diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,1 +1,1 @@
-Please see https://github.com/haskell/cabal/blob/master/release-notes/cabal-install-3.16.1.0.md
+Please see https://github.com/haskell/cabal/blob/master/release-notes/cabal-install-3.18.1.0.md
diff --git a/LICENSE b/LICENSE
--- a/LICENSE
+++ b/LICENSE
@@ -1,4 +1,4 @@
-Copyright (c) 2003-2025, Cabal Development Team.
+Copyright (c) 2003-2026, Cabal Development Team.
 See the AUTHORS file for the full list of copyright holders.
 
 See */LICENSE for the copyright holders of the subcomponents.
diff --git a/cabal-install-solver.cabal b/cabal-install-solver.cabal
--- a/cabal-install-solver.cabal
+++ b/cabal-install-solver.cabal
@@ -1,6 +1,6 @@
 cabal-version: 3.6
 name:          cabal-install-solver
-version:       3.16.1.0
+version:       3.18.1.0
 synopsis:      The solver component of cabal-install
 description:
   The solver component used in the cabal-install command-line program.
@@ -11,7 +11,7 @@
 license-file:  LICENSE
 author:        Cabal Development Team (see AUTHORS file)
 maintainer:    Cabal Development Team <cabal-devel@haskell.org>
-copyright:     2003-2025, Cabal Development Team
+copyright:     2003-2026, Cabal Development Team
 category:      Distribution
 build-type:    Simple
 extra-doc-files:
@@ -95,18 +95,19 @@
     Distribution.Solver.Types.SolverId
     Distribution.Solver.Types.SolverPackage
     Distribution.Solver.Types.SourcePackage
+    Distribution.Solver.Types.SummarizedMessage
     Distribution.Solver.Types.Variable
 
   build-depends:
     , array         >=0.4      && <0.6
-    , base          >=4.13     && <4.22
+    , base          >=4.17     && <4.24
     , bytestring    >=0.10.6.0 && <0.13
-    , Cabal         ^>=3.16.1.0
-    , Cabal-syntax  ^>=3.16.1.0
+    , Cabal         ^>=3.18
+    , Cabal-syntax  ^>=3.18
     , containers    >=0.5.6.2  && <0.9
     , edit-distance ^>= 0.2.2
     , directory     >= 1.3.7.0  && < 1.4
-    , filepath      ^>=1.4.0.0 || ^>=1.5.0.0
+    , filepath      >= 1.3.0.1  && < 1.6
     , mtl           >=2.0      && <2.4
     , network-uri   >= 2.6.0.2 && < 2.7
     , pretty        ^>=1.1
@@ -132,7 +133,7 @@
      UnitTests.Distribution.Solver.Modular.MessageUtils
 
    build-depends:
-     , base        >= 4.13  && <4.22
+     , base        >= 4.17  && <4.24
      , Cabal-syntax
      , cabal-install-solver
      , tasty       >= 1.2.3 && <1.6
diff --git a/src/Distribution/Solver/Modular.hs b/src/Distribution/Solver/Modular.hs
--- a/src/Distribution/Solver/Modular.hs
+++ b/src/Distribution/Solver/Modular.hs
@@ -27,8 +27,14 @@
          ( convCP )
 import qualified Distribution.Solver.Modular.ConflictSet as CS
 import Distribution.Solver.Modular.Dependency
-import Distribution.Solver.Modular.Flag
-import Distribution.Solver.Modular.Index
+    ( Var(..),
+      showVar,
+      ConflictMap,
+      ConflictSet,
+      showConflictSet,
+      RevDepMap )
+import Distribution.Solver.Modular.Flag ( SN(SN), FN(FN) )
+import Distribution.Solver.Modular.Index ( Index )
 import Distribution.Solver.Modular.IndexConversion
          ( convPIs )
 import Distribution.Solver.Modular.Log
@@ -36,25 +42,38 @@
 import Distribution.Solver.Modular.Package
          ( PN )
 import Distribution.Solver.Modular.RetryLog
+    ( RetryLog,
+      toProgress,
+      fromProgress,
+      retry,
+      failWith,
+      continueWith )
 import Distribution.Solver.Modular.Solver
          ( SolverConfig(..), PruneAfterFirstSuccess(..), solve )
 import Distribution.Solver.Types.DependencyResolver
+    ( DependencyResolver )
 import Distribution.Solver.Types.LabeledPackageConstraint
+    ( LabeledPackageConstraint, unlabelPackageConstraint )
 import Distribution.Solver.Types.PackageConstraint
-import Distribution.Solver.Types.PackagePath
+    ( PackageConstraint(..), scopeToPackageName )
+import Distribution.Solver.Types.PackagePath ( QPN )
 import Distribution.Solver.Types.PackagePreferences
+    ( PackagePreferences )
 import Distribution.Solver.Types.PkgConfigDb
          ( PkgConfigDb )
 import Distribution.Solver.Types.Progress
-import Distribution.Solver.Types.Variable
+    ( Progress(..), foldProgress )
+import Distribution.Solver.Types.SummarizedMessage
+    ( SummarizedMessage(StringMsg) )
+import Distribution.Solver.Types.Variable ( Variable(..) )
 import Distribution.System
          ( Platform(..) )
 import Distribution.Simple.Setup
          ( BooleanFlag(..) )
 import Distribution.Simple.Utils
-         ( ordNubBy )
+    ( ordNubBy )
 import Distribution.Verbosity
-
+import Distribution.Solver.Modular.Message ( renderSummarizedMessage )
 
 -- | Ties the two worlds together: classic cabal-install vs. the modular
 -- solver. Performs the necessary translations before and after.
@@ -120,21 +139,21 @@
        -> (PN -> PackagePreferences)
        -> Map PN [LabeledPackageConstraint]
        -> Set PN
-       -> Progress String String (Assignment, RevDepMap)
+       -> Progress SummarizedMessage String (Assignment, RevDepMap)
 solve' sc cinfo idx pkgConfigDB pprefs gcs pns =
     toProgress $ retry (runSolver printFullLog sc) createErrorMsg
   where
     runSolver :: Bool -> SolverConfig
-              -> RetryLog String SolverFailure (Assignment, RevDepMap)
+              -> RetryLog SummarizedMessage SolverFailure (Assignment, RevDepMap)
     runSolver keepLog sc' =
         displayLogMessages keepLog $
         solve sc' cinfo idx pkgConfigDB pprefs gcs pns
 
     createErrorMsg :: SolverFailure
-                   -> RetryLog String String (Assignment, RevDepMap)
+                   -> RetryLog SummarizedMessage String (Assignment, RevDepMap)
     createErrorMsg failure@(ExhaustiveSearch cs cm) =
       if asBool $ minimizeConflictSet sc
-      then continueWith ("Found no solution after exhaustively searching the "
+      then continueWith (mkStringMsg $ "Found no solution after exhaustively searching the "
                           ++ "dependency tree. Rerunning the dependency solver "
                           ++ "to minimize the conflict set ({"
                           ++ showConflictSet cs ++ "}).") $
@@ -155,7 +174,7 @@
            rerunSolverForErrorMsg cs ++ finalErrorMsg sc failure
     createErrorMsg failure@BackjumpLimitReached     =
         continueWith
-             ("Backjump limit reached. Rerunning dependency solver to generate "
+             (mkStringMsg $ "Backjump limit reached. Rerunning dependency solver to generate "
               ++ "a final conflict set for the search tree containing the "
               ++ "first backjump.") $
         retry (runSolver printFullLog sc { pruneAfterFirstSuccess = PruneAfterFirstSuccess True }) $
@@ -181,13 +200,16 @@
           -- original goal order.
           goalOrder' = preferGoalsFromConflictSet cs <> fromMaybe mempty (goalOrder sc)
 
-      in unlines ("Could not resolve dependencies:" : messages (toProgress (runSolver True sc')))
+      in unlines ("Could not resolve dependencies:" : map renderSummarizedMessage (messages (toProgress (runSolver True sc'))))
 
-    printFullLog = solverVerbosity sc >= verbose
+    printFullLog = solverVerbosity sc >= Verbose
 
     messages :: Progress step fail done -> [step]
     messages = foldProgress (:) (const []) (const [])
 
