     | (_, pkgs) <- SI.allPackagesBySourcePackageIdAndLibName idx
     , (maybeShadow, pkg) <- zip (id : repeat shadow) pkgs ]
   where
 
     -- shadowing is recorded in the package info
-    shadow (pn, i, PInfo fdeps comps fds _)
-      | sip = (pn, i, PInfo fdeps comps fds (Just Shadowed))
+    shadow (pn, i, instId, PInfo fdeps comps fds _)
+      | sip = (pn, i, instId, PInfo fdeps comps fds (Just Shadowed))
     shadow x                                     = x
 
 -- | Extract/recover the package ID from an installed package info, and convert it to a solver's I.
 convId :: IPI.InstalledPackageInfo -> (PN, I)
-convId ipi = (pn, I ver $ Inst $ IPI.installedUnitId ipi)
-  where MungedPackageId mpn ver = mungedId ipi
-        -- HACK. See Note [Index conversion with internal libraries]
-        pn = encodeCompatPackageName mpn
+convId ipi = ( pkgName spi
+             , I (pkgVersion spi) $ Inst $ IPI.installedUnitId ipi
+             )
+ where spi = IPI.sourcePackageId ipi
 
 -- | Convert a single installed package into the solver-specific format.
-convIP :: SI.InstalledPackageIndex -> IPI.InstalledPackageInfo -> (PN, I, PInfo)
+convIP
+  :: SI.InstalledPackageIndex
+  -> IPI.InstalledPackageInfo
+  -> (PN, I, InstanceUnitId, PInfo)
 convIP idx ipi =
   case traverse (convIPId (DependencyReason pn M.empty S.empty) comp idx) (IPI.depends ipi) of
-        Left u    -> (pn, i, PInfo [] M.empty M.empty (Just (Broken u)))
-        Right fds -> (pn, i, PInfo fds components M.empty Nothing)
+        Left u    -> (pn, i, IPI.installedInstanceUnitId ipi, PInfo [] M.empty M.empty (Just (Broken u)))
+        Right fds -> (pn, i, IPI.installedInstanceUnitId ipi, PInfo fds components M.empty Nothing)
  where
-  -- TODO: Handle sub-libraries and visibility.
   components =
-      M.singleton (ExposedLib LMainLibName)
+      M.singleton (ExposedLib $ IPI.sourceLibName ipi)
                   ComponentInfo {
                       compIsVisible = IsVisible True
                     , compIsBuildable = IsBuildable True
@@ -111,45 +185,22 @@ convIP idx ipi =
   comp = componentNameToComponent $ CLibName $ IPI.sourceLibName ipi
 -- TODO: Installed packages should also store their encapsulations!
 
--- Note [Index conversion with internal libraries]
--- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
--- Something very interesting happens when we have internal libraries
--- in our index.  In this case, we maybe have p-0.1, which itself
--- depends on the internal library p-internal ALSO from p-0.1.
--- Here's the danger:
---
---      - If we treat both of these packages as having PN "p",
---        then the solver will try to pick one or the other,
---        but never both.
---
---      - If we drop the internal packages, now p-0.1 has a
---        dangling dependency on an "installed" package we know
---        nothing about. Oops.
---
--- An expedient hack is to put p-internal into cabal-install's
--- index as a MUNGED package name, so that it doesn't conflict
--- with anyone else (except other instances of itself).  But
--- yet, we ought NOT to say that PNs in the solver are munged
--- package names, because they're not; for source packages,
--- we really will never see munged package names.
---
--- The tension here is that the installed package index is actually
--- per library, but the solver is per package.  We need to smooth
--- it over, and munging the package names is a pretty good way to
--- do it.
-
 -- | Convert dependencies specified by an installed package id into
 -- flagged dependencies of the solver.
 --
 -- May return Nothing if the package can't be found in the index. That
 -- indicates that the original package having this dependency is broken
 -- and should be ignored.
-convIPId :: DependencyReason PN -> Component -> SI.InstalledPackageIndex -> UnitId -> Either UnitId (FlaggedDep PN)
+convIPId :: DependencyReason PN
+  -> Component
+  -> SI.InstalledPackageIndex
+  -> UnitId
+  -> Either UnitId (FlaggedDep PN)
 convIPId dr comp idx ipid =
   case SI.lookupUnitId idx ipid of
     Nothing  -> Left ipid
     Just ipi -> let (pn, i) = convId ipi
-                    name = ExposedLib LMainLibName  -- TODO: Handle sub-libraries.
+                    name = ExposedLib $ IPI.sourceLibName ipi
                 in  Right (D.Simple (LDep dr (Dep (PkgComponent pn name) (Fixed i))) comp)
                 -- NB: something we pick up from the
                 -- InstalledPackageIndex is NEVER an executable
diff --git a/cabal-install-solver/src/Distribution/Solver/Modular/Message.hs b/cabal-install-solver/src/Distribution/Solver/Modular/Message.hs
index 5dbcce9194c..03c8afb2663 100644
--- a/cabal-install-solver/src/Distribution/Solver/Modular/Message.hs
+++ b/cabal-install-solver/src/Distribution/Solver/Modular/Message.hs
@@ -289,7 +289,7 @@ showOptions q [x] = showOption q x
 showOptions q xs = showQPN q ++ "; " ++ (L.intercalate ", "
   [if isJust linkedTo
     then showOption q x
-    else showI i -- Don't show the package, just the version
+    else showI q i -- Don't show the package, just the version
   | x@(POption i linkedTo) <- xs
   ])
 
