Add test and comment for `PATH#existing`.

2 files changed, 5 insertions, 0 deletions

diff --git a/Library/Homebrew/PATH.rb b/Library/Homebrew/PATH.rb
index f1c0ebb5d..de7167eb4 100644
--- a/Library/Homebrew/PATH.rb
+++ b/Library/Homebrew/PATH.rb
@@ -59,6 +59,7 @@ class PATH
 
   def existing
     existing_path = select(&File.method(:directory?))
+    # return nil instead of empty PATH, to unset environment variables
     existing_path unless existing_path.empty?
   end
 
diff --git a/Library/Homebrew/test/PATH_spec.rb b/Library/Homebrew/test/PATH_spec.rb
index b20a3d3ac..68233c23c 100644
--- a/Library/Homebrew/test/PATH_spec.rb
+++ b/Library/Homebrew/test/PATH_spec.rb
@@ -107,5 +107,9 @@ describe PATH do
       expect(path.existing.to_ary).to eq(["/path1"])
       expect(path.to_ary).to eq(["/path1", "/path2"])
     end
+
+    it "returns nil instead of an empty #{described_class}" do
+      expect(described_class.new.existing).to be nil
+    end
   end
 end