+mkStringMsg :: String -> SummarizedMessage
+mkStringMsg msg = StringMsg msg
+
 -- | Try to remove variables from the given conflict set to create a minimal
 -- conflict set.
 --
@@ -219,11 +241,11 @@
 --    solver to add new unnecessary variables to the conflict set. This function
 --    discards the result from any run that adds new variables to the conflict
 --    set, but the end result may not be completely minimized.
-tryToMinimizeConflictSet :: forall a . (SolverConfig -> RetryLog String SolverFailure a)
+tryToMinimizeConflictSet :: forall a . (SolverConfig -> RetryLog SummarizedMessage SolverFailure a)
                          -> SolverConfig
                          -> ConflictSet
                          -> ConflictMap
-                         -> RetryLog String SolverFailure a
+                         -> RetryLog SummarizedMessage SolverFailure a
 tryToMinimizeConflictSet runSolver sc cs cm =
     foldl (\r v -> retryNoSolution r $ tryToRemoveOneVar v)
           (fromProgress $ Fail $ ExhaustiveSearch cs cm)
@@ -249,14 +271,14 @@
     tryToRemoveOneVar :: Var QPN
                       -> ConflictSet
                       -> ConflictMap
-                      -> RetryLog String SolverFailure a
+                      -> RetryLog SummarizedMessage SolverFailure a
     tryToRemoveOneVar v smallestKnownCS smallestKnownCM
         -- Check whether v is still present, because it may have already been
         -- removed in a previous solver rerun.
       | not (v `CS.member` smallestKnownCS) =
           fromProgress $ Fail $ ExhaustiveSearch smallestKnownCS smallestKnownCM
       | otherwise =
-        continueWith ("Trying to remove variable " ++ varStr ++ " from the "
+        continueWith (mkStringMsg $ "Trying to remove variable " ++ varStr ++ " from the "
                       ++ "conflict set.") $
         retry (runSolver sc') $ \case
             err@(ExhaustiveSearch cs' _)
@@ -268,14 +290,14 @@
                                   ++ "conflict set."
                   in -- Use the new conflict set, even if v wasn't removed,
                      -- because other variables may have been removed.
-                     failWith (msg ++ " Continuing with " ++ showCS cs' ++ ".") err
+                     failWith (mkStringMsg $ msg ++ " Continuing with " ++ showCS cs' ++ ".") err
               | otherwise ->
-                  failWith ("Failed to find a smaller conflict set. The new "
+                  failWith (mkStringMsg $ "Failed to find a smaller conflict set. The new "
                              ++ "conflict set is not a subset of the previous "
                              ++ "conflict set: " ++ showCS cs') $
                   ExhaustiveSearch smallestKnownCS smallestKnownCM
             BackjumpLimitReached ->
-                failWith "Reached backjump limit while minimizing conflict set."
+                failWith (mkStringMsg "Reached backjump limit while minimizing conflict set.")
                          BackjumpLimitReached
       where
         varStr = "\"" ++ showVar v ++ "\""
@@ -321,7 +343,7 @@
           ++ showCS cm cs
           ++ flagSuggestion
         where
-          showCS = if solverVerbosity sc > normal
+          showCS = if solverVerbosity sc > Normal
                    then CS.showCSWithFrequency
                    else CS.showCSSortedByFrequency
           flagSuggestion =
diff --git a/src/Distribution/Solver/Modular/Assignment.hs b/src/Distribution/Solver/Modular/Assignment.hs
--- a/src/Distribution/Solver/Modular/Assignment.hs
+++ b/src/Distribution/Solver/Modular/Assignment.hs
@@ -27,6 +27,7 @@
 import Distribution.Solver.Modular.Flag
 import Distribution.Solver.Modular.LabeledGraph
 import Distribution.Solver.Modular.Package
+import Distribution.Simple.Utils (ordNub)
 
 -- | A (partial) package assignment. Qualified package names
 -- are associated with instances.
@@ -53,7 +54,7 @@
     vm  :: Vertex -> ((), QPN, [(Component, QPN)])
     cvm :: QPN -> Maybe Vertex
     -- Note that the RevDepMap contains duplicate dependencies. Therefore the nub.
-    (g, vm, cvm) = graphFromEdges (L.map (\ (x, xs) -> ((), x, nub xs))
+    (g, vm, cvm) = graphFromEdges (L.map (\ (x, xs) -> ((), x, ordNub xs))
                                   (M.toList rdm))
     tg :: Graph Component
     tg = transposeG g
@@ -68,13 +69,12 @@
     -- complete flag assignment by package.
     fapp :: Map QPN FlagAssignment
     fapp = M.fromListWith mappend $
-           L.map (\ ((FN qpn fn), b) -> (qpn, mkFlagAssignment [(fn, b)])) $
-           M.toList $
-           fa
+           L.map (\ (FN qpn fn, b) -> (qpn, mkFlagAssignment [(fn, b)])) $
+           M.toList fa
     -- Stanzas per package.
     sapp :: Map QPN OptionalStanzaSet
     sapp = M.fromListWith mappend
-         $ L.map (\ ((SN qpn sn), b) -> (qpn, if b then optStanzaSetSingleton sn else mempty))
+         $ L.map (\ (SN qpn sn, b) -> (qpn, if b then optStanzaSetSingleton sn else mempty))
          $ M.toList sa
     -- Dependencies per package.
     depp :: QPN -> [(Component, PI QPN)]
diff --git a/src/Distribution/Solver/Modular/Builder.hs b/src/Distribution/Solver/Modular/Builder.hs
--- a/src/Distribution/Solver/Modular/Builder.hs
+++ b/src/Distribution/Solver/Modular/Builder.hs
@@ -1,4 +1,6 @@
 {-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TupleSections #-}
+
 module Distribution.Solver.Modular.Builder (
     buildTree
   , splits -- for testing
@@ -215,7 +217,7 @@
 addLinking :: LinkingState -> TreeF () c a -> TreeF () c (Linker a)
 -- The only nodes of interest are package nodes
 addLinking ls (PChoiceF qpn@(Q pp pn) rdm gr cs) =
-  let linkedCs = fmap (\bs -> Linker bs ls) $
+  let linkedCs = fmap (`Linker` ls) $
                  W.fromList $ concatMap (linkChoices ls qpn) (W.toList cs)
       unlinkedCs = W.mapWithKey goP cs
       allCs = unlinkedCs `W.union` linkedCs
@@ -226,7 +228,7 @@
       goP (POption i Nothing) bs = Linker bs $ M.insertWith (++) (pn, i) [pp] ls
       goP _                   _  = alreadyLinked
   in PChoiceF qpn rdm gr allCs
-addLinking ls t = fmap (\bs -> Linker bs ls) t
+addLinking ls t = fmap (`Linker` ls) t
 
 linkChoices :: forall a w . LinkingState
             -> QPN
@@ -252,7 +254,7 @@
     build Linker {
         buildState = BS {
             index = idx
-          , rdeps = M.fromList (L.map (\ qpn -> (qpn, []))              qpns)
+          , rdeps = M.fromList (L.map (, []) qpns)
           , open  = L.map topLevelGoal qpns
           , next  = Goals
           , qualifyOptions = defaultQualifyOptions idx
diff --git a/src/Distribution/Solver/Modular/Cycles.hs b/src/Distribution/Solver/Modular/Cycles.hs
--- a/src/Distribution/Solver/Modular/Cycles.hs
+++ b/src/Distribution/Solver/Modular/Cycles.hs
@@ -23,11 +23,11 @@
     -- Only check children of choice nodes.
     go :: Tree d c -> Tree d c
     go (PChoice qpn rdm gr                         cs) =
-        PChoice qpn rdm gr     $ fmap (checkChild qpn)   (fmap go cs)
+        PChoice qpn rdm gr     $ fmap (checkChild qpn . go) cs
     go (FChoice qfn@(FN qpn _) rdm gr w m d cs) =
-        FChoice qfn rdm gr w m d $ fmap (checkChild qpn) (fmap go cs)
+        FChoice qfn rdm gr w m d $ fmap (checkChild qpn . go) cs
     go (SChoice qsn@(SN qpn _) rdm gr w     cs) =
-        SChoice qsn rdm gr w   $ fmap (checkChild qpn)   (fmap go cs)
+        SChoice qsn rdm gr w   $ fmap (checkChild qpn . go) cs
     go (GoalChoice rdm cs) = GoalChoice rdm (fmap go cs)
     go x@(Fail _ _) = x
     go x@(Done _ _) = x
diff --git a/src/Distribution/Solver/Modular/Dependency.hs b/src/Distribution/Solver/Modular/Dependency.hs
--- a/src/Distribution/Solver/Modular/Dependency.hs
+++ b/src/Distribution/Solver/Modular/Dependency.hs
@@ -147,10 +147,10 @@
 -- | Print the reason that a dependency was introduced.
 showDependencyReason :: DependencyReason QPN -> String
 showDependencyReason (DependencyReason qpn flags stanzas) =
-    intercalate " " $
+    unwords $
         showQPN qpn
       : map (uncurry showFlagValue) (M.toList flags)
-     ++ map (\s -> showSBool s True) (S.toList stanzas)
+     ++ map (`showSBool` True) (S.toList stanzas)
 
 -- | Options for goal qualification (used in 'qualifyDeps')
 --
diff --git a/src/Distribution/Solver/Modular/Explore.hs b/src/Distribution/Solver/Modular/Explore.hs
--- a/src/Distribution/Solver/Modular/Explore.hs
+++ b/src/Distribution/Solver/Modular/Explore.hs
@@ -194,7 +194,7 @@
         where f k             r = r (A pa (M.insert qfn k fa) sa)
     go (SChoice qsn rdm y t     ts) (A pa fa sa) = SChoice qsn rdm y t     $ W.mapWithKey f (fmap go ts)
         where f k             r = r (A pa fa (M.insert qsn k sa))
-    go (GoalChoice  rdm         ts) a            = GoalChoice  rdm         $ fmap ($ a) (fmap go ts)
+    go (GoalChoice  rdm         ts) a            = GoalChoice  rdm         $ fmap (`go` a) ts
 
 -- | A tree traversal that simultaneously propagates conflict sets up
 -- the tree from the leaves and creates a log.
diff --git a/src/Distribution/Solver/Modular/IndexConversion.hs b/src/Distribution/Solver/Modular/IndexConversion.hs
--- a/src/Distribution/Solver/Modular/IndexConversion.hs
+++ b/src/Distribution/Solver/Modular/IndexConversion.hs
@@ -42,6 +42,9 @@
 import Distribution.Solver.Modular.Tree
 import Distribution.Solver.Modular.Version
 
+import qualified Distribution.Compat.Lens as L
+import qualified Distribution.Types.BuildInfo.Lens as L
+
 -- | Convert both the installed package index and the source package
 -- index into one uniform solver index.
 --
@@ -182,7 +185,7 @@
 
 
     conv :: Monoid a => Component -> (a -> BuildInfo) -> DependencyReason PN ->
-            CondTree ConfVar [Dependency] a -> FlaggedDeps PN
+            CondTree ConfVar a -> FlaggedDeps PN
     conv comp getInfo dr =
         convCondTree M.empty dr pkg os arch cinfo pn fds comp getInfo solveExes .
         addBuildableCondition getInfo
@@ -251,7 +254,7 @@
                           -> CompilerInfo
                           -> [LabeledPackageConstraint]
                           -> (a -> Bool)
-                          -> CondTree ConfVar [Dependency] a
+                          -> CondTree ConfVar a
                           -> Maybe Bool
 testConditionForComponent os arch cinfo constraints p tree =
     case go $ extractCondition p tree of
@@ -329,8 +332,8 @@
                 Component ->
                 (a -> BuildInfo) ->
                 SolveExecutables ->
-                CondTree ConfVar [Dependency] a -> FlaggedDeps PN
-convCondTree flags dr pkg os arch cinfo pn fds comp getInfo solveExes@(SolveExecutables solveExes') (CondNode info ds branches) =
+                CondTree ConfVar a -> FlaggedDeps PN
+convCondTree flags dr pkg os arch cinfo pn fds comp getInfo solveExes@(SolveExecutables solveExes') (CondNode info branches) =
              -- Merge all library and build-tool dependencies at every level in
              -- the tree of flagged dependencies. Otherwise 'extractCommon'
              -- could create duplicate dependencies, and the number of
@@ -338,7 +341,7 @@
              -- of the tree.
              mergeSimpleDeps $
                  [ D.Simple singleDep comp
-                 | dep <- ds
+                 | dep <- L.view L.targetBuildDepends (getInfo info)
                  , singleDep <- convLibDeps dr dep ]  -- unconditional package dependencies
 
