Do not permanently overwrite Parameters#[]

When stubbing the `ActionController::Parameters#[]` method, do not
permanently overwrite it. Restore the method after verifying simulating
the controller action.

lib/shoulda/matchers/action_controller/strong_parameters_matcher.rb

@@ -65,9 +65,13 @@ module Shoulda
 
         def simulate_controller_action
           ensure_action_and_verb_present!
-          stubbed_model_attributes
+          stub_model_attributes
 
-          context.send(verb, action)
+          begin
+            context.send(verb, action)
+          ensure
+            unstub_model_attributes
+          end
 
           verify_permit_call
         end
@@ -84,17 +88,26 @@ module Shoulda
           attributes & @model_attrs.shoulda_permitted_params
         end
 
-        def stubbed_model_attributes
+        def stub_model_attributes
           @model_attrs = self.class.stubbed_parameters_class.new(arbitrary_attributes)
 
           local_model_attrs = @model_attrs
           ::ActionController::Parameters.class_eval do
+            alias_method :'shoulda_original_[]', :[]
+
             define_method :[] do |*args|
               local_model_attrs
             end
           end
         end
 
+        def unstub_model_attributes
+          ::ActionController::Parameters.class_eval do
+            alias_method :[], :'shoulda_original_[]'
+            undef_method :'shoulda_original_[]'
+          end
+        end
+
         def ensure_action_and_verb_present!
           if action.blank?
             raise ActionNotDefinedError

spec/shoulda/matchers/action_controller/strong_parameters_matcher_spec.rb

@@ -102,6 +102,35 @@ describe Shoulda::Matchers::ActionController::StrongParametersMatcher do
       expect { matcher.matches? }
         .to raise_error(Shoulda::Matchers::ActionController::StrongParametersMatcher::VerbNotDefinedError)
     end
+
+    context 'Stubbing ActionController::Parameters#[]' do
+      it "does not permanently stub []" do
+        controller_for_resource_with_strong_parameters(action: :create) do
+          params.require(:user).permit(:name)
+        end
+
+        described_class.new([:name]).in_context(self).for(:create).matches?
+
+        param = ActionController::Parameters.new(name: 'Ralph')[:name]
+        expect(param.singleton_class).not_to include(
+          Shoulda::Matchers::ActionController::StrongParametersMatcher::StubbedParameters
+        )
+      end
+
+      it 'prevents permanently overwriting [] on error' do
+        stub_controller_with_exception
+
+        begin
+          described_class.new([:name]).in_context(self).for(:create).matches?
+        rescue SimulatedError
+        end
+
+        param = ActionController::Parameters.new(name: 'Ralph')[:name]
+        expect(param.singleton_class).not_to include(
+          Shoulda::Matchers::ActionController::StrongParametersMatcher::StubbedParameters
+        )
+      end
+    end
   end
 
   describe "failure message" do
@@ -157,4 +186,20 @@ describe Shoulda::Matchers::ActionController::StrongParametersMatcher do
       end
     end
   end
+
+  def stub_controller_with_exception
+    controller = define_controller('Examples') do
+      def create
+        raise SimulatedError
+      end
+    end
+
+    setup_rails_controller_test(controller)
+
+    define_routes do
+      get 'examples', to: 'examples#create'
+    end
+  end
+
+  class SimulatedError < StandardError; end
 end