Add support for instance_of argument matcher

Author: wata727

lib/super_diff/rspec.rb

@@ -68,6 +68,12 @@ def self.an_instance_of_something?(value)
         value.base_matcher.is_a?(::RSpec::Matchers::BuiltIn::BeAnInstanceOf)
     end
 
+    # HINT: `an_instance_of` is a matcher in the rspec-expectations gem.
+    #       `instance_of` is an argument matcher in the rspec-mocks gem.
+    def self.instance_of_something?(value)
+      value.is_a?(::RSpec::Mocks::ArgumentMatchers::InstanceOf)
+    end
+
     def self.a_value_within_something?(value)
       fuzzy_object?(value) &&
         value.base_matcher.is_a?(::RSpec::Matchers::BuiltIn::BeWithin)

lib/super_diff/rspec/object_inspection/inspectors/instance_of.rb

@@ -4,15 +4,20 @@ module ObjectInspection
       module Inspectors
         class InstanceOf < SuperDiff::ObjectInspection::Inspectors::Base
           def self.applies_to?(value)
-            SuperDiff::RSpec.an_instance_of_something?(value)
+            SuperDiff::RSpec.an_instance_of_something?(value) || SuperDiff::RSpec.instance_of_something?(value)
           end
 
           protected
 
           def inspection_tree
             SuperDiff::ObjectInspection::InspectionTree.new do
-              add_text do |aliased_matcher|
-                "#<an instance of #{aliased_matcher.expected}>"
+              add_text do |value|
+                klass = if SuperDiff::RSpec.an_instance_of_something?(value)
+                          value.expected
+                        else
+                          value.instance_variable_get(:@klass)
+                        end
+                "#<an instance of #{klass}>"
               end
             end
           end