               ++ L.map (\e -> D.Simple (LDep dr (Ext  e)) comp) (allExtensions bi) -- unconditional extension dependencies
@@ -461,7 +464,7 @@
            -> Component
            -> (a -> BuildInfo)
            -> SolveExecutables
-           -> CondBranch ConfVar [Dependency] a
+           -> CondBranch ConfVar a
            -> FlaggedDeps PN
 convBranch flags dr pkg os arch cinfo pn fds comp getInfo solveExes (CondBranch c' t' mf') =
     go c'
diff --git a/src/Distribution/Solver/Modular/LabeledGraph.hs b/src/Distribution/Solver/Modular/LabeledGraph.hs
--- a/src/Distribution/Solver/Modular/LabeledGraph.hs
+++ b/src/Distribution/Solver/Modular/LabeledGraph.hs
@@ -52,7 +52,7 @@
     sorted_edges = sortBy lt edges0
     edges1       = zip [0..] sorted_edges
 
-    graph        = array bounds0 [(v, (mapMaybe mk_edge ks))
+    graph        = array bounds0 [(v, mapMaybe mk_edge ks)
                                  | (v, (_, _, ks)) <- edges1]
     key_map      = array bounds0 [(v, k                    )
                                  | (v, (_, k, _ )) <- edges1]
diff --git a/src/Distribution/Solver/Modular/Linking.hs b/src/Distribution/Solver/Modular/Linking.hs
--- a/src/Distribution/Solver/Modular/Linking.hs
+++ b/src/Distribution/Solver/Modular/Linking.hs
@@ -145,7 +145,7 @@
   deriving (Functor, Applicative, Monad)
 
 instance MonadState ValidateState UpdateState where
-  get    = UpdateState $ get
+  get    = UpdateState get
   put st = UpdateState $ do
              expensiveAssert (lgInvariant $ vsLinks st) $ return ()
              put st
diff --git a/src/Distribution/Solver/Modular/Log.hs b/src/Distribution/Solver/Modular/Log.hs
--- a/src/Distribution/Solver/Modular/Log.hs
+++ b/src/Distribution/Solver/Modular/Log.hs
@@ -7,11 +7,14 @@
 import Distribution.Solver.Compat.Prelude
 
 import Distribution.Solver.Types.Progress
-
-import Distribution.Solver.Modular.Dependency
-import Distribution.Solver.Modular.Message
+    ( Progress(Done, Fail), foldProgress )
+import Distribution.Solver.Modular.ConflictSet
+    ( ConflictMap, ConflictSet )
 import Distribution.Solver.Modular.RetryLog
-
+    ( RetryLog, toProgress, fromProgress )
+import Distribution.Solver.Modular.Message (Message, summarizeMessages)
+import Distribution.Solver.Types.SummarizedMessage
+    ( SummarizedMessage(..) )
 -- | Information about a dependency solver failure.
 data SolverFailure =
     ExhaustiveSearch ConflictSet ConflictMap
@@ -22,10 +25,10 @@
 -- 'keepLog'), for efficiency.
 displayLogMessages :: Bool
                    -> RetryLog Message SolverFailure a
-                   -> RetryLog String SolverFailure a
+                   -> RetryLog SummarizedMessage SolverFailure a
 displayLogMessages keepLog lg = fromProgress $
     if keepLog
-    then showMessages progress
+    then summarizeMessages progress
     else foldProgress (const id) Fail Done progress
   where
     progress = toProgress lg
diff --git a/src/Distribution/Solver/Modular/Message.hs b/src/Distribution/Solver/Modular/Message.hs
--- a/src/Distribution/Solver/Modular/Message.hs
+++ b/src/Distribution/Solver/Modular/Message.hs
@@ -1,9 +1,9 @@
 {-# LANGUAGE BangPatterns #-}
-{-# LANGUAGE LambdaCase #-}
 
 module Distribution.Solver.Modular.Message (
     Message(..),
-    showMessages
+    summarizeMessages,
+    renderSummarizedMessage,
   ) where
 
 import qualified Data.List as L
@@ -14,26 +14,50 @@
 import Data.Maybe (catMaybes, mapMaybe, isJust)
 import Prelude hiding (pi)
 
-import Distribution.Pretty (prettyShow) -- from Cabal
+import Distribution.Pretty ( prettyShow ) -- from Cabal
 
 import qualified Distribution.Solver.Modular.ConflictSet as CS
 import Distribution.Solver.Modular.Dependency
-import Distribution.Solver.Modular.Flag ( QFN, QSN )
-import qualified Distribution.Solver.Modular.Flag as Flag ( showQFN, showQFNBool, showQSN, showQSNBool )
+    ( Var(P),
+      ConflictSet,
+      showConflictSet,
+      QGoalReason,
+      GoalReason(DependencyGoal, UserGoal),
+      Goal(Goal),
+      DependencyReason(DependencyReason),
+      ExposedComponent(..),
+      PkgComponent(PkgComponent),
+      CI(Constrained, Fixed),
+      showDependencyReason )
+import Distribution.Solver.Modular.Flag
+    ( QSN, QFN, showQFNBool, showQSNBool, showQFN, showQSN )
 import Distribution.Solver.Modular.MessageUtils
-         (showUnsupportedExtension, showUnsupportedLanguage)
+    ( showUnsupportedExtension, showUnsupportedLanguage )
 import Distribution.Solver.Modular.Package
+    ( PI(PI), showI, showPI )
 import Distribution.Solver.Modular.Tree
-         ( FailReason(..), POption(..), ConflictingDep(..) )
+    ( FailReason(..), POption(..), ConflictingDep(..) )
 import Distribution.Solver.Modular.Version
+    ( VR, Ver, showVer, showVR, (.||.) )
+
 import Distribution.Solver.Types.ConstraintSource
+    ( ConstraintSource (..), showConstraintSource )
 import Distribution.Solver.Types.PackagePath
+    ( QPN, Qualified(Q), showQPN )
 import Distribution.Solver.Types.Progress
-import Distribution.Solver.Types.ProjectConfigPath (docProjectConfigPathFailReason)
+    ( Progress(..) )
+import Distribution.Solver.Types.ProjectConfigPath
+    ( docProjectConfigPathFailReason)
+import Distribution.Solver.Types.SummarizedMessage
+    ( Entry(..), EntryAtLevel(..), SummarizedMessage(..) )
 import Distribution.Types.LibraryName
+    ( LibraryName(LSubLibName, LMainLibName) )
 import Distribution.Types.UnqualComponentName
-import Text.PrettyPrint (nest, render)
+    ( unUnqualComponentName )
 
+import Text.PrettyPrint ( nest, render )
+
+-- A data type to hold log information from the modular solver.
 data Message =
     Enter           -- ^ increase indentation level
   | Leave           -- ^ decrease indentation level
@@ -45,51 +69,81 @@
   | Success
   | Failure ConflictSet FailReason
 
