Don't break when data-dir is null.

Cabal/Distribution/Simple/Glob.hs

@@ -142,9 +142,19 @@ matchFileGlob verbosity version = matchDirFileGlob verbosity version "."
 
 -- The returned values do not include the supplied @dir@ prefix.
 matchDirFileGlob :: Verbosity -> Version -> FilePath -> FilePath -> IO [FilePath]
-matchDirFileGlob verbosity version dir filepath = case parseFileGlob version filepath of
+matchDirFileGlob verbosity version rawDir filepath = case parseFileGlob version filepath of
   Left err -> die' verbosity $ explainGlobSyntaxError filepath err
   Right pat -> do
+    -- The default data-dir is null. Our callers -should- be
+    -- converting that to '.' themselves, but it's a certainty that
+    -- some future call-site will forget and trigger a really
+    -- hard-to-debug failure if we don't check for that here.
+    when (null rawDir) $
+      warn verbosity $
+           "Null dir passed to matchDirFileGlob; interpreting it "
+        ++ "as '.'. This is probably an internal error."
+    let dir = if null rawDir then "." else rawDir
+    debug verbosity $ "Expanding glob '" ++ filepath ++ "' in directory '" ++ dir ++ "'."
     -- This function might be called from the project root with dir as
     -- ".". Walking the tree starting there involves going into .git/
     -- and dist-newstyle/, which is a lot of work for no reward, so

Cabal/Distribution/Simple/Install.hs

@@ -235,7 +235,10 @@ copyComponent _ _ _ (CTest _) _ _ = return ()
 installDataFiles :: Verbosity -> PackageDescription -> FilePath -> IO ()
 installDataFiles verbosity pkg_descr destDataDir =
   flip traverse_ (dataFiles pkg_descr) $ \ file -> do
-    let srcDataDir = dataDir pkg_descr
+    let srcDataDirRaw = dataDir pkg_descr
+        srcDataDir = if null srcDataDirRaw
+          then "."
+          else srcDataDirRaw
     files <- matchDirFileGlob verbosity (specVersion pkg_descr) srcDataDir file
     let dir = takeDirectory file
     createDirectoryIfMissingVerbose verbosity True (destDataDir </> dir)

Cabal/Distribution/Simple/SrcDist.hs

@@ -209,8 +209,12 @@ listPackageSourcesOrdinary verbosity pkg_descr pps =
     -- Data files.
   , fmap concat
     . for (dataFiles pkg_descr) $ \filename ->
-        fmap (fmap (dataDir pkg_descr </>)) $
-          matchDirFileGlob verbosity (specVersion pkg_descr) (dataDir pkg_descr) filename
+        let srcDataDirRaw = dataDir pkg_descr
+            srcDataDir = if null srcDataDirRaw
+              then "."
+              else srcDataDirRaw
+        in fmap (fmap (srcDataDir </>)) $
+             matchDirFileGlob verbosity (specVersion pkg_descr) srcDataDir filename
 
     -- Extra doc files.
   , fmap concat

cabal-testsuite/PackageTests/Regression/T5318/Main.hs

@@ -0,0 +1 @@
+main = putStrLn "hi"

cabal-testsuite/PackageTests/Regression/T5318/empty-data-dir.cabal

@@ -0,0 +1,10 @@
+cabal-version: 2.0
+name: empty-data-dir
+version: 0
+build-type: Simple
+data-files: foo.dat
+
+executable foo
+  default-language: Haskell2010
+  build-depends: base
+  main-is: Main.hs

cabal-testsuite/PackageTests/Regression/T5318/foo.dat

@@ -0,0 +1 @@
+Hello!

cabal-testsuite/PackageTests/Regression/T5318/install.out

@@ -0,0 +1,8 @@
+# cabal install
+Resolving dependencies...
+Configuring empty-data-dir-0...
+Preprocessing executable 'foo' for empty-data-dir-0..
+Building executable 'foo' for empty-data-dir-0..
+Installing executable foo in <PATH>
+Warning: The directory <ROOT>/install.dist/home/.cabal/bin is not in the system search path.
+Installed empty-data-dir-0

cabal-testsuite/PackageTests/Regression/T5318/install.test.hs

@@ -0,0 +1,3 @@
+import Test.Cabal.Prelude
+main = cabalTest $
+  cabal "install" []

cabal-testsuite/PackageTests/Regression/T5318/sdist-list-sources.out

@@ -0,0 +1,3 @@
+# cabal sdist
+List of package sources written to file '<TMPDIR>/sources'
+List of package sources written to file '<TMPDIR>/sources'

cabal-testsuite/PackageTests/Regression/T5318/sdist-list-sources.test.hs

@@ -0,0 +1,7 @@
+import Test.Cabal.Prelude
+main = cabalTest $ do
+  tmpdir <- fmap testTmpDir getTestEnv
+  let fn = tmpdir </> "sources"
+  cabal "sdist" ["--list-sources=" ++ fn]
+  -- --list-sources outputs with slashes on posix and backslashes on Windows. 'normalise' converts our needle to the necessary format.
+  assertFileDoesContain fn $ normalise "foo.dat"