Verifying null objects only respond to defined methods.

* No additional documentation, since this is the least surprising
  behaviour.

Fixes rspec#392.

Changelog.md

@@ -26,6 +26,8 @@ Enhancements:
 * Yield the receiver to `any_instance` implementation blocks (Sam Phippen).
 * Provide `instance_double` and `class_double` to create verifying doubles,
   ported from `rspec-fire` (Xavier Shay).
+* `as_null_object` on a verifying double only responds to defined methods
+  (Xavier Shay).
 * Improved performance of double creation, particularly those with many
   attributes. (Xavier Shay)
 * Default value of `transfer_nested_constants` option for constant stubbing can

lib/rspec/mocks/verifying_double.rb

@@ -4,11 +4,22 @@
 module RSpec
   module Mocks
 
+    module VerifyingDouble
+      def method_missing(message, *args, &block)
+        # Null object conditional is an optimization. If not a null object,
+        # validity of method expectations will have been checked at definition
+        # time.
+        __mock_proxy.ensure_implemented(message) if null_object?
+        super
+      end
+    end
+
     # A mock providing a custom proxy that can verify the validity of any
     # method stubs or expectations against the public instance methods of the
     # given class.
     class InstanceVerifyingMock
       include TestDouble
+      include VerifyingDouble
 
       def initialize(doubled_module, *args)
         @doubled_module = doubled_module
@@ -32,6 +43,7 @@ def __build_mock_proxy
     # constants to work.
     class ClassVerifyingMock < Module
       include TestDouble
+      include VerifyingDouble
 
       def initialize(doubled_module, *args)
         @doubled_module = doubled_module

lib/rspec/mocks/verifying_proxy.rb

@@ -58,8 +58,6 @@ def method_double
         end
       end
 
-    protected
-
       def ensure_implemented(method_name)
         @doubled_module.when_loaded do |original_module|
           unless original_module.__send__(@method_checker, method_name)

spec/rspec/mocks/verifying_double_spec.rb

@@ -107,6 +107,13 @@ def prevents(&block)
             o = instance_double(LoadedClass, :defined_instance_method => 1)
             expect(o.defined_instance_method).to eq(1)
           end
+
+          it 'only allows defined methods for null objects' do
+            o = instance_double('LoadedClass').as_null_object
+
+            expect(o.defined_instance_method).to eq(o)
+            prevents { o.undefined_method }
+          end
         end
       end
 
@@ -186,6 +193,13 @@ def prevents(&block)
               expect(dbl1).to receive(:undefined_class_method)
             }
           end
+
+          it 'only allows defined methods for null objects' do
+            o = class_double('LoadedClass').as_null_object
+
+            expect(o.defined_class_method).to eq(o)
+            prevents { o.undefined_method }
+          end
         end
       end