--- | Transforms the structured message type to actual messages (strings).
+renderSummarizedMessage :: SummarizedMessage -> String
+renderSummarizedMessage (SummarizedMsg i) = displayMessageAtLevel i
+renderSummarizedMessage (StringMsg s) = s
+
+displayMessageAtLevel :: EntryAtLevel -> String
+displayMessageAtLevel (AtLevel l msg) =
+  let s = show l
+  in  "[" ++ replicate (3 - length s) '_' ++ s ++ "] " ++ displayMessage msg
+
+displayMessage :: Entry -> String
+displayMessage (EntryPackageGoal qpn gr) = "next goal: " ++ showQPN qpn ++ showGR gr
+displayMessage (EntryRejectF qfn b c fr) = "rejecting: " ++ showQFNBool qfn b ++ showFR c fr
+displayMessage (EntryRejectS qsn b c fr) = "rejecting: " ++ showQSNBool qsn b ++ showFR c fr
+displayMessage (EntrySkipping cs) = "skipping: " ++ showConflicts cs
+displayMessage (EntryTryingF qfn b) = "trying: " ++ showQFNBool qfn b
+displayMessage (EntryTryingP qpn i) = "trying: " ++ showOption qpn i
+displayMessage (EntryTryingNewP qpn i gr) = "trying: " ++ showOption qpn i ++ showGR gr
+displayMessage (EntryTryingS qsn b) = "trying: " ++ showQSNBool qsn b
+displayMessage (EntryUnknownPackage qpn gr) = "unknown package: " ++ showQPN qpn ++ showGR gr
+displayMessage EntrySuccess = "done"
+displayMessage (EntryFailure c fr) = "fail" ++ showFR c fr
+displayMessage (EntrySkipMany qsn b cs) = "skipping: " ++ showOptions qsn b ++ " " ++ showConflicts cs
+-- Instead of displaying `aeson-1.0.2.1, aeson-1.0.2.0, aeson-1.0.1.0, ...`,
+-- the following line aims to display `aeson: 1.0.2.1, 1.0.2.0, 1.0.1.0, ...`.
 --
+displayMessage (EntryRejectMany qpn is c fr) = "rejecting: " ++ showOptions qpn is ++ showFR c fr
+
+-- | Transforms the structured message type to actual messages (SummarizedMessage s).
+--
 -- The log contains level numbers, which are useful for any trace that involves
 -- backtracking, because only the level numbers will allow to keep track of
 -- backjumps.
-showMessages :: Progress Message a b -> Progress String a b
-showMessages = go 0
+summarizeMessages :: Progress Message a b -> Progress SummarizedMessage a b
+summarizeMessages = go 0
   where
     -- 'go' increments the level for a recursive call when it encounters
     -- 'TryP', 'TryF', or 'TryS' and decrements the level when it encounters 'Leave'.
-    go :: Int -> Progress Message a b -> Progress String a b
+    go :: Int -> Progress Message a b -> Progress SummarizedMessage a b
     go !_ (Done x)                           = Done x
     go !_ (Fail x)                           = Fail x
+
     -- complex patterns
     go !l (Step (TryP qpn i) (Step Enter (Step (Failure c fr) (Step Leave ms)))) =
         goPReject l qpn [i] c fr ms
+
     go !l (Step (TryP qpn i) (Step Enter (Step (Skip conflicts) (Step Leave ms)))) =
         goPSkip l qpn [i] conflicts ms
+
     go !l (Step (TryF qfn b) (Step Enter (Step (Failure c fr) (Step Leave ms)))) =
-        (atLevel l $ blurbQFNBool Rejecting qfn b ++ showFR c fr) (go l ms)
+        Step (SummarizedMsg $ AtLevel l (EntryRejectF qfn b c fr)) (go l ms)
+
     go !l (Step (TryS qsn b) (Step Enter (Step (Failure c fr) (Step Leave ms)))) =
-        (atLevel l $ blurbQSNBool Rejecting qsn b ++ showFR c fr) (go l ms)
+        Step (SummarizedMsg $ AtLevel l (EntryRejectS qsn b c fr)) (go l ms)
+
+    -- "Trying ..." message when a new goal is started
     go !l (Step (Next (Goal (P _  ) gr)) (Step (TryP qpn' i) ms@(Step Enter (Step (Next _) _)))) =
-        (atLevel l $ blurbOption Trying qpn' i ++ showGR gr) (go l ms)
+        Step (SummarizedMsg $ AtLevel l (EntryTryingNewP qpn' i gr)) (go l ms)
+
     go !l (Step (Next (Goal (P qpn) gr)) (Step (Failure _c UnknownPackage) ms)) =
-        atLevel l ("unknown package: " ++ showQPN qpn ++ showGR gr) $ go l ms
+        Step (SummarizedMsg $ AtLevel l (EntryUnknownPackage qpn gr)) (go l ms)
+
     -- standard display
     go !l (Step Enter                    ms) = go (l+1) ms
     go !l (Step Leave                    ms) = go (l-1) ms
-    go !l (Step (TryP qpn i)             ms) = (atLevel l $ blurbOption Trying qpn i) (go l ms)
-    go !l (Step (TryF qfn b)             ms) = (atLevel l $ blurbQFNBool Trying qfn b) (go l ms)
-    go !l (Step (TryS qsn b)             ms) = (atLevel l $ blurbQSNBool Trying qsn b) (go l ms)
-    go !l (Step (Next (Goal (P qpn) gr)) ms) = (atLevel l $ showPackageGoal qpn gr) (go l ms)
-    go !l (Step (Next _)                 ms) = go l     ms -- ignore flag goals in the log
-    go !l (Step (Skip conflicts)         ms) =
-        -- 'Skip' should always be handled by 'goPSkip' in the case above.
-        (atLevel l $ blurb Skipping ++ showConflicts conflicts) (go l ms)
-    go !l (Step (Success)                ms) = (atLevel l $ "done") (go l ms)
-    go !l (Step (Failure c fr)           ms) = (atLevel l $ showFailure c fr) (go l ms)
 
-    showPackageGoal :: QPN -> QGoalReason -> String
-    showPackageGoal qpn gr = "next goal: " ++ showQPN qpn ++ showGR gr
+    go !l (Step (TryP qpn i)             ms) = Step (SummarizedMsg $ AtLevel l (EntryTryingP qpn i)) (go l ms)
+    go !l (Step (TryF qfn b)             ms) = Step (SummarizedMsg $ AtLevel l (EntryTryingF qfn b)) (go l ms)
+    go !l (Step (TryS qsn b)             ms) = Step (SummarizedMsg $ AtLevel l (EntryTryingS qsn b)) (go l ms)
+    go !l (Step (Next (Goal (P qpn) gr)) ms) = Step (SummarizedMsg $ AtLevel l (EntryPackageGoal qpn gr)) (go l ms)
+    go !l (Step (Next _)                 ms) = go l ms -- ignore flag goals in the log
 
-    showFailure :: ConflictSet -> FailReason -> String
-    showFailure c fr = "fail" ++ showFR c fr
+    -- 'Skip' should always be handled by 'goPSkip' in the case above.
+    go !l (Step (Skip conflicts)         ms) = Step (SummarizedMsg $ AtLevel l (EntrySkipping conflicts)) (go l ms)
+    go !l (Step Success                  ms) = Step (SummarizedMsg $ AtLevel l EntrySuccess) (go l ms)
+    go !l (Step (Failure c fr)           ms) = Step (SummarizedMsg $ AtLevel l (EntryFailure c fr)) (go l ms)
 
     -- special handler for many subsequent package rejections
     goPReject :: Int
@@ -98,14 +152,13 @@
               -> ConflictSet
               -> FailReason
               -> Progress Message a b
-              -> Progress String a b
+              -> Progress SummarizedMessage a b
     goPReject l qpn is c fr (Step (TryP qpn' i) (Step Enter (Step (Failure _ fr') (Step Leave ms))))
       | qpn == qpn' && fr == fr' =
         -- By prepending (i : is) we reverse the order of the instances.
         goPReject l qpn (i : is) c fr ms
     goPReject l qpn is c fr ms =
-        (atLevel l $ blurbOptions Rejecting qpn (reverse is) ++ showFR c fr)
-        (go l ms)
+        Step (SummarizedMsg $ AtLevel l (EntryRejectMany qpn (reverse is) c fr)) (go l ms)
 
     -- Handle many subsequent skipped package instances.
     goPSkip :: Int
@@ -113,25 +166,18 @@
             -> [POption]
             -> Set CS.Conflict
             -> Progress Message a b
-            -> Progress String a b
+            -> Progress SummarizedMessage a b
     goPSkip l qpn is conflicts (Step (TryP qpn' i) (Step Enter (Step (Skip conflicts') (Step Leave ms))))
       | qpn == qpn' && conflicts == conflicts' =
         -- By prepending (i : is) we reverse the order of the instances.
         goPSkip l qpn (i : is) conflicts ms
     goPSkip l qpn is conflicts ms =
-      let msg = blurbOptions Skipping qpn (reverse is) ++ showConflicts conflicts
-      in atLevel l msg (go l ms)
-
-    -- write a message with the current level number
-    atLevel :: Int -> String -> Progress String a b -> Progress String a b
-    atLevel l x xs =
-      let s = show l
-      in  Step ("[" ++ replicate (3 - length s) '_' ++ s ++ "] " ++ x) xs
+       Step (SummarizedMsg $ AtLevel l (EntrySkipMany qpn (reverse is) conflicts)) (go l ms)
 
 -- | Display the set of 'Conflicts' for a skipped package version.
 showConflicts :: Set CS.Conflict -> String
 showConflicts conflicts =
-    " (has the same characteristics that caused the previous version to fail: "
+    "(has the same characteristics that caused the previous version to fail: "
      ++ conflictMsg ++ ")"
   where
     conflictMsg :: String
@@ -213,29 +259,6 @@
   , versionConflict :: Maybe VR
   }
 
-data ProgressAction =
-    Trying
-  | Skipping
-  | Rejecting
-
-blurb :: ProgressAction -> String
-blurb = \case
-  Trying -> "trying: "
-  Skipping -> "skipping: "
-  Rejecting -> "rejecting: "
-
-blurbQFNBool :: ProgressAction -> QFN -> Bool -> String
-blurbQFNBool a q b = blurb a ++ Flag.showQFNBool q b
-
-blurbQSNBool :: ProgressAction -> QSN -> Bool -> String
-blurbQSNBool a q b = blurb a ++ Flag.showQSNBool q b
-
-blurbOption :: ProgressAction -> QPN -> POption -> String
-blurbOption a q p = blurb a ++ showOption q p
-
-blurbOptions :: ProgressAction -> QPN -> [POption] -> String
-blurbOptions a q ps = blurb a ++ showOptions q ps
-
 showOption :: QPN -> POption -> String
 showOption qpn@(Q _pp pn) (POption i linkedTo) =
   case linkedTo of
@@ -263,12 +286,12 @@
 showOptions :: QPN -> [POption] -> String
 showOptions _ [] = "unexpected empty list of versions"
 showOptions q [x] = showOption q x
-showOptions q xs = showQPN q ++ "; " ++ (L.intercalate ", "
+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
   | x@(POption i linkedTo) <- xs
-  ])
+  ]
 
 showGR :: QGoalReason -> String
 showGR UserGoal            = " (user goal)"
@@ -295,6 +318,7 @@
 showFR _ (GlobalConstraintVersion vr (ConstraintSourceProjectConfig pc)) = '\n' : (render . nest 6 $ docProjectConfigPathFailReason vr pc)
 showFR _ (GlobalConstraintVersion vr src) = " (" ++ constraintSource src ++ " requires " ++ prettyShow vr ++ ")"
 showFR _ (GlobalConstraintInstalled src)  = " (" ++ constraintSource src ++ " requires installed instance)"
+showFR _ (GlobalConstraintInstalledSpecificUnitId unitId src)  = " (" ++ constraintSource src ++ " requires installed instance with unit id " ++ prettyShow unitId ++ ")"
 showFR _ (GlobalConstraintSource src)     = " (" ++ constraintSource src ++ " requires source instance)"
 showFR _ (GlobalConstraintFlag src)       = " (" ++ constraintSource src ++ " requires opposite flag selection)"
 showFR _ ManualFlag                       = " (manual flag can only be changed explicitly)"
@@ -306,8 +330,8 @@
 -- The following are internal failures. They should not occur. In the
 -- interest of not crashing unnecessarily, we still just print an error
 -- message though.
-showFR _ (MalformedFlagChoice qfn)        = " (INTERNAL ERROR: MALFORMED FLAG CHOICE: " ++ Flag.showQFN qfn ++ ")"
-showFR _ (MalformedStanzaChoice qsn)      = " (INTERNAL ERROR: MALFORMED STANZA CHOICE: " ++ Flag.showQSN qsn ++ ")"
+showFR _ (MalformedFlagChoice qfn)        = " (INTERNAL ERROR: MALFORMED FLAG CHOICE: " ++ showQFN qfn ++ ")"
+showFR _ (MalformedStanzaChoice qsn)      = " (INTERNAL ERROR: MALFORMED STANZA CHOICE: " ++ showQSN qsn ++ ")"
 showFR _ EmptyGoalChoice                  = " (INTERNAL ERROR: EMPTY GOAL CHOICE)"
 
 showExposedComponent :: ExposedComponent -> String
@@ -332,7 +356,9 @@
                          componentStr ++ showVR vr
 