@@ -351,7 +351,7 @@ showConflictingDep (ConflictingDep dr (PkgComponent qpn comp) ci) =
                        ExposedLib (LSubLibName lib) -> " (lib " ++ unUnqualComponentName lib ++ ")"
   in case ci of
        Fixed i        -> (if qpn /= qpn' then showDependencyReason dr ++ " => " else "") ++
-                         showQPN qpn ++ componentStr ++ "==" ++ showI i
+                         showQPN qpn ++ componentStr ++ "==" ++ showI qpn i
        Constrained vr -> showDependencyReason dr ++ " => " ++ showQPN qpn ++
                          componentStr ++ showVR vr
 
diff --git a/cabal-install-solver/src/Distribution/Solver/Modular/Package.hs b/cabal-install-solver/src/Distribution/Solver/Modular/Package.hs
index 876ac2d790c..9cda3734465 100644
--- a/cabal-install-solver/src/Distribution/Solver/Modular/Package.hs
+++ b/cabal-install-solver/src/Distribution/Solver/Modular/Package.hs
@@ -22,12 +22,16 @@ module Distribution.Solver.Modular.Package
 import Prelude ()
 import Distribution.Solver.Compat.Prelude
 
+import qualified Data.List as L
+import qualified Data.Set as S
+
 import Distribution.Package -- from Cabal
 import Distribution.Pretty (prettyShow)
 
 import Distribution.Solver.Modular.Version
 import Distribution.Solver.Types.PackagePath
 
+
 -- | A package name.
 type PN = PackageName
 
@@ -49,7 +53,10 @@ type PId = UnitId
 -- package instance via its 'PId'.
 --
 -- TODO: More information is needed about the repo.
-data Loc = Inst PId | InRepo
+data Loc
+  = Inst PId
+  | InstGroup PId (Set PId)
+  | InRepo
   deriving (Eq, Ord, Show)
 
 -- | Instance. A version number and a location.
@@ -57,14 +64,29 @@ data I = I Ver Loc
   deriving (Eq, Ord, Show)
 
 -- | String representation of an instance.
-showI :: I -> String
-showI (I v InRepo)   = showVer v
-showI (I v (Inst uid)) = showVer v ++ "/installed" ++ extractPackageAbiHash uid
-  where
-    extractPackageAbiHash xs =
-      case first reverse $ break (=='-') $ reverse (prettyShow xs) of
-        (ys, []) -> ys
-        (ys, _)  -> '-' : ys
+showI :: QPN -> I -> String
+showI _qpn (I v InRepo)   = showVer v
+showI qpn (I v (Inst uid)) =
+  let
+    uidPrefix = showQPN qpn <> "-" <> showVer v <> "-"
+    renderUid u =
+      case L.stripPrefix uidPrefix (prettyShow u) of
+        Nothing -> showVer v
+        Just stripped -> stripped
+  in
+    showVer v <> "/installed-" <> renderUid uid
+showI qpn (I v (InstGroup uid subUids)) =
+  let
+    uidPrefix = showQPN qpn <> "-" <> showVer v <> "-"
+    renderUid u =
+      case L.stripPrefix uidPrefix (prettyShow u) of
+        Nothing -> prettyShow u
+        Just stripped -> stripped
+  in
+     showI qpn (I v (Inst uid))
+  <> " installed package group ["
+  <> unwords (map renderUid $ S.toList subUids)
+  <> "]"
 
