Add explicit failure modes

cabal-install/src/Distribution/Client/CmdErrorMessages.hs

@@ -300,6 +300,12 @@ renderTargetProblem verb _ (TargetProblemNoSuchComponent pkgid cname) =
   ++ "for the project plan, which would suggest an inconsistency "
   ++ "between readTargetSelectors and resolveTargets."
 
+renderTargetProblem verb _ (TargetProblemNotSinglePackage []) =
+    "Internal error when trying to " ++ verb ++ ". Found no package."
+renderTargetProblem verb _ (TargetProblemNotSinglePackage pkgids) =
+    "Internal error when trying to " ++ verb ++ ". Found multiple packages:\n"
+  ++ unlines (map prettyShow pkgids)
+
 
 ------------------------------------------------------------
 -- Renderering error messages for TargetProblemNoneEnabled

cabal-install/src/Distribution/Client/ProjectOrchestration.hs

@@ -576,8 +576,7 @@ resolveTargets selectPackageTargets selectComponentTarget
       = Left (TargetProblemNoSuchPackage pkgid)
 
     checkTarget (TargetPackage _ pkgids _)
-      = error ("TODO: add support for multiple packages in a directory.  Got\n"
-              ++ unlines (map prettyShow pkgids))
+      = Left (TargetProblemNotSinglePackage pkgids)
       -- For the moment this error cannot happen here, because it gets
       -- detected when the package config is being constructed. This case
       -- will need handling properly when we do add support.

cabal-install/src/Distribution/Client/TargetProblem.hs

@@ -44,6 +44,7 @@ data TargetProblem a
     -- directly then of course it can.
     | TargetProblemNoSuchPackage           PackageId
     | TargetProblemNoSuchComponent         PackageId ComponentName
+    | TargetProblemNotSinglePackage        [PackageId]
 
       -- | A custom target problem
     | CustomTargetProblem a

cabal-install/src/Distribution/Client/TargetSelector.hs

@@ -470,8 +470,11 @@ resolveTargetSelectors (KnownTargets{knownPackagesAll = []}) [] _ =
 resolveTargetSelectors (KnownTargets{knownPackagesPrimary = []}) [] _ =
     ([TargetSelectorNoTargetsInCwd], [])
 
-resolveTargetSelectors (KnownTargets{knownPackagesPrimary}) [] _ =
-    ([], [TargetPackage TargetImplicitCwd pkgids Nothing])
+resolveTargetSelectors (KnownTargets{knownPackagesPrimary}) [] _
+  | [pkgid] <- pkgids =
+    ([], [TargetPackage TargetImplicitCwd [pkgid] Nothing])
+  | otherwise =
+    ([TargetSelectorNotSinglePackage pkgids], [])
   where
     pkgids = [ pinfoId | KnownPackage{pinfoId} <- knownPackagesPrimary ]
 
@@ -593,6 +596,7 @@ data TargetSelectorProblem
    | TargetSelectorUnrecognised String
      -- ^ Syntax error when trying to parse a target string.
    | TargetSelectorNoCurrentPackage TargetString
+   | TargetSelectorNotSinglePackage [PackageId]
    | TargetSelectorNoTargetsInCwd
    | TargetSelectorNoTargetsInProject
   deriving (Show, Eq)
@@ -805,7 +809,29 @@ reportTargetSelectorProblems verbosity problems = do
     case [ () | TargetSelectorNoTargetsInProject <- problems ] of
       []  -> return ()
       _:_ ->
-        die' verbosity $
+        die' verbosity noPackageErrorMessage
+
+    case [ pkgids | TargetSelectorNotSinglePackage pkgids <- problems ] of
+      []      -> return ()
+      mpkgids ->
+        let
+          allpkgids = concat mpkgids
+        in
+          case allpkgids of
+            [] ->
+              die' verbosity noPackageErrorMessage
+            pkgids ->
+              die' verbosity $
+                  "Multiple packages have been found:\n  "
+               ++ unlines (map prettyShow pkgids)
+
+
+        where
+
+
+    fail "reportTargetSelectorProblems: internal error"
+  where
+    noPackageErrorMessage =
             "There is no <pkgname>.cabal package file or cabal.project file. "
          ++ "To build packages locally you need at minimum a <pkgname>.cabal "
          ++ "file. You can use 'cabal init' to create one.\n"
@@ -815,9 +841,6 @@ reportTargetSelectorProblems verbosity problems = do
          ++ "packages in your project and all other build configuration. "
          ++ "See the Cabal user guide for full details."
 
-    fail "reportTargetSelectorProblems: internal error"
-
-
 ----------------------------------
 -- Syntax type
 --