 -- $setup
+-- >>> import Distribution.Solver.Modular.Package
 -- >>> import Distribution.Solver.Types.PackagePath
+-- >>> import Distribution.Types.PackageName
 -- >>> import Distribution.Types.Version
 -- >>> import Distribution.Types.UnitId
 -- >>> let foobarPN = PackagePath DefaultNamespace QualToplevel
diff --git a/src/Distribution/Solver/Modular/Package.hs b/src/Distribution/Solver/Modular/Package.hs
--- a/src/Distribution/Solver/Modular/Package.hs
+++ b/src/Distribution/Solver/Modular/Package.hs
@@ -10,6 +10,7 @@
   , PN
   , QPV
   , instI
+  , instUid
   , makeIndependent
   , primaryPP
   , setupPP
@@ -76,6 +77,10 @@
 instI :: I -> Bool
 instI (I _ (Inst _)) = True
 instI _              = False
+
+instUid :: UnitId -> I -> Bool
+instUid uid (I _ (Inst uid')) = uid == uid'
+instUid _ _ = False
 
 -- | Is the package in the primary group of packages.  This is used to
 -- determine (1) if we should try to establish stanza preferences
diff --git a/src/Distribution/Solver/Modular/Preference.hs b/src/Distribution/Solver/Modular/Preference.hs
--- a/src/Distribution/Solver/Modular/Preference.hs
+++ b/src/Distribution/Solver/Modular/Preference.hs
@@ -135,7 +135,7 @@
     preferred pn opt =
       let PackagePreferences vrs _ _ = pcs pn
       in fromIntegral . negate . L.length $
-         L.filter (flip checkVR (version opt)) vrs
+         L.filter (`checkVR` version opt) vrs
 
     -- Prefer installed packages over non-installed packages.
     installed :: POption -> Weight
@@ -190,6 +190,9 @@
     go _       PackagePropertyInstalled
         | instI i       = r
         | otherwise     = Fail c (GlobalConstraintInstalled src)
+    go _       (PackagePropertyInstalledSpecificUnitId unitId)
+        | instUid unitId i       = r
+        | otherwise     = Fail c (GlobalConstraintInstalledSpecificUnitId unitId src)
     go _       PackagePropertySource
         | not (instI i) = r
         | otherwise     = Fail c (GlobalConstraintSource src)
diff --git a/src/Distribution/Solver/Modular/Solver.hs b/src/Distribution/Solver/Modular/Solver.hs
--- a/src/Distribution/Solver/Modular/Solver.hs
+++ b/src/Distribution/Solver/Modular/Solver.hs
@@ -71,7 +71,7 @@
   enableBackjumping      :: EnableBackjumping,
   solveExecutables       :: SolveExecutables,
   goalOrder              :: Maybe (Variable QPN -> Variable QPN -> Ordering),
-  solverVerbosity        :: Verbosity,
+  solverVerbosity        :: VerbosityLevel,
   pruneAfterFirstSuccess :: PruneAfterFirstSuccess
 }
 
diff --git a/src/Distribution/Solver/Modular/Tree.hs b/src/Distribution/Solver/Modular/Tree.hs
--- a/src/Distribution/Solver/Modular/Tree.hs
+++ b/src/Distribution/Solver/Modular/Tree.hs
@@ -118,6 +118,7 @@
                 | UnknownPackage
                 | GlobalConstraintVersion VR ConstraintSource
                 | GlobalConstraintInstalled ConstraintSource
+                | GlobalConstraintInstalledSpecificUnitId UnitId ConstraintSource
                 | GlobalConstraintSource ConstraintSource
                 | GlobalConstraintFlag ConstraintSource
                 | ManualFlag
@@ -163,10 +164,10 @@
 inn (FailF       c x           ) = Fail       c x
 
 innM :: Monad m => TreeF d c (m (Tree d c)) -> m (Tree d c)
-innM (PChoiceF    p s i       ts) = liftM (PChoice    p s i      ) (sequence ts)
-innM (FChoiceF    p s i b m d ts) = liftM (FChoice    p s i b m d) (sequence ts)
-innM (SChoiceF    p s i b     ts) = liftM (SChoice    p s i b    ) (sequence ts)
-innM (GoalChoiceF   s         ts) = liftM (GoalChoice   s        ) (sequence ts)
+innM (PChoiceF    p s i       ts) = PChoice    p s i       <$> sequence ts
+innM (FChoiceF    p s i b m d ts) = FChoice    p s i b m d <$> sequence ts
+innM (SChoiceF    p s i b     ts) = SChoice    p s i b     <$> sequence ts
+innM (GoalChoiceF   s         ts) = GoalChoice   s         <$> sequence ts
 innM (DoneF       x s           ) = return $ Done     x s
 innM (FailF       c x           ) = return $ Fail     c x
 
diff --git a/src/Distribution/Solver/Modular/Validate.hs b/src/Distribution/Solver/Modular/Validate.hs
--- a/src/Distribution/Solver/Modular/Validate.hs
+++ b/src/Distribution/Solver/Modular/Validate.hs
@@ -566,7 +566,7 @@
 validateTree :: CompilerInfo -> Index -> Maybe PkgConfigDb -> Tree d c -> Tree d c
 validateTree cinfo idx pkgConfigDb t = runValidate (validate t) VS {
     supportedExt        = maybe (const True) -- if compiler has no list of extensions, we assume everything is supported
-                                (\ es -> let s = S.fromList es in \ x -> S.member x s)
+                                (\ es -> let s = S.fromList es in (`S.member` s))
                                 (compilerInfoExtensions cinfo)
   , supportedLang       = maybe (const True)
                                 (flip L.elem) -- use list lookup because language list is small and no Ord instance
diff --git a/src/Distribution/Solver/Types/ComponentDeps.hs b/src/Distribution/Solver/Types/ComponentDeps.hs
--- a/src/Distribution/Solver/Types/ComponentDeps.hs
+++ b/src/Distribution/Solver/Types/ComponentDeps.hs
@@ -1,5 +1,7 @@
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE TupleSections #-}
+{-# LANGUAGE LambdaCase #-}
 
 -- | Fine-grained package dependencies
 --
@@ -30,7 +32,6 @@
   , fromInstalled
     -- ** Deconstructing ComponentDeps
   , toList
-  , flatDeps
   , nonSetupDeps
   , libraryDeps
   , setupDeps
@@ -43,7 +44,6 @@
 import Distribution.Solver.Compat.Prelude hiding (empty,toList,zip)
 
 import qualified Data.Map as Map
-import Data.Foldable (fold)
 
 import Distribution.Pretty (Pretty (..))
 import qualified Distribution.Types.ComponentName as CN
@@ -119,7 +119,7 @@
 -------------------------------------------------------------------------------}
 
 empty :: ComponentDeps a
-empty = ComponentDeps $ Map.empty
+empty = ComponentDeps Map.empty
 
 fromList :: Monoid a => [ComponentDep a] -> ComponentDeps a
 fromList = ComponentDeps . Map.fromListWith mappend
@@ -142,8 +142,8 @@
     ComponentDeps $
       Map.mergeWithKey
         (\_ a b -> Just (a,b))
-        (fmap (\a -> (a, mempty)))
-        (fmap (\b -> (mempty, b)))
+        (fmap (, mempty))
+        (fmap (mempty, ))
         d1 d2
 
 -- | Keep only selected components (and their associated deps info).
@@ -175,14 +175,6 @@
 toList :: ComponentDeps a -> [ComponentDep a]
 toList = Map.toList . unComponentDeps
 
--- | All dependencies of a package.
---
--- This is just a synonym for 'fold', but perhaps a use of 'flatDeps' is more
--- obvious than a use of 'fold', and moreover this avoids introducing lots of
--- @#ifdef@s for 7.10 just for the use of 'fold'.
-flatDeps :: Monoid a => ComponentDeps a -> a
-flatDeps = fold
-
 -- | All dependencies except the setup dependencies.
 --
 -- Prior to the introduction of setup dependencies in version 1.24 this
@@ -193,9 +185,10 @@
 -- | Library dependencies proper only.  (Includes dependencies
 -- of internal libraries.)
 libraryDeps :: Monoid a => ComponentDeps a -> a
-libraryDeps = select (\c -> case c of ComponentSubLib _ -> True
-                                      ComponentLib -> True
-                                      _ -> False)
+libraryDeps = select (\case
+  ComponentSubLib _ -> True
+  ComponentLib -> True
+  _ -> False)
 
 -- | List components
 components :: ComponentDeps a -> Set Component
diff --git a/src/Distribution/Solver/Types/ConstraintSource.hs b/src/Distribution/Solver/Types/ConstraintSource.hs
--- a/src/Distribution/Solver/Types/ConstraintSource.hs
+++ b/src/Distribution/Solver/Types/ConstraintSource.hs
@@ -63,6 +63,7 @@
   deriving (Show, Eq, Generic)
 
 instance Binary ConstraintSource
+instance NFData ConstraintSource
 instance Structured ConstraintSource
 
