Fixing an issue with a spoiling of the object singleton classes

lib/minitest/mock.rb

@@ -277,10 +277,13 @@ class Object
 
   def stub name, val_or_callable, *block_args, **block_kwargs, &block
     new_name = "__minitest_stub__#{name}"
+    singleton_has_stubbing_method = singleton_methods.map(&:to_s).include?( name.to_s )
 
     metaclass = class << self; self; end
 
     if respond_to? name and not methods.map(&:to_s).include? name.to_s then
+      # this mean that 'name' is a public method which is expected
+      # to be defined dynamically somewhere inside ancestors chain
       metaclass.send :define_method, name do |*args, **kwargs|
         super(*args, **kwargs)
       end
@@ -320,8 +323,8 @@ def stub name, val_or_callable, *block_args, **block_kwargs, &block
 
     block[self]
   ensure
-    metaclass.send :undef_method, name
-    metaclass.send :alias_method, name, new_name
-    metaclass.send :undef_method, new_name
+    metaclass.send :remove_method, name
+    metaclass.send :alias_method, name, new_name if singleton_has_stubbing_method
+    metaclass.send :remove_method, new_name
   end
 end

test/minitest/test_minitest_mock.rb

@@ -762,6 +762,41 @@ def test_mock_with_yield_kwargs
 
   alias test_stub_value__old test_stub_value # TODO: remove/rename
 
+  def test_instance_singleton_class_is_not_dirty
+    _class = Class.new do
+      def clean; :dirty end
+    end
+
+    _module = Module.new do
+      def clean; :clean end
+    end
+
+    shirt, t_shirt = _class.new, _class.new
+
+    assert_equal( t_shirt.clean, :dirty )
+    assert_equal( shirt.clean, :dirty )
+
+    shirt.stub(:clean, :clean) do |s|
+      assert_equal( s.clean, :clean )
+    end
+
+    _class.prepend(_module)
+
+    assert_equal( t_shirt.clean, :clean )
+    assert_equal( shirt.clean, :clean )
+  end
+
+  def test_class_singleton_class_will_not_loose_its_method
+    _class = Class.new do
+      def self.clean; :dirty end
+    end
+
+    _class.stub(:clean, :clean) do |s|
+      assert_equal( s.clean, :clean )
+    end
+    # method is still present and unchanged
+    assert_equal( _class.clean, :dirty )
+  end
   ## Permutation Sets:
 
   # [:value, :lambda]