 -- | Package instance. A package name and an instance.
 data PI qpn = PI qpn I
@@ -72,7 +94,7 @@ data PI qpn = PI qpn I
 
 -- | String representation of a package instance.
 showPI :: PI QPN -> String
-showPI (PI qpn i) = showQPN qpn ++ "-" ++ showI i
+showPI (PI qpn i) = showQPN qpn ++ "-" ++ showI qpn i
 
 instI :: I -> Bool
 instI (I _ (Inst _)) = True
diff --git a/cabal-install-solver/src/Distribution/Solver/Modular/Validate.hs b/cabal-install-solver/src/Distribution/Solver/Modular/Validate.hs
index ed01234bdba..88d494d4396 100644
--- a/cabal-install-solver/src/Distribution/Solver/Modular/Validate.hs
+++ b/cabal-install-solver/src/Distribution/Solver/Modular/Validate.hs
@@ -129,6 +129,7 @@ type PPreAssignment = Map QPN MergedPkgDep
 
 -- | A dependency on a component, including its DependencyReason.
 data PkgDep = PkgDep (DependencyReason QPN) (PkgComponent QPN) CI
+  deriving Show
 
 -- | Map from component name to one of the reasons that the component is
 -- required.
@@ -146,6 +147,7 @@ type ComponentDependencyReasons = Map ExposedComponent (DependencyReason QPN)
 data MergedPkgDep =
     MergedDepFixed ExposedComponent (DependencyReason QPN) I
   | MergedDepConstrained [VROrigin]
+  deriving Show
 
 -- | Version ranges paired with origins.
 type VROrigin = (VR, ExposedComponent, DependencyReason QPN)
diff --git a/cabal-install-solver/src/Distribution/Solver/Types/InstSolverPackage.hs b/cabal-install-solver/src/Distribution/Solver/Types/InstSolverPackage.hs
index 871a0dd15a9..719ac0ea2ef 100644
--- a/cabal-install-solver/src/Distribution/Solver/Types/InstSolverPackage.hs
+++ b/cabal-install-solver/src/Distribution/Solver/Types/InstSolverPackage.hs
@@ -6,12 +6,9 @@ module Distribution.Solver.Types.InstSolverPackage
 import Distribution.Solver.Compat.Prelude
 import Prelude ()
 
-import Distribution.Package ( Package(..), HasMungedPackageId(..), HasUnitId(..) )
+import Distribution.Package ( Package(..), HasUnitId(..) )
 import Distribution.Solver.Types.ComponentDeps ( ComponentDeps )
 import Distribution.Solver.Types.SolverId
-import Distribution.Types.MungedPackageId
-import Distribution.Types.PackageId
-import Distribution.Types.MungedPackageName
 import Distribution.InstalledPackageInfo (InstalledPackageInfo)
 
 -- | An 'InstSolverPackage' is a pre-existing installed package
@@ -27,13 +24,7 @@ instance Binary InstSolverPackage
 instance Structured InstSolverPackage
 
 instance Package InstSolverPackage where
-    packageId i =
-        -- HACK! See Note [Index conversion with internal libraries]
-        let MungedPackageId mpn v = mungedId i
-        in PackageIdentifier (encodeCompatPackageName mpn) v
-
-instance HasMungedPackageId InstSolverPackage where
-    mungedId = mungedId . instSolverPkgIPI
+    packageId = packageId . instSolverPkgIPI
 
 instance HasUnitId InstSolverPackage where
     installedUnitId = installedUnitId . instSolverPkgIPI
diff --git a/cabal-install/src/Distribution/Client/Config.hs b/cabal-install/src/Distribution/Client/Config.hs
index dd353e422a6..b050ef7a5a8 100644
--- a/cabal-install/src/Distribution/Client/Config.hs
+++ b/cabal-install/src/Distribution/Client/Config.hs
@@ -527,6 +527,7 @@ instance Semigroup SavedConfig where
           , configDeterministic = combine configDeterministic
           , configIPID = combine configIPID
           , configCID = combine configCID
+          , configIUID = combine configIUID
           , configUserInstall = combine configUserInstall
           , -- TODO: NubListify
             configPackageDBs = lastNonEmpty configPackageDBs
diff --git a/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs b/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
index 2168e9054c7..d29b5f24db2 100644
--- a/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
+++ b/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
@@ -1105,6 +1105,7 @@ convertToLegacyAllPackageConfig
           , configDeterministic = mempty
           , configIPID = mempty
           , configCID = mempty
+          , configIUID = mempty
           , configConfigurationsFlags = mempty
           , configTests = mempty
           , configCoverage = mempty -- TODO: don't merge
@@ -1182,6 +1183,7 @@ convertToLegacyPerPackageConfig PackageConfig{..} =
         , configExtraIncludeDirs = fmap makeSymbolicPath packageConfigExtraIncludeDirs
         , configIPID = mempty
         , configCID = mempty
+        , configIUID = mempty
         , configDeterministic = mempty
         , configConfigurationsFlags = packageConfigFlagAssignment
         , configTests = packageConfigTests
diff --git a/cabal-install/src/Distribution/Client/ProjectPlanning.hs b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
index 87b41a0be73..d2f7e86f694 100644
--- a/cabal-install/src/Distribution/Client/ProjectPlanning.hs
+++ b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
@@ -1673,18 +1673,19 @@ elaborateInstallPlan
 
