Fix filtering package in root of project (#1041)

When a package is not in a subdirectory, but instead
in the root of the project the cleaning of the package
source was broken.

This might explain why some `cabal.project` file
based projects might also see be affected by #1013.

Author: hamishmack

lib/clean-cabal-component.nix

@@ -37,9 +37,16 @@ let
       then "/"
       else ""
     ) + normalizeRelativePath path;
-  combinePaths = a: b: if isAbsolutePath b
-    then b
-    else normalizePath (a + "/" + b);
+  combinePaths = a: b:
+    if isAbsolutePath b
+      then b
+    else normalizePath (
+      if a == ""
+        then b
+      else if b == ""
+        then a
+      else a + "/" + b
+    );
   # Like normalizePath but with a trailing / when needed
   normalizeDir = dir:
     let p = normalizePath dir;

lib/default.nix

@@ -314,14 +314,17 @@ in {
   rootAndSubDir = src: rec {
     subDir = src.origSubDir or "";
     root =
-      # Use `cleanSourceWith` to make sure the `filter` is still used
-      if src ? origSrc && src ? filter && subDir != ""
-        then haskellLib.cleanSourceWith {
-          name = src.name or "source" + "-root";
-          src = src.origSrc;
-          # Not passing src.origSubDir so that the result points `origSrc`
-          inherit (src) filter;
-        }
-        else src.origSrc or src;
+      if subDir == ""
+        then src # if there was no subdir use the original src
+        else
+          # Use `cleanSourceWith` to make sure the `filter` is still used
+          if src ? origSrc && src ? filter
+            then haskellLib.cleanSourceWith {
+              name = src.name or "source" + "-root";
+              src = src.origSrc;
+              # Not passing src.origSubDir so that the result points `origSrc`
+              inherit (src) filter;
+            }
+            else src.origSrc or src;  # If there is a subDir and origSrc (but no filter) use origSrc
   };
 }