3 files changed, 37 insertions, 12 deletions
diff --git a/Library/Homebrew/cask/lib/hbc/dsl/conflicts_with.rb b/Library/Homebrew/cask/lib/hbc/dsl/conflicts_with.rb
index 948348239..dfaa55a5c 100644
--- a/Library/Homebrew/cask/lib/hbc/dsl/conflicts_with.rb
+++ b/Library/Homebrew/cask/lib/hbc/dsl/conflicts_with.rb
@@ -10,25 +10,20 @@ module Hbc
         :java,
       ]
 
-      attr_accessor(*VALID_KEYS)
-      attr_accessor :pairs
+      attr_reader *VALID_KEYS
 
       def initialize(pairs = {})
         @pairs = pairs
+
+        VALID_KEYS.each do |key|
+          instance_variable_set("@#{key}", Set.new)
+        end
+
         pairs.each do |key, value|
           raise "invalid conflicts_with key: '#{key.inspect}'" unless VALID_KEYS.include?(key)
-          writer_method = "#{key}=".to_sym
-          send(writer_method, value)
+          instance_variable_set("@#{key}", instance_variable_get("@#{key}").merge([*value]))
         end
       end
-
-      def to_yaml
-        @pairs.to_yaml
-      end
-
-      def to_s
-        @pairs.inspect
-      end
     end
   end
 end
diff --git a/Library/Homebrew/cask/lib/hbc/exceptions.rb b/Library/Homebrew/cask/lib/hbc/exceptions.rb
index 1a246be65..f7f9e43b6 100644
--- a/Library/Homebrew/cask/lib/hbc/exceptions.rb
+++ b/Library/Homebrew/cask/lib/hbc/exceptions.rb
@@ -17,6 +17,19 @@ module Hbc
     end
   end
 
+  class CaskConflictError < AbstractCaskErrorWithToken
+    attr_reader :conflicting_cask
+
+    def initialize(token, conflicting_cask)
+      super(token)
+      @conflicting_cask = conflicting_cask
+    end
+
+    def to_s
+      "Cask '#{token}' conflicts with '#{conflicting_cask}'."
+    end
+  end
+
   class CaskUnavailableError < AbstractCaskErrorWithToken
     def to_s
       "Cask '#{token}' is unavailable" << (reason.empty? ? "." : ": #{reason}")
diff --git a/Library/Homebrew/cask/lib/hbc/installer.rb b/Library/Homebrew/cask/lib/hbc/installer.rb
index 53210ed4b..37cc4e561 100644
--- a/Library/Homebrew/cask/lib/hbc/installer.rb
+++ b/Library/Homebrew/cask/lib/hbc/installer.rb
@@ -86,6 +86,8 @@ module Hbc
         raise CaskAlreadyInstalledError, @cask
       end
 
+      check_conflicts
+
       print_caveats
       fetch
       uninstall_existing_cask if @reinstall
@@ -98,6 +100,21 @@ module Hbc
       puts summary
     end
 
+    def check_conflicts
+      return unless @cask.conflicts_with
+
+      @cask.conflicts_with.cask.each do |conflicting_cask|
+        begin
+          conflicting_cask = CaskLoader.load(conflicting_cask)
+          if conflicting_cask.installed?
+            raise CaskConflictError.new(@cask, conflicting_cask)
+          end
+        rescue CaskUnavailableError
+          next # Ignore conflicting Casks that do not exist.
+        end
+      end
+    end
+
     def reinstall
       odebug "Hbc::Installer#reinstall"
       @reinstall = true