 -- | Description of a 'ConstraintSource'.
diff --git a/src/Distribution/Solver/Types/DependencyResolver.hs b/src/Distribution/Solver/Types/DependencyResolver.hs
--- a/src/Distribution/Solver/Types/DependencyResolver.hs
+++ b/src/Distribution/Solver/Types/DependencyResolver.hs
@@ -2,17 +2,22 @@
     ( DependencyResolver
     ) where
 
-import Distribution.Solver.Compat.Prelude
+import Distribution.Solver.Compat.Prelude ( Maybe, String, Set )
 import Prelude ()
 
 import Distribution.Solver.Types.LabeledPackageConstraint
+    ( LabeledPackageConstraint )
 import Distribution.Solver.Types.PkgConfigDb ( PkgConfigDb )
 import Distribution.Solver.Types.PackagePreferences
+    ( PackagePreferences )
 import Distribution.Solver.Types.PackageIndex ( PackageIndex )
 import Distribution.Solver.Types.Progress
+    ( Progress )
 import Distribution.Solver.Types.ResolverPackage
-import Distribution.Solver.Types.SourcePackage
-
+    ( ResolverPackage )
+import Distribution.Solver.Types.SourcePackage ( SourcePackage )
+import Distribution.Solver.Types.SummarizedMessage
+    ( SummarizedMessage(..) )
 import Distribution.Simple.PackageIndex ( InstalledPackageIndex )
 import Distribution.Package ( PackageName )
 import Distribution.Compiler ( CompilerInfo )
@@ -34,4 +39,4 @@
                            -> (PackageName -> PackagePreferences)
                            -> [LabeledPackageConstraint]
                            -> Set PackageName
-                           -> Progress String String [ResolverPackage loc]
+                           -> Progress SummarizedMessage String [ResolverPackage loc]
diff --git a/src/Distribution/Solver/Types/OptionalStanza.hs b/src/Distribution/Solver/Types/OptionalStanza.hs
--- a/src/Distribution/Solver/Types/OptionalStanza.hs
+++ b/src/Distribution/Solver/Types/OptionalStanza.hs
@@ -57,6 +57,7 @@
     }
 
 instance Binary OptionalStanza
+instance NFData OptionalStanza
 instance Structured OptionalStanza
 
 -------------------------------------------------------------------------------
diff --git a/src/Distribution/Solver/Types/PackageConstraint.hs b/src/Distribution/Solver/Types/PackageConstraint.hs
--- a/src/Distribution/Solver/Types/PackageConstraint.hs
+++ b/src/Distribution/Solver/Types/PackageConstraint.hs
@@ -19,7 +19,7 @@
 import Distribution.Solver.Compat.Prelude
 import Prelude ()
 
-import Distribution.Package                        (PackageName)
+import Distribution.Package                        (PackageName, UnitId)
 import Distribution.PackageDescription             (FlagAssignment, dispFlagAssignment)
 import Distribution.Pretty                         (flatStyle, Pretty(pretty))
 import Distribution.Types.PackageVersionConstraint (PackageVersionConstraint (..))
@@ -90,17 +90,20 @@
 data PackageProperty
    = PackagePropertyVersion   VersionRange
    | PackagePropertyInstalled
+   | PackagePropertyInstalledSpecificUnitId UnitId
    | PackagePropertySource
    | PackagePropertyFlags     FlagAssignment
    | PackagePropertyStanzas   [OptionalStanza]
   deriving (Eq, Show, Generic)
 
 instance Binary PackageProperty
+instance NFData PackageProperty
 instance Structured PackageProperty
 
 instance Pretty PackageProperty where
   pretty (PackagePropertyVersion verrange) = pretty verrange
   pretty PackagePropertyInstalled          = Disp.text "installed"
+  pretty (PackagePropertyInstalledSpecificUnitId unitId) = Disp.text "installed(" <> pretty unitId <> Disp.text ")"
   pretty PackagePropertySource             = Disp.text "source"
   pretty (PackagePropertyFlags flags)      = dispFlagAssignment flags
   pretty (PackagePropertyStanzas stanzas)  =
@@ -137,7 +140,8 @@
 packageConstraintToDependency (PackageConstraint scope prop) = toDep prop
   where
     toDep (PackagePropertyVersion vr) = Just $ PackageVersionConstraint (scopeToPackageName scope) vr
-    toDep (PackagePropertyInstalled)  = Nothing
-    toDep (PackagePropertySource)     = Nothing
+    toDep PackagePropertyInstalled  = Nothing
+    toDep (PackagePropertyInstalledSpecificUnitId {}) = Nothing
+    toDep PackagePropertySource       = Nothing
     toDep (PackagePropertyFlags _)    = Nothing
     toDep (PackagePropertyStanzas _)  = Nothing
diff --git a/src/Distribution/Solver/Types/PackageIndex.hs b/src/Distribution/Solver/Types/PackageIndex.hs
--- a/src/Distribution/Solver/Types/PackageIndex.hs
+++ b/src/Distribution/Solver/Types/PackageIndex.hs
@@ -22,6 +22,8 @@
   -- * Updates
   merge,
   override,
+  OverrideOrMerge(..),
+  overrideOrMerge,
   insert,
   deletePackageName,
   deletePackageId,
@@ -120,7 +122,7 @@
 -- case-sensitively.
 --
 lookup :: PackageIndex pkg -> PackageName -> [pkg]
-lookup (PackageIndex m) name = fromMaybe [] $ Map.lookup name m
+lookup (PackageIndex m) name = Map.findWithDefault [] name m
 
 --
 -- * Construction
@@ -181,6 +183,28 @@
   expensiveAssert (invariant i1 && invariant i2) $
     mkPackageIndex (Map.unionWith (\_l r -> r) m1 m2)
 
+data OverrideOrMerge = Override | Merge
+  deriving (Eq, Show)
+
+-- | Combined override-or-merge of two indexes.
+--
+-- For any package, either 'override' or 'merge' the packages from the second
+-- index into the first based on the supplied predicate.
+--
+overrideOrMerge ::
+     Package pkg
+  => (PackageName -> OverrideOrMerge)
+  -> PackageIndex pkg
+  -> PackageIndex pkg
+  -> PackageIndex pkg
+overrideOrMerge strategy i1@(PackageIndex m1) i2@(PackageIndex m2) =
+  expensiveAssert (invariant i1 && invariant i2) $
+    mkPackageIndex (Map.unionWithKey overridePkg m1 m2)
+  where
+    overridePkg name l r = case strategy name of
+      Override -> r
+      Merge -> mergeBuckets l r
+
 -- | Inserts a single package into the index.
 --
 -- This is equivalent to (but slightly quicker than) using 'mappend' or
@@ -227,8 +251,8 @@
 deleteDependency :: Package pkg
                  => PackageName -> VersionRange -> PackageIndex pkg
                  -> PackageIndex pkg
-deleteDependency name verstionRange =
-  delete name (\pkg -> packageVersion pkg `withinRange` verstionRange)
+deleteDependency name versionRange =
+  delete name (\pkg -> packageVersion pkg `withinRange` versionRange)
 
 --
 -- * Bulk queries
diff --git a/src/Distribution/Solver/Types/PkgConfigDb.hs b/src/Distribution/Solver/Types/PkgConfigDb.hs
--- a/src/Distribution/Solver/Types/PkgConfigDb.hs
+++ b/src/Distribution/Solver/Types/PkgConfigDb.hs
@@ -82,19 +82,18 @@
               . filter (either (const True) (not . null))
               -- Try decoding strictly; if it fails, put the lenient
               -- decoding in a Left for later reporting.
-              . map (\bsname ->
-                       let sbsname = LBS.toStrict bsname
-                       in case T.decodeUtf8' sbsname of
-                            Left _ -> Left (T.unpack (decodeUtf8LenientCompat sbsname))
-                            Right name -> Right (T.unpack name))
               -- The output of @pkg-config --list-all@ also includes a
               -- description for each package, which we do not need.
               -- We don't use Data.Char.isSpace because that would also
               -- include 0xA0, the non-breaking space, which can occur
               -- in multi-byte UTF-8 sequences.
-              . map (LBS.takeWhile (not . isAsciiSpace))
+              . map ((\bsname ->
+                       let sbsname = LBS.toStrict bsname
+                       in case T.decodeUtf8' sbsname of
+                            Left _ -> Left (T.unpack (decodeUtf8LenientCompat sbsname))
+                            Right name -> Right (T.unpack name)) . LBS.takeWhile (not . isAsciiSpace))
               $ pkgList
-        when (not (null failedPkgNames)) $
+        unless (null failedPkgNames) $
           info verbosity ("Some pkg-config packages have names containing invalid unicode: " ++ intercalate ", " failedPkgNames)
         (outs, _errs, exitCode) <-
                      getProgramInvocationOutputAndErrors verbosity
@@ -110,7 +109,7 @@
           -- one package version, so if the returned list is shorter than the
           -- requested one, we fall back to querying one by one.
           do
-            info verbosity ("call to pkg-config --modversion on all packages failed. Falling back to querying pkg-config individually on each package")
+            info verbosity "call to pkg-config --modversion on all packages failed. Falling back to querying pkg-config individually on each package"
             Just . pkgConfigDbFromList . catMaybes <$> mapM (getIndividualVersion pkgConfig) pkgNames
   where
     -- For when pkg-config invocation fails (possibly because of a
diff --git a/src/Distribution/Solver/Types/Progress.hs b/src/Distribution/Solver/Types/Progress.hs
--- a/src/Distribution/Solver/Types/Progress.hs
+++ b/src/Distribution/Solver/Types/Progress.hs
@@ -1,10 +1,13 @@
+{-# LANGUAGE DeriveFunctor #-}
+{-# LANGUAGE FlexibleInstances #-}
 module Distribution.Solver.Types.Progress
     ( Progress(..)
     , foldProgress
+    , step
     ) where
 
 import Prelude ()
-import Distribution.Solver.Compat.Prelude hiding (fail)
+import Distribution.Solver.Compat.Prelude
 