       preexistingInstantiatedPkgs :: Map UnitId FullUnitId
       preexistingInstantiatedPkgs =
-        Map.fromList (mapMaybe f (SolverInstallPlan.toList solverPlan))
+        Map.fromList (concat $ mapMaybe f (SolverInstallPlan.toList solverPlan))
         where
           f (SolverInstallPlan.PreExisting inst)
             | let ipkg = instSolverPkgIPI inst
             , not (IPI.indefinite ipkg) =
-                Just
-                  ( IPI.installedUnitId ipkg
-                  , ( FullUnitId
-                        (IPI.installedComponentId ipkg)
-                        (Map.fromList (IPI.instantiatedWith ipkg))
-                    )
-                  )
+                let mkEntry x =
+                      ( IPI.installedUnitId x
+                      , ( FullUnitId
+                            (IPI.installedComponentId x)
+                            (Map.fromList (IPI.instantiatedWith x))
+                        )
+                      )
+                 in Just $ (mkEntry ipkg) : (map mkEntry (IPI.installedSublibs ipkg))
           f _ = Nothing
 
       elaboratedInstallPlan
@@ -1693,7 +1694,9 @@ elaborateInstallPlan
         flip InstallPlan.fromSolverInstallPlanWithProgress solverPlan $ \mapDep planpkg ->
           case planpkg of
             SolverInstallPlan.PreExisting pkg ->
-              return [InstallPlan.PreExisting (instSolverPkgIPI pkg)]
+              return $
+                [InstallPlan.PreExisting (instSolverPkgIPI pkg)]
+                  ++ map InstallPlan.PreExisting (IPI.installedSublibs (instSolverPkgIPI pkg))
             SolverInstallPlan.Configured pkg ->
               let inplace_doc
                     | shouldBuildInplaceOnly pkg = text "inplace"
@@ -1704,7 +1707,28 @@ elaborateInstallPlan
                         <+> text "package"
                         <+> quotes (pretty (packageId pkg))
                     )
-                    $ map InstallPlan.Configured <$> elaborateSolverToComponents mapDep pkg
+                    $ map InstallPlan.Configured . setCommonInstanceUnitId <$> elaborateSolverToComponents mapDep pkg
+
+      -- Set a common 'InstanceUnitId' for all components so they can be
+      -- identified as belonging to the same group.
+      --
+      -- If the package has a main library, its 'InstanceUnitId' is used.
+      -- Otherwise, an arbitrary component's 'InstanceUnitId' is used.
+      setCommonInstanceUnitId
+        :: [ElaboratedConfiguredPackage]
+        -> [ElaboratedConfiguredPackage]
+      setCommonInstanceUnitId [] = []
+      setCommonInstanceUnitId [single] = [single]
+      setCommonInstanceUnitId elabPkgs@(elabPkg : _rest) =
+        let
+          assignAll instanceUnitId = map (\x -> x{elabInstanceUnitId = instanceUnitId}) elabPkgs
+         in
+          case find (matchElabPkg (== (CLibName LMainLibName))) elabPkgs of
+            Nothing ->
+              -- no main library, use arbitrary (first one) for elabInstanceUnitID assignment
+              assignAll (elabInstanceUnitId elabPkg)
+            Just mainLib ->
+              assignAll (elabInstanceUnitId mainLib)
 
