Introduce checks for upper dependencies in setup.

setup depends are now checked for having upper dependencies.
Upper bounds are mandatory for base and Cabal libraries, and
are optional (but emit warning) for other libraries.
Implicit dependencies are not being checked.

Cabal/Distribution/PackageDescription/Check.hs

@@ -152,6 +152,7 @@ checkPackage gpkg mpkg =
   ++ checkUnusedFlags gpkg
   ++ checkUnicodeXFields gpkg
   ++ checkPathsModuleExtensions pkg
+  ++ checkSetupVersions gpkg
   where
     pkg = fromMaybe (flattenPackageDescription gpkg) mpkg
 
@@ -1525,21 +1526,7 @@ checkPackageVersions pkg =
 
   ]
   where
-    -- TODO: What we really want to do is test if there exists any
-    -- configuration in which the base version is unbounded above.
-    -- However that's a bit tricky because there are many possible
-    -- configurations. As a cheap easy and safe approximation we will
-    -- pick a single "typical" configuration and check if that has an
-    -- open upper bound. To get a typical configuration we finalise
-    -- using no package index and the current platform.
-    finalised = finalizePD
-                              mempty defaultComponentRequestedSpec (const True)
-                              buildPlatform
-                              (unknownCompilerInfo
-                                (CompilerId buildCompilerFlavor nullVersion)
-                                NoAbiTag)
-                              [] pkg
-    baseDependency = case finalised of
+    baseDependency = case typicalPkg pkg of
       Right (pkg', _) | not (null baseDeps) ->
           foldr intersectVersionRanges anyVersion baseDeps
         where
@@ -1552,13 +1539,6 @@ checkPackageVersions pkg =
       -- then we will just skip the check, since boundedAbove noVersion = True
       _          -> noVersion
 
-    boundedAbove :: VersionRange -> Bool
-    boundedAbove vr = case asVersionIntervals vr of
-      []        -> True -- this is the inconsistent version range.
-      intervals -> case last intervals of
-        (_,   UpperBound _ _) -> True
-        (_, NoUpperBound    ) -> False
-
 
 checkConditionals :: GenericPackageDescription -> [PackageCheck]
 checkConditionals pkg =
@@ -2136,6 +2116,41 @@ checkTarPath path
       ++ "Files with an empty name cannot be stored in a tar archive or in "
       ++ "standard file systems."
 
+-- | Check that setup dependencies, have proper upper bounds.
+-- @base@ and @Cabal@ upper dependencies are mandatory and emit
+-- error, all other packages emit warning.
+checkSetupVersions :: GenericPackageDescription -> [PackageCheck]
+checkSetupVersions pkg =
+    [ emitMessage $ unPackageName name
+    | (name, vr) <- Map.toList deps
+    , not (boundedAbove vr)
+    ]
+  where
+    criticalPkgs = ["Cabal", "base"]
+    deps = case typicalPkg pkg of
+      Right (pkgs', _) ->
+        Map.fromListWith intersectVersionRanges
+          [ (pname, vr)
+          | Just spi <- pure $ setupBuildInfo pkgs'
+          , Dependency pname vr <- setupDepends spi
+          ]
+      _ -> Map.empty
+    emitMessage nm
+      | nm `elem` criticalPkgs = emitError nm
+      | otherwise = emitWarning nm
+    emitError nm =
+      PackageDistInexcusable $
+           "The dependency 'build-depends: '"++nm++"' does not specify an "
+        ++ "upper bound on the version number. Each major release of the "
+        ++ "'"++nm++"' package changes the API in various ways and most "
+        ++ "packages will need some changes to compile with it.If you are "
+        ++ "not sure what upper bound to use then use the next major "
+        ++ "version."
+    emitWarning nm =
+      PackageDistSuspiciousWarn $
+           "The dependency 'build-depends: '"++nm++"' does not specify an upper "
+        ++ "bound on the version number. "
+
 -- ------------------------------------------------------------
 -- * Utils
 -- ------------------------------------------------------------
@@ -2156,3 +2171,29 @@ fileExtensionSupportedLanguage path =
     extension = takeExtension path
     isHaskell = extension `elem` [".hs", ".lhs"]
     isC       = isJust (filenameCDialect extension)
+
+boundedAbove :: VersionRange -> Bool
+boundedAbove vr = case asVersionIntervals vr of
+  []        -> True -- this is the inconsistent version range.
+  intervals -> case last intervals of
+    (_,   UpperBound _ _) -> True
+    (_, NoUpperBound    ) -> False
+
+
+--
+-- TODO: What we really want to do is test if there exists any
+-- configuration in which the base version is unbounded above.
+-- However that's a bit tricky because there are many possible
+-- configurations. As a cheap easy and safe approximation we will
+-- pick a single "typical" configuration and check if that has an
+-- open upper bound. To get a typical configuration we finalise
+-- using no package index and the current platform.
+typicalPkg :: GenericPackageDescription
+           -> Either [Dependency] (PackageDescription, FlagAssignment)
+typicalPkg = finalizePD
+  mempty defaultComponentRequestedSpec (const True)
+  buildPlatform
+  (unknownCompilerInfo
+    (CompilerId buildCompilerFlavor nullVersion)
+      NoAbiTag)
+  []