 -- | A type to represent the unfolding of an expensive long running
 -- calculation that may fail. We may get intermediate steps before the final
@@ -13,14 +16,10 @@
 data Progress step fail done = Step step (Progress step fail done)
                              | Fail fail
                              | Done done
+  deriving (Functor)
 
--- This Functor instance works around a bug in GHC 7.6.3.
--- See https://gitlab.haskell.org/ghc/ghc/-/issues/7436#note_66637.
--- The derived functor instance caused a space leak in the solver.
-instance Functor (Progress step fail) where
-  fmap f (Step s p) = Step s (fmap f p)
-  fmap _ (Fail x)   = Fail x
-  fmap f (Done r)   = Done (f r)
+step :: step -> Progress step fail ()
+step s = Step s (Done ())
 
 -- | Consume a 'Progress' calculation. Much like 'foldr' for lists but with two
 -- base cases, one for a final result and one for failure.
@@ -31,18 +30,21 @@
 --
 foldProgress :: (step -> a -> a) -> (fail -> a) -> (done -> a)
              -> Progress step fail done -> a
-foldProgress step fail done = fold
-  where fold (Step s p) = step s (fold p)
-        fold (Fail f)   = fail f
-        fold (Done r)   = done r
+foldProgress step' fail' done' = fold
+  where fold (Step s p) = step' s (fold p)
+        fold (Fail f)   = fail' f
+        fold (Done r)   = done' r
 
 instance Monad (Progress step fail) where
   return   = pure
   p >>= f  = foldProgress Step Fail f p
 
+instance MonadFail (Progress step String) where
+  fail = Fail
+
 instance Applicative (Progress step fail) where
   pure a  = Done a
-  p <*> x = foldProgress Step Fail (flip fmap x) p
+  p <*> x = foldProgress Step Fail (`fmap` x) p
 
 instance Monoid fail => Alternative (Progress step fail) where
   empty   = Fail mempty
diff --git a/src/Distribution/Solver/Types/ProjectConfigPath.hs b/src/Distribution/Solver/Types/ProjectConfigPath.hs
--- a/src/Distribution/Solver/Types/ProjectConfigPath.hs
+++ b/src/Distribution/Solver/Types/ProjectConfigPath.hs
@@ -5,16 +5,17 @@
     (
     -- * Project Config Path Manipulation
       ProjectConfigPath(..)
+    , compareLexicographically
+    , compareSegmentally
     , projectConfigPathRoot
     , nullProjectConfigPath
     , consProjectConfigPath
     , unconsProjectConfigPath
+    , currentProjectConfigPath
 
     -- * Messages
     , docProjectConfigPath
-    , docProjectConfigFiles
-    , cyclicalImportMsg
-    , untrimmedUriImportMsg
+    , docProjectImportedBy
     , docProjectConfigPathFailReason
     , quoteUntrimmed
 
@@ -42,7 +43,6 @@
 import Distribution.Pretty (prettyShow, Pretty(..))
 import Distribution.Utils.String (trim)
 import Text.PrettyPrint
-import Distribution.Simple.Utils (ordNub)
 import Distribution.System (OS(Windows), buildOS)
 
 -- | Path to a configuration file, either a singleton project root, or a longer
@@ -57,10 +57,10 @@
 -- List elements are relative to each other but once canonicalized, elements are
 -- relative to the directory of the project root.
 newtype ProjectConfigPath = ProjectConfigPath (NonEmpty FilePath)
-    deriving (Eq, Show, Generic)
+    deriving (Eq, Generic)
 
-instance Pretty ProjectConfigPath where
-  pretty = docProjectConfigPath
+instance Pretty ProjectConfigPath where pretty = docProjectConfigPath
+instance Show ProjectConfigPath where show = prettyShow
 
 -- | Sorts URIs after local file paths and longer file paths after shorter ones
 -- as measured by the number of path segments. If still equal, then sorting is
@@ -77,44 +77,93 @@
 -- >>> let abBwd = ProjectConfigPath $ "a\\b.config" :| []
 -- >>> compare abFwd abBwd
 -- EQ
+--
+-- >>> let abc = ProjectConfigPath $ "a/b/c.config" :| []
+-- >>> let yz = ProjectConfigPath $ "y/z.config" :| []
+-- >>> (compare abc yz, let xs = [abc, yz] in xs == sort xs)
+-- (GT,False)
+--
+-- >>> let abc = ProjectConfigPath $ "C.config" :| ["B.config", "A.project"]
+-- >>> let bcd = ProjectConfigPath $ "D.config" :| ["C.config", "B.project"]
+-- >>> (compare abc bcd, let xs = [abc, bcd] in xs == sort xs)
+-- (LT,True)
+--
+-- >>> let abc = ProjectConfigPath $ "C.config" :| ["B.config", "A.project"]
+-- >>> let yz = ProjectConfigPath $ "Z.config" :| ["Y.project"]
+-- >>> (compare abc yz, let xs = [abc, yz] in xs == sort xs)
+-- (GT,False)
 instance Ord ProjectConfigPath where
-    compare pa@(ProjectConfigPath (NE.toList -> as)) pb@(ProjectConfigPath (NE.toList -> bs)) =
+    compare = compareSegmentally
+
+-- | A comparison that puts projects first, URLs last and sorts the other paths
+-- lexically.
+compareLexicographically :: ProjectConfigPath -> ProjectConfigPath -> Ordering
+compareLexicographically (ProjectConfigPath as) (ProjectConfigPath bs) =
         case (as, bs) of
+            -- Single element paths are projects, they should always sort first.
+            (a :| [], b :| []) -> compare (splitPath a) (splitPath b)
+            (_ :| [], _) -> LT
+            (_, _ :| []) -> GT
+
+            (a :| aImporters, b :| bImporters) -> case (parseAbsoluteURI a, parseAbsoluteURI b) of
+                (Just ua, Just ub) -> compare ua ub P.<> compare aImporters bImporters
+                (Just _, Nothing) -> GT
+                (Nothing, Just _) -> LT
+                (Nothing, Nothing) -> compare (splitPath a) (splitPath b) P.<> compare aImporters bImporters
+
+-- | A comparison that puts projects first, URLs last and sorts the other paths
+-- by putting longer paths after shorter ones as measured by the number of path
+-- segments. If still equal, then sorting is lexical.
+compareSegmentally:: ProjectConfigPath -> ProjectConfigPath -> Ordering
+compareSegmentally pa@(ProjectConfigPath as) pb@(ProjectConfigPath bs) =
+        case (as, bs) of
             -- There should only ever be one root project path, only one path
             -- with length 1. Comparing it to itself should be EQ. Don't assume
             -- this though, do a comparison anyway when both sides have length
             -- 1.  The root path, the project itself, should always be the first
             -- path in a sorted listing.
-            ([a], [b]) -> compare (splitPath a) (splitPath b)
-            ([_], _) -> LT
-            (_, [_]) -> GT
+            (a :| [], b :| []) ->
+                let aPaths = splitPath a
+                    bPaths = splitPath b
+                in
+                    compare (length aPaths) (length bPaths)
+                    P.<> compare aPaths bPaths
 
-            (a:_, b:_) -> case (parseAbsoluteURI a, parseAbsoluteURI b) of
+            (_ :| [], _) -> LT
+            (_, _ :| []) -> GT
+
+            (a :| _, b :| _) -> case (parseAbsoluteURI a, parseAbsoluteURI b) of
                 (Just ua, Just ub) -> compare ua ub P.<> compare aImporters bImporters
                 (Just _, Nothing) -> GT
                 (Nothing, Just _) -> LT
-                (Nothing, Nothing) -> compare (splitPath a) (splitPath b) P.<> compare aImporters bImporters
-            _ ->
-                compare (length as) (length bs)
-                P.<> compare (length aPaths) (length bPaths)
-                P.<> compare aPaths bPaths
+                (Nothing, Nothing) ->
+                    let aPaths = splitPath a
+                        bPaths = splitPath b
+                    in
+                        compare (length as) (length bs)
+                        P.<> compare asPaths bsPaths
+                        P.<> compare (length aPaths) (length bPaths)
+                        P.<> compare aPaths bPaths
+                        P.<> compare aImporters bImporters
         where
-            splitPath = FP.splitPath . normSep where
-                normSep p =
-                    if buildOS == Windows
-                        then
-                            Windows.joinPath $ Windows.splitDirectories
-                            [if Posix.isPathSeparator c then Windows.pathSeparator else c| c <- p]
-                        else
-                            Posix.joinPath $ Posix.splitDirectories
-                            [if Windows.isPathSeparator c then Posix.pathSeparator else c| c <- p]
-
-            aPaths = splitPath <$> as
-            bPaths = splitPath <$> bs
+            asPaths = splitPath <$> as
+            bsPaths = splitPath <$> bs
             aImporters = snd $ unconsProjectConfigPath pa
             bImporters = snd $ unconsProjectConfigPath pb
 
+splitPath :: FilePath -> [FilePath]
+splitPath = FP.splitPath . normSep
+    where
+        normSep p =
+            if buildOS == Windows then
+                Windows.joinPath $ Windows.splitDirectories
+                [if Posix.isPathSeparator c then Windows.pathSeparator else c| c <- p]
+            else
+                Posix.joinPath $ Posix.splitDirectories
+                [if Windows.isPathSeparator c then Posix.pathSeparator else c| c <- p]
+
 instance Binary ProjectConfigPath
+instance NFData ProjectConfigPath
 instance Structured ProjectConfigPath
 
 -- | Renders the path like this;
@@ -131,66 +180,16 @@
 docProjectConfigPath (ProjectConfigPath (p :| ps)) = vcat $ quoteUntrimmed p :
     [ text " " <+> text "imported by:" <+> quoteUntrimmed l | l <- ps ]
 
