avoid invalid filepaths on Windows (#9254)

* avoid invalid filepaths on Windows

* comment bad Arbitrary instances

Cabal-QuickCheck/src/Test/QuickCheck/Instances/Cabal.hs

@@ -5,8 +5,8 @@ module Test.QuickCheck.Instances.Cabal () where
 
 import Control.Applicative        (liftA2)
 import Data.Bits                  (shiftR)
-import Data.Char                  (isAlphaNum, isDigit)
-import Data.List                  (intercalate)
+import Data.Char                  (isAlphaNum, isDigit, toLower)
+import Data.List                  (intercalate, (\\))
 import Data.List.NonEmpty         (NonEmpty (..))
 import Distribution.Utils.Generic (lowercase)
 import Test.QuickCheck
@@ -405,6 +405,10 @@ instance (Arbitrary a, Ord a) => Arbitrary (NubList a) where
 -- InstallDirs
 -------------------------------------------------------------------------------
 
+-- these are wrong because they bottom out in String. We should really use
+-- the modern FilePath at some point, so we get QC instances that don't include
+-- invalid characters or path components
+
 instance Arbitrary a => Arbitrary (InstallDirs a) where
     arbitrary = InstallDirs
         <$> arbitrary <*> arbitrary <*> arbitrary <*> arbitrary --  4
@@ -506,11 +510,20 @@ arbitraryShortToken :: Gen String
 arbitraryShortToken = arbitraryShortStringWithout "{}[]"
 
 arbitraryShortPath :: Gen String
-arbitraryShortPath = arbitraryShortStringWithout "{}[],"
+arbitraryShortPath = arbitraryShortStringWithout "{}[],<>:|*?" `suchThat` (not . winDevice)
+    where
+        -- split path components on dots
+        -- no component can be empty or a device name
+        -- this blocks a little too much (both "foo..bar" and "foo.con" are legal)
+        -- but for QC being a little conservative isn't harmful
+        winDevice = any (any (`elem` ["","con", "aux", "prn", "com", "lpt", "nul"]) . splitBy ".") . splitBy "\\/" . map toLower
+        splitBy _ "" = []
+        splitBy seps str = let (part,rest) = break (`elem` seps) str
+                            in part : if length rest == 1 then [""] else splitBy seps (drop 1 rest)
 
 arbitraryShortStringWithout :: String -> Gen String
 arbitraryShortStringWithout excludeChars =
-    shortListOf1 5 $ elements [c | c <- ['#' ..  '~' ], c `notElem` excludeChars ]
+    shortListOf1 5 $ elements $ ['#' .. '~'] \\ excludeChars
 
 -- |
 intSqrt :: Int -> Int