       -- NB: We don't INSTANTIATE packages at this point.  That's
       -- a post-pass.  This makes it simpler to compute dependencies.
@@ -1819,6 +1843,7 @@ elaborateInstallPlan
                   elab0
                     { elabModuleShape = emptyModuleShape
                     , elabUnitId = notImpl "elabUnitId"
+                    , elabInstanceUnitId = notImpl "elabInstanceUnitId"
                     , elabComponentId = notImpl "elabComponentId"
                     , elabLinkedInstantiatedWith = Map.empty
                     , elabInstallDirs = notImpl "elabInstallDirs"
@@ -1878,6 +1903,7 @@ elaborateInstallPlan
                   toConfiguredComponent
                     pd
                     (error "Distribution.Client.ProjectPlanning.cc_cid: filled in later")
+                    (error "Distribution.Client.ProjectPlanning.cc_instance_unit_id: filled in later")
                     (Map.unionWith Map.union external_lib_cc_map cc_map)
                     (Map.unionWith Map.union external_exe_cc_map cc_map)
                     comp
@@ -1927,7 +1953,11 @@ elaborateInstallPlan
                               elaboratedSharedConfig
                               elab1 -- knot tied
                           )
-                    cc = cc0{cc_ann_id = fmap (const cid) (cc_ann_id cc0)}
+                    cc =
+                      cc0
+                        { cc_ann_id = fmap (const cid) (cc_ann_id cc0)
+                        , cc_instance_unit_id = mkInstanceUnitId $ mkUnitId $ unComponentId cid
+                        }
                 infoProgress $ dispConfiguredComponent cc
 
                 -- 4. Perform mix-in linking
@@ -1955,6 +1985,7 @@ elaborateInstallPlan
                     elab1
                       { elabModuleShape = lc_shape lc
                       , elabUnitId = abstractUnitId (lc_uid lc)
+                      , elabInstanceUnitId = lc_instance_id lc
                       , elabComponentId = lc_cid lc
                       , elabLinkedInstantiatedWith = Map.fromList (lc_insts lc)
                       , elabPkgOrComp =
@@ -2123,6 +2154,7 @@ elaborateInstallPlan
               elab0
                 { elabUnitId = newSimpleUnitId pkgInstalledId
                 , elabComponentId = pkgInstalledId
+                , elabInstanceUnitId = mkInstanceUnitId $ newSimpleUnitId pkgInstalledId
                 , elabLinkedInstantiatedWith = Map.empty
                 , elabPkgOrComp = ElabPackage $ ElaboratedPackage{..}
                 , elabModuleShape = modShape
@@ -2227,6 +2259,7 @@ elaborateInstallPlan
 
             -- These get filled in later
             elabUnitId = error "elaborateSolverToCommon: elabUnitId"
+            elabInstanceUnitId = error "elaborateSolverToCommon: elabInstanceUnitId"
             elabComponentId = error "elaborateSolverToCommon: elabComponentId"
             elabInstantiatedWith = Map.empty
             elabLinkedInstantiatedWith = error "elaborateSolverToCommon: elabLinkedInstantiatedWith"
@@ -2676,12 +2709,7 @@ shouldBeLocal (SpecificSourcePackage pkg) = case srcpkgSource pkg of
 
 -- | Given a 'ElaboratedPlanPackage', report if it matches a 'ComponentName'.
 matchPlanPkg :: (ComponentName -> Bool) -> ElaboratedPlanPackage -> Bool
-matchPlanPkg p = InstallPlan.foldPlanPackage (p . ipiComponentName) (matchElabPkg p)
-
--- | Get the appropriate 'ComponentName' which identifies an installed
--- component.
-ipiComponentName :: IPI.InstalledPackageInfo -> ComponentName
-ipiComponentName = CLibName . IPI.sourceLibName
+matchPlanPkg p = InstallPlan.foldPlanPackage (p . IPI.sourceComponentName) (matchElabPkg p)
 
 -- | Given a 'ElaboratedConfiguredPackage', report if it matches a
 -- 'ComponentName'.
@@ -2713,7 +2741,7 @@ mkCCMapping =
     ( \ipkg ->
         ( packageName ipkg
         , Map.singleton
-            (ipiComponentName ipkg)
+            (IPI.sourceComponentName ipkg)
             -- TODO: libify
             ( AnnotatedId
                 { ann_id = IPI.installedComponentId ipkg
@@ -3175,7 +3203,7 @@ availableInstalledTargets ipkg =
       status = TargetBuildable (unitid, cname) TargetRequestedByDefault
       target = AvailableTarget (packageId ipkg) cname status False
       fake = False
-   in [(packageId ipkg, cname, fake, target)]
+   in (packageId ipkg, cname, fake, target) : (concatMap availableInstalledTargets (IPI.installedSublibs ipkg))
 