+-- | Render the paths which imports this config.
+docProjectImportedBy :: ProjectConfigPath -> Doc
+docProjectImportedBy (ProjectConfigPath (_ :| [])) = text ""
+docProjectImportedBy (ProjectConfigPath (_ :| ps)) = vcat $
+    [ text " " <+> text "imported by:" <+> quoteUntrimmed l | l <- ps ]
+
 -- | If the path has leading or trailing spaces then show it quoted.
 quoteUntrimmed :: FilePath -> Doc
 quoteUntrimmed s = if trim s /= s then quotes (text s) else text s
 
--- | Renders the paths as a list without showing which path imports another,
--- like this;
---
--- >- cabal.project
--- >- project-cabal/constraints.config
--- >- project-cabal/ghc-latest.config
--- >- project-cabal/ghc-options.config
--- >- project-cabal/pkgs.config
--- >- project-cabal/pkgs/benchmarks.config
--- >- project-cabal/pkgs/buildinfo.config
--- >- project-cabal/pkgs/cabal.config
--- >- project-cabal/pkgs/install.config
--- >- project-cabal/pkgs/integration-tests.config
--- >- project-cabal/pkgs/tests.config
---
---
--- >>> :{
---   do
---     let ps =
---              [ ProjectConfigPath ("cabal.project" :| [])
---              , ProjectConfigPath ("project-cabal/constraints.config" :| ["cabal.project"])
---              , ProjectConfigPath ("project-cabal/ghc-latest.config" :| ["cabal.project"])
---              , ProjectConfigPath ("project-cabal/ghc-options.config" :| ["cabal.project"])
---              , ProjectConfigPath ("project-cabal/pkgs.config" :| ["cabal.project"])
---              , ProjectConfigPath ("project-cabal/pkgs/benchmarks.config" :| ["project-cabal/pkgs.config","cabal.project"])
---              , ProjectConfigPath ("project-cabal/pkgs/buildinfo.config" :| ["project-cabal/pkgs.config","cabal.project"])
---              , ProjectConfigPath ("project-cabal/pkgs/cabal.config" :| ["project-cabal/pkgs.config","cabal.project"])
---              , ProjectConfigPath ("project-cabal/pkgs/install.config" :| ["project-cabal/pkgs.config","cabal.project"])
---              , ProjectConfigPath ("project-cabal/pkgs/integration-tests.config" :| ["project-cabal/pkgs.config","cabal.project"])
---              , ProjectConfigPath ("project-cabal/pkgs/tests.config" :| ["project-cabal/pkgs.config","cabal.project"])
---              ]
---     return . render $ docProjectConfigFiles ps
--- :}
--- "- cabal.project\n- project-cabal/constraints.config\n- project-cabal/ghc-latest.config\n- project-cabal/ghc-options.config\n- project-cabal/pkgs.config\n- project-cabal/pkgs/benchmarks.config\n- project-cabal/pkgs/buildinfo.config\n- project-cabal/pkgs/cabal.config\n- project-cabal/pkgs/install.config\n- project-cabal/pkgs/integration-tests.config\n- project-cabal/pkgs/tests.config"
-docProjectConfigFiles :: [ProjectConfigPath] -> Doc
-docProjectConfigFiles ps = vcat
-    [ text "-" <+> text p
-    | p <- ordNub [ p | ProjectConfigPath (p :| _) <- ps ]
-    ]
-
--- | A message for a cyclical import, a "cyclical import of".
-cyclicalImportMsg :: ProjectConfigPath -> Doc
-cyclicalImportMsg path@(ProjectConfigPath (duplicate :| _)) =
-    vcat
-    [ text "cyclical import of" <+> text duplicate <> semi
-    , nest 2 (docProjectConfigPath path)
-    ]
-
--- | A message for an import that has leading or trailing spaces.
-untrimmedUriImportMsg :: Doc -> ProjectConfigPath -> Doc
-untrimmedUriImportMsg intro path =
-    vcat
-    [ intro <+> text "import has leading or trailing whitespace" <> semi
-    , nest 2 (docProjectConfigPath path)
-    ]
-
 docProjectConfigPathFailReason :: VR -> ProjectConfigPath -> Doc
 docProjectConfigPathFailReason vr pcp
     | ProjectConfigPath (p :| []) <- pcp =
@@ -236,6 +235,9 @@
 -- | Split the path into the importee and the importer path.
 unconsProjectConfigPath :: ProjectConfigPath -> (FilePath, Maybe ProjectConfigPath)
 unconsProjectConfigPath ps = fmap ProjectConfigPath <$> NE.uncons (coerce ps)
+
+currentProjectConfigPath :: ProjectConfigPath -> FilePath
+currentProjectConfigPath (ProjectConfigPath (p :| _)) = p
 
 -- | Make paths relative to the directory of the root of the project, not
 -- relative to the file they were imported from.
diff --git a/src/Distribution/Solver/Types/ResolverPackage.hs b/src/Distribution/Solver/Types/ResolverPackage.hs
--- a/src/Distribution/Solver/Types/ResolverPackage.hs
+++ b/src/Distribution/Solver/Types/ResolverPackage.hs
@@ -17,6 +17,7 @@
 import Distribution.Compat.Graph (IsNode(..))
 import Distribution.Package (Package(..), HasUnitId(..))
 import Distribution.Simple.Utils (ordNub)
+import Data.Foldable (fold)
 
 -- | The dependency resolver picks either pre-existing installed packages
 -- or it picks source packages along with package configuration.
@@ -48,5 +49,5 @@
   nodeKey (Configured spkg) = PlannedId (packageId spkg)
   -- Use dependencies for ALL components
   nodeNeighbors pkg =
-    ordNub $ CD.flatDeps (resolverPackageLibDeps pkg) ++
-             CD.flatDeps (resolverPackageExeDeps pkg)
+    ordNub $ fold (resolverPackageLibDeps pkg) ++
+             fold (resolverPackageExeDeps pkg)
diff --git a/src/Distribution/Solver/Types/Settings.hs b/src/Distribution/Solver/Types/Settings.hs
--- a/src/Distribution/Solver/Types/Settings.hs
+++ b/src/Distribution/Solver/Types/Settings.hs
@@ -95,6 +95,18 @@
 instance Structured OnlyConstrained
 instance Structured SolveExecutables
 
+instance NFData ReorderGoals
+instance NFData CountConflicts
+instance NFData FineGrainedConflicts
+instance NFData IndependentGoals
+instance NFData PreferOldest
+instance NFData MinimizeConflictSet
+instance NFData AvoidReinstalls
+instance NFData ShadowPkgs
+instance NFData StrongFlags
+instance NFData AllowBootLibInstalls
+instance NFData OnlyConstrained
+
 instance Pretty OnlyConstrained where
   pretty OnlyConstrainedAll  = PP.text "all"
   pretty OnlyConstrainedNone = PP.text "none"
@@ -105,3 +117,26 @@
     , P.string "none" >> return OnlyConstrainedNone
     ]
 
+instance Parsec ReorderGoals where
+  parsec = ReorderGoals <$> parsec
+
+instance Parsec CountConflicts where
+  parsec = CountConflicts <$> parsec
+
+instance Parsec FineGrainedConflicts where
+  parsec = FineGrainedConflicts <$> parsec
+
+instance Parsec MinimizeConflictSet where
+  parsec = MinimizeConflictSet <$> parsec
+
+instance Parsec StrongFlags where
+  parsec = StrongFlags <$> parsec
+
+instance Parsec AllowBootLibInstalls where
+  parsec = AllowBootLibInstalls <$> parsec
+
+instance Parsec PreferOldest where
+  parsec = PreferOldest <$> parsec
+
+instance Parsec IndependentGoals where
+  parsec = IndependentGoals <$> parsec
diff --git a/src/Distribution/Solver/Types/SummarizedMessage.hs b/src/Distribution/Solver/Types/SummarizedMessage.hs
new file mode 100644
--- /dev/null
+++ b/src/Distribution/Solver/Types/SummarizedMessage.hs
@@ -0,0 +1,48 @@
+module Distribution.Solver.Types.SummarizedMessage
+    ( Entry(..)
+    , EntryAtLevel(..)
+    , SummarizedMessage(..)
+    ) where
+
+import Prelude ()
+import Distribution.Solver.Compat.Prelude hiding (fail)
+
+import Distribution.Solver.Modular.Tree
+    ( FailReason(..), POption(..) )
+import Distribution.Solver.Types.PackagePath ( QPN )
+import Distribution.Solver.Modular.Flag ( QSN, QFN )
+import Distribution.Solver.Modular.Dependency
+    ( ConflictSet, QGoalReason, GoalReason )
+import qualified Distribution.Solver.Modular.ConflictSet as CS
+
+-- The following types are used to encode log messages from the
+-- dependency solver so they can be easily displayed in the module
+-- `Distribution.Solver.Modular.Message`.
+--
+--  These types are an intermediate representation of the solver log.
+-- The log is converted from a list of Message to a list of
+-- SummarizedMessage to a list of String. Message is very similar to
+-- the structure of the search tree but difficult to read, and
+-- SummarizedMessage has the structure that is shown to users.
+
+-- Encoding of solver messages.
+data Entry
+  = EntryPackageGoal QPN QGoalReason
+  | EntryRejectF QFN Bool ConflictSet FailReason
+  | EntryRejectS QSN Bool ConflictSet FailReason
+  | EntrySkipping (Set CS.Conflict)
+  | EntryTryingF QFN Bool
+  | EntryTryingP QPN POption
+  | EntryTryingNewP QPN POption (GoalReason QPN)
+  | EntryTryingS QSN Bool
+  | EntryRejectMany QPN [POption] ConflictSet FailReason
+  | EntrySkipMany QPN [POption] (Set CS.Conflict)
+  | EntryUnknownPackage QPN (GoalReason QPN)
+  | EntrySuccess
+  | EntryFailure ConflictSet FailReason
+
+-- Encode the level at which the solver message occurred.
+data EntryAtLevel = AtLevel Int Entry
+
+-- Messages from the solver.
+data SummarizedMessage = SummarizedMsg EntryAtLevel | StringMsg String