Add spec for conflicts_with cask.

Library/Homebrew/test/cask/conflicts_with_spec.rb

@@ -0,0 +1,23 @@
+describe "conflicts_with", :cask do
+  describe "conflicts_with cask" do
+    let(:local_caffeine) {
+      Hbc::CaskLoader.load_from_file(TEST_FIXTURE_DIR/"cask/Casks/local-caffeine.rb")
+    }
+
+    let(:with_conflicts_with) {
+      Hbc::CaskLoader.load_from_file(TEST_FIXTURE_DIR/"cask/Casks/with-conflicts-with.rb")
+    }
+
+    it "installs the dependency of a Cask and the Cask itself", :focus do
+      Hbc::Installer.new(local_caffeine).install
+
+      expect(local_caffeine).to be_installed
+
+      expect {
+        Hbc::Installer.new(with_conflicts_with).install
+      }.to raise_error(Hbc::CaskConflictError, "Cask 'with-conflicts-with' conflicts with 'local-caffeine'.")
+
+      expect(with_conflicts_with).not_to be_installed
+    end
+  end
+end

Library/Homebrew/test/support/fixtures/cask/Casks/with-conflicts-with.rb

@@ -5,7 +5,7 @@ cask 'with-conflicts-with' do
   url "file://#{TEST_FIXTURE_DIR}/cask/caffeine.zip"
   homepage 'http://example.com/with-conflicts-with'
 
-  conflicts_with formula: 'unar'
+  conflicts_with cask: 'local-caffeine'
 
   app 'Caffeine.app'
 end