 availableSourceTargets
   :: ElaboratedConfiguredPackage
@@ -3637,10 +3665,11 @@ pruneInstallPlanPass1 pkgs
         ]
 
     availablePkgs =
-      Set.fromList
-        [ installedUnitId pkg
-        | InstallPlan.PreExisting pkg <- pkgs
-        ]
+      Set.fromList $
+        concat
+          [ (installedUnitId pkg) : (map installedUnitId (IPI.installedSublibs pkg))
+          | InstallPlan.PreExisting pkg <- pkgs
+          ]
 
 {-
 Note [Pruning for Multi Repl]
@@ -4122,6 +4151,7 @@ setupHsConfigureFlags
       configCID = case elabPkgOrComp of
         ElabPackage _ -> mempty
         ElabComponent _ -> toFlag elabComponentId
+      configIUID = toFlag elabInstanceUnitId
 
       configProgramPaths = Map.toList elabProgramPaths
       configProgramArgs = Map.toList elabProgramArgs
diff --git a/cabal-install/src/Distribution/Client/ProjectPlanning/Types.hs b/cabal-install/src/Distribution/Client/ProjectPlanning/Types.hs
index adbd8a85f5e..ed56682b388 100644
--- a/cabal-install/src/Distribution/Client/ProjectPlanning/Types.hs
+++ b/cabal-install/src/Distribution/Client/ProjectPlanning/Types.hs
@@ -203,6 +203,9 @@ instance Structured ElaboratedSharedConfig
 data ElaboratedConfiguredPackage = ElaboratedConfiguredPackage
   { elabUnitId :: UnitId
   -- ^ The 'UnitId' which uniquely identifies this item in a build plan
+  , elabInstanceUnitId :: InstanceUnitId
+  -- ^ The 'InstanceUnitId' which uniquely identifies package instance
+  -- (equal to elabUnitId for main component, sub components inherit it from main)
   , elabComponentId :: ComponentId
   , elabInstantiatedWith :: Map ModuleName Module
   , elabLinkedInstantiatedWith :: Map ModuleName OpenModule
diff --git a/cabal-install/src/Distribution/Client/Setup.hs b/cabal-install/src/Distribution/Client/Setup.hs
index ad043c68b7c..49476965d57 100644
--- a/cabal-install/src/Distribution/Client/Setup.hs
+++ b/cabal-install/src/Distribution/Client/Setup.hs
@@ -719,6 +719,7 @@ filterConfigureFlags' flags cabalLibVersion
       flags_latest
         { configBytecodeLib = NoFlag
         , configInstallDirs = (configInstallDirs flags){bytecodelibdir = NoFlag}
+        , configIUID = NoFlag
         }
 
     flags_3_13_0 =
diff --git a/cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs b/cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs
index 9ea020bd512..ef1d8da532b 100644
--- a/cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs
+++ b/cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs
@@ -932,7 +932,7 @@ tests =
       , runTest $
           let db =
                 [ Right $ exAv "my-package" 1 [ExFix "other-package" 3]
-                , Left $ exInst "other-package" 2 "other-package-AbCdEfGhIj0123456789" []
+                , Left $ exInst "other-package" 2 "other-package-2-AbCdEfGhIj0123456789" []
                 ]
               msg = "rejecting: other-package-2/installed-AbCdEfGhIj0123456789"
            in mkTest db "show full installed package ABI hash (issue #5892)" ["my-package"] $
diff --git a/cabal.project b/cabal.project
index a2075cfdc29..cc64225a0e4 100644
--- a/cabal.project
+++ b/cabal.project
@@ -12,3 +12,5 @@ package cabal-install
 
 package Cabal
   flags: +git-rev
+
+multi-repl: True
diff --git a/changelog.d/pr-11788.md b/changelog.d/pr-11788.md
new file mode 100644
index 00000000000..aef943b1184
--- /dev/null
+++ b/changelog.d/pr-11788.md
@@ -0,0 +1,11 @@
+---
+synopsis: Group installed multilib packages
+packages: [Cabal, Cabal-syntax, cabal-install, cabal-install-solver]
+prs: 11788
+---
+
+Introduce `InstanceUnitId` (`instance-id`) field for `InstalledPackageInfo` uniquely identifying
+group of components.
+
+Group components in solvers index conversion pass, allowing us to treat installed packages
+similar to source packages.