Make Module#{public,private,protected,module_function} return arguments

Previously, each of these methods returned self, but it is
more useful to return arguments, to allow for simpler method
decorators, such as:

```ruby
cached private def foo; some_long_calculation; end
```

Where cached sets up caching for the method.

For each of these methods, the following behavior is used:

1) No arguments returns nil
2) Single argument is returned
3) Multiple arguments are returned as an array

The single argument case is really the case we are trying to
optimize for, for the same reason that def was changed to return
a symbol for the method.

Idea and initial patch from Herwin Quarantainenet.

Implements [Feature #12495]

core/main/private_spec.rb

@@ -32,8 +32,16 @@
     end
   end
 
-  it "returns Object" do
-    eval("private :main_public_method", TOPLEVEL_BINDING).should equal(Object)
+  ruby_version_is ''...'3.1' do
+    it "returns Object" do
+      eval("private :main_public_method", TOPLEVEL_BINDING).should equal(Object)
+    end
+  end
+
+  ruby_version_is '3.1' do
+    it "returns argument" do
+      eval("private :main_public_method", TOPLEVEL_BINDING).should equal(:main_public_method)
+    end
   end
 
   it "raises a NameError when at least one of given method names is undefined" do

core/main/public_spec.rb

@@ -32,10 +32,19 @@
     end
   end
 
-  it "returns Object" do
-    eval("public :main_private_method", TOPLEVEL_BINDING).should equal(Object)
+  ruby_version_is ''...'3.1' do
+    it "returns Object" do
+      eval("public :main_private_method", TOPLEVEL_BINDING).should equal(Object)
+    end
   end
 
+  ruby_version_is '3.1' do
+    it "returns argument" do
+      eval("public :main_private_method", TOPLEVEL_BINDING).should equal(:main_private_method)
+    end
+  end
+
+
   it "raises a NameError when given an undefined name" do
     -> do
       eval "public :main_undefined_method", TOPLEVEL_BINDING

core/module/module_function_spec.rb

@@ -38,14 +38,23 @@ def test3() end
     m.respond_to?(:test3).should == false
   end
 
-  it "returns the current module" do
-    x = nil
-    m = Module.new do
-      def test()  end
-      x = module_function :test
+  ruby_version_is ""..."3.1" do
+    it "returns self" do
+      Module.new do
+        def foo; end
+        module_function(:foo).should equal(self)
+      end
     end
+  end
 
-    x.should == m
+  ruby_version_is "3.1" do
+    it "returns argument or arguments if given" do
+      Module.new do
+        def foo; end
+        module_function(:foo).should equal(:foo)
+        module_function(:foo, :foo).should == [:foo, :foo]
+      end
+    end
   end
 
   it "creates an independent copy of the method, not a redirect" do
@@ -160,13 +169,20 @@ def test2() end
     m.respond_to?(:test2).should == true
   end
 
-  it "returns the current module" do
-    x = nil
-    m = Module.new {
-      x = module_function
-    }
+  ruby_version_is ""..."3.1" do
+    it "returns self" do
+      Module.new do
+        module_function.should equal(self)
+      end
+    end
+  end
 
-    x.should == m
+  ruby_version_is "3.1" do
+    it "returns nil" do
+      Module.new do
+        module_function.should equal(nil)
+      end
+    end
   end
 
   it "stops creating module functions if the body encounters another toggle " \

core/module/private_spec.rb

@@ -38,11 +38,25 @@ class << obj
                   :module_specs_public_method_on_object_for_kernel_private)
   end
 
-  it "returns self" do
-    (class << Object.new; self; end).class_eval do
-      def foo; end
-      private(:foo).should equal(self)
-      private.should equal(self)
+  ruby_version_is ""..."3.1" do
+    it "returns self" do
+      (class << Object.new; self; end).class_eval do
+        def foo; end
+        private(:foo).should equal(self)
+        private.should equal(self)
+      end
+    end
+  end
+
+  ruby_version_is "3.1" do
+    it "returns argument or arguments if given" do
+      (class << Object.new; self; end).class_eval do
+        def foo; end
+        private(:foo).should equal(:foo)
+        private([:foo, :foo]).should == [:foo, :foo]
+        private(:foo, :foo).should == [:foo, :foo]
+        private.should equal(nil)
+      end
     end
   end
 

core/module/protected_spec.rb

@@ -39,11 +39,25 @@ class << ModuleSpecs::Parent
                   :module_specs_public_method_on_object_for_kernel_protected)
   end
 
-  it "returns self" do
-    (class << Object.new; self; end).class_eval do
-      def foo; end
-      protected(:foo).should equal(self)
-      protected.should equal(self)
+  ruby_version_is ""..."3.1" do
+    it "returns self" do
+      (class << Object.new; self; end).class_eval do
+        def foo; end
+        protected(:foo).should equal(self)
+        protected.should equal(self)
+      end
+    end
+  end
+
+  ruby_version_is "3.1" do
+    it "returns argument or arguments if given" do
+      (class << Object.new; self; end).class_eval do
+        def foo; end
+        protected(:foo).should equal(:foo)
+        protected([:foo, :foo]).should == [:foo, :foo]
+        protected(:foo, :foo).should == [:foo, :foo]
+        protected.should equal(nil)
+      end
     end
   end
 

core/module/public_spec.rb

@@ -27,12 +27,25 @@
                   :module_specs_private_method_on_object_for_kernel_public)
   end
 
-  it "returns self" do
-    (class << Object.new; self; end).class_eval do
-      def foo; end
-      private :foo
-      public(:foo).should equal(self)
-      public.should equal(self)
+  ruby_version_is ""..."3.1" do
+    it "returns self" do
+      (class << Object.new; self; end).class_eval do
+        def foo; end
+        public(:foo).should equal(self)
+        public.should equal(self)
+      end
+    end
+  end
+
+  ruby_version_is "3.1" do
+    it "returns argument or arguments if given" do
+      (class << Object.new; self; end).class_eval do
+        def foo; end
+        public(:foo).should equal(:foo)
+        public([:foo, :foo]).should == [:foo, :foo]
+        public(:foo, :foo).should == [:foo, :foo]
+        public.should equal(nil)
+      end
     end
   end