Adding test case for https://github.com/kazu-yamamoto/ghc-mod/issues/336

ISSUE336.md

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+https://github.com/kazu-yamamoto/ghc-mod/issues/336
+
+```
+{-# LANGUAGE DataKinds, TypeFamilies, TypeOperators, GADTs, KindSignatures #-}
+
+module Vect where
+
+data Nat = Z | S Nat
+
+type family   (n :: Nat) :+ (m :: Nat) :: Nat
+type instance Z :+ m = m
+type instance S n :+ m = S (n :+ m)
+
+data Vect :: Nat -> * -> * where
+  VNil :: Vect Z a
+  (:::) :: a -> Vect n a -> Vect (S n) a
+
+vAppend :: Vect n a -> Vect m a -> Vect (n :+ m) a
+vAppend x y = _vAppend_body
+
+lAppend :: [a] -> [a] -> [a]
+lAppend x y = _lAppend_body
+
+data MyList a = Nil | Cons a (MyList a)
+
+mlAppend :: MyList a -> MyList a -> MyList a
+mlAppend x y = _mlAppend_body
+```

test/CaseSplitSpec.hs

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+{-# LANGUAGE CPP #-}
+module CaseSplitSpec where
+
+import Control.Applicative ((<$>))
+import Data.List (isPrefixOf)
+import Language.Haskell.GhcMod
+#if __GLASGOW_HASKELL__ < 706
+import System.Environment.Executable (getExecutablePath)
+#else
+import System.Environment (getExecutablePath)
+#endif
+import System.Exit
+import System.FilePath
+import System.Process
+import Test.Hspec
+import TestUtils
+import Dir
+
+spec :: Spec
+spec = do
+    describe "case split" $ do
+        it "does not blow up on HsWithBndrs panic" $ do
+            withDirectory_ "test/data/case-split" $ do
+                res <- runD $ splits "Vect.hs" 24 10
+                res `shouldBe` "9 5 11 40 \"Int -> a -> a -> a\"\n7 1 11 40 \"Int -> Integer\"\n"

test/data/case-split/Vect.hs

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+{-# LANGUAGE DataKinds, TypeFamilies, TypeOperators, GADTs, KindSignatures #-}
+
+module Vect where
+
+data Nat = Z | S Nat
+
+type family   (n :: Nat) :+ (m :: Nat) :: Nat
+type instance Z :+ m = m
+type instance S n :+ m = S (n :+ m)
+
+data Vect :: Nat -> * -> * where
+  VNil :: Vect Z a
+  (:::) :: a -> Vect n a -> Vect (S n) a
+
+vAppend :: Vect n a -> Vect m a -> Vect (n :+ m) a
+vAppend x y = _vAppend_body
+
+lAppend :: [a] -> [a] -> [a]
+lAppend x y = _lAppend_body
+
+data MyList a = Nil | Cons a (MyList a)
+
+mlAppend :: MyList a -> MyList a -> MyList a
+mlAppend x y = _mlAppend_body