Cherry-pick #4094 onto 1.24.

Cabal/Cabal.cabal

@@ -80,6 +80,8 @@ extra-source-files:
   tests/PackageTests/BuildDeps/TargetSpecificDeps3/MyLibrary.hs
   tests/PackageTests/BuildDeps/TargetSpecificDeps3/lemon.hs
   tests/PackageTests/BuildDeps/TargetSpecificDeps3/my.cabal
+  tests/PackageTests/BuildTargetErrors/BuildTargetErrors.cabal
+  tests/PackageTests/BuildTargetErrors/Main.hs
   tests/PackageTests/BuildTestSuiteDetailedV09/Dummy2.hs
   tests/PackageTests/BuildableField/BuildableField.cabal
   tests/PackageTests/BuildableField/Main.hs

Cabal/Distribution/PackageDescription/Configuration.hs

@@ -229,7 +229,7 @@ resolveWithFlags dom os arch impl constrs trees checkDeps =
     -- dependencies to dependency maps.
     simplifiedTrees :: [CondTree FlagName DependencyMap PDTagged]
     simplifiedTrees = map ( mapTreeConstrs toDepMap  -- convert to maps
-                          . addBuildableCondition pdTaggedBuildInfo
+                          . addBuildableConditionPDTagged
                           . mapTreeConds (fst . simplifyWithSysParams os arch impl))
                           trees
 
@@ -278,13 +278,6 @@ resolveWithFlags dom os arch impl constrs trees checkDeps =
     env :: FlagAssignment -> FlagName -> Either FlagName Bool
     env flags flag = (maybe (Left flag) Right . lookup flag) flags
 
-    pdTaggedBuildInfo :: PDTagged -> BuildInfo
-    pdTaggedBuildInfo (Lib l) = libBuildInfo l
-    pdTaggedBuildInfo (Exe _ e) = buildInfo e
-    pdTaggedBuildInfo (Test _ t) = testBuildInfo t
-    pdTaggedBuildInfo (Bench _ b) = benchmarkBuildInfo b
-    pdTaggedBuildInfo PDNull = mempty
-
 -- | Transforms a 'CondTree' by putting the input under the "then" branch of a
 -- conditional that is True when Buildable is True. If 'addBuildableCondition'
 -- can determine that Buildable is always True, it returns the input unchanged.
@@ -298,6 +291,35 @@ addBuildableCondition getInfo t =
     Lit False -> CondNode mempty mempty []
     c         -> CondNode mempty mempty [(c, t, Nothing)]
 
+-- | This is a special version of 'addBuildableCondition' for the 'PDTagged'
+-- type.
+--
+-- It is not simply a specialisation. It is more complicated than it
+-- ought to be because of the way the 'PDTagged' monoid instance works. The
+-- @mempty = 'PDNull'@ forgets the component type, which has the effect of
+-- completely deleting components that are not buildable.
+--
+-- See <https://github.com/haskell/cabal/pull/4094> for more details.
+--
+addBuildableConditionPDTagged :: (Eq v, Monoid c) =>
+                                 CondTree v c PDTagged
+                              -> CondTree v c PDTagged
+addBuildableConditionPDTagged t =
+    case extractCondition (buildable . getInfo) t of
+      Lit True  -> t
+      Lit False -> deleteConstraints t
+      c         -> CondNode mempty mempty [(c, t, Just (deleteConstraints t))]
+  where
+    deleteConstraints = mapTreeConstrs (const mempty)
+
+    getInfo :: PDTagged -> BuildInfo
+    getInfo (Lib l) = libBuildInfo l
+    getInfo (Exe _ e) = buildInfo e
+    getInfo (Test _ test) = testBuildInfo test
+    getInfo (Bench _ b) = benchmarkBuildInfo b
+    getInfo PDNull = mempty
+
+
 -- Note: extracting buildable conditions.
 -- --------------------------------------
 --

Cabal/changelog

@@ -1,4 +1,7 @@
 -*-change-log-*-
+1.24.1.1 Ryan Thomas <ryan@ryant.org> ??? 2016
+	* Fixed a bug in the handling of non-buildable components (#4094).
+
 1.24.1.0 Ryan Thomas <ryan@ryant.org> October 2016
 	* API addition: 'differenceVersionRanges' (#3519).
 	* Fixed reexported-modules display mangling (#3928).

Cabal/tests/PackageTests/BuildTargetErrors/BuildTargetErrors.cabal

@@ -0,0 +1,10 @@
+name: BuildTargetErrors
+version: 1.0
+build-type: Simple
+cabal-version: >= 1.9
+
+library
+
+executable not-buildable-exe
+    main-is: Main.hs
+    buildable: False

Cabal/tests/PackageTests/BuildTargetErrors/Main.hs

@@ -0,0 +1 @@
+main = return ()

Cabal/tests/PackageTests/Tests.hs

@@ -279,6 +279,13 @@ tests config = do
                     "tests/PackageTests/GhcPkgGuess/SymlinkGhcVersion/ghc" $
                     ghc_pkg_guess "ghc"
 
+  -- Test error message we report when a non-buildable target is
+  -- requested to be built
+  tc "BuildTargetErrors" $ do
+    cabal "configure" []
+    assertOutputContains "the component is marked as disabled"
+        =<< shouldFail (cabal' "build" ["not-buildable-exe"])
+
   where
     ghc_pkg_guess bin_name = do
         cwd <- packageDir