Implicitly set responder on partial mocks

We now automatically set a responder on mock object which are used for
partial mocks.

Having made the change above, I had to set include_all to true for the
call to Object#respond_to? in Mock#check_responder_responds_to in order
to fix a load of broken tests.

The legacy_behaviour_for_array_flatten condition in
Mock#check_responder_responds_to is needed to avoid a regression of #580
in Ruby < v2.3.

Hopefully this is a small step towards having
Configuration.prevent(:stubbing_non_existent_method) check Method#arity
and/or Method#parameters (#149) and rationalising
Configuration.stubbing_non_existent_method= & Mock#responds_like (#531).

lib/mocha/class_methods.rb

@@ -12,7 +12,7 @@ def initialize(klass)
 
       def mocha(instantiate = true)
         if instantiate
-          @mocha ||= Mocha::Mockery.instance.mock_impersonating_any_instance_of(@stubba_object)
+          @mocha ||= Mocha::Mockery.instance.mock_impersonating_any_instance_of(@stubba_object).responds_like_instance_of(@stubba_object)
         else
           defined?(@mocha) ? @mocha : nil
         end

lib/mocha/mock.rb

@@ -8,6 +8,7 @@
 require 'mocha/parameters_matcher'
 require 'mocha/argument_iterator'
 require 'mocha/expectation_error_factory'
+require 'mocha/ruby_version'
 
 module Mocha
   # Traditional mock object.
@@ -381,9 +382,13 @@ def raise_unexpected_invocation_error(invocation, matching_expectation)
     end
 
     def check_responder_responds_to(symbol)
-      if @responder && !@responder.respond_to?(symbol) # rubocop:disable Style/GuardClause
-        raise NoMethodError, "undefined method `#{symbol}' for #{mocha_inspect} which responds like #{@responder.mocha_inspect}"
-      end
+      return unless @responder
+
+      legacy_behaviour_for_array_flatten = !RUBY_V23_PLUS && !@responder.respond_to?(symbol) && (symbol == :to_ary)
+
+      return if @responder.respond_to?(symbol, true) && !legacy_behaviour_for_array_flatten
+
+      raise NoMethodError, "undefined method `#{symbol}' for #{mocha_inspect} which responds like #{@responder.mocha_inspect}"
     end
 
     def check_expiry

lib/mocha/object_methods.rb

@@ -14,7 +14,7 @@ module ObjectMethods
     # @private
     def mocha(instantiate = true)
       if instantiate
-        @mocha ||= Mocha::Mockery.instance.mock_impersonating(self)
+        @mocha ||= Mocha::Mockery.instance.mock_impersonating(self).responds_like(self)
       else
         defined?(@mocha) ? @mocha : nil
       end

lib/mocha/ruby_version.rb

@@ -1,3 +1,4 @@
 module Mocha
+  RUBY_V23_PLUS = Gem::Version.new(RUBY_VERSION.dup) >= Gem::Version.new('2.3')
   RUBY_V27_PLUS = Gem::Version.new(RUBY_VERSION.dup) >= Gem::Version.new('2.7')
 end

test/acceptance/responds_like_test.rb

@@ -131,14 +131,14 @@ def foo; end
       assert_passed(test_result)
     end
 
-    def test_mock_which_responds_like_object_with_protected_method_raises_no_method_error_when_method_is_not_stubbed
+    def test_mock_which_responds_like_object_with_protected_method_raises_unexpected_invocation_exception_when_method_is_not_stubbed
       object = Class.new do
         def foo; end
         protected :foo
       end.new
       test_result = run_as_test do
         m = mock.responds_like(object)
-        assert_raises(NoMethodError) { m.foo } # vs Minitest::Assertion for public method
+        assert_raises(Minitest::Assertion) { m.foo }
       end
       assert_passed(test_result)
     end
@@ -169,15 +169,15 @@ def foo; end
       assert_passed(test_result)
     end
 
-    def test_mock_which_responds_like_object_with_protected_method_raises_no_method_error_when_method_is_stubbed
+    def test_mock_which_responds_like_object_with_protected_method_does_not_raise_exception_when_method_is_stubbed
       object = Class.new do
         def foo; end
         protected :foo
       end.new
       test_result = run_as_test do
         m = mock.responds_like(object)
         m.stubs(:foo)
-        assert_raises(NoMethodError) { m.foo } # vs no exception for public method
+        assert_nil m.foo
       end
       assert_passed(test_result)
     end
@@ -197,14 +197,14 @@ def foo; end
       assert_passed(test_result)
     end
 
-    def test_mock_which_responds_like_object_with_private_method_raises_no_method_error_when_method_is_not_stubbed
+    def test_mock_which_responds_like_object_with_private_method_raises_unexpected_invocation_exception_when_method_is_not_stubbed
       object = Class.new do
         def foo; end
         private :foo
       end.new
       test_result = run_as_test do
         m = mock.responds_like(object)
-        assert_raises(NoMethodError) { m.foo } # vs Minitest::Assertion for public method
+        assert_raises(Minitest::Assertion) { m.foo }
       end
       assert_passed(test_result)
     end
@@ -235,15 +235,15 @@ def foo; end
       assert_passed(test_result)
     end
 
-    def test_mock_which_responds_like_object_with_private_method_raises_no_method_error_when_method_is_stubbed
+    def test_mock_which_responds_like_object_with_private_method_does_not_raise_exception_when_method_is_stubbed
       object = Class.new do
         def foo; end
         private :foo
       end.new
       test_result = run_as_test do
         m = mock.responds_like(object)
         m.stubs(:foo)
-        assert_raises(NoMethodError) { m.foo } # vs no exception for public method
+        assert_nil m.foo
       end
       assert_passed(test_result)
     end