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]

2 files changed, 21 insertions, 4 deletions

diff --git a/spec/ruby/core/main/private_spec.rb b/spec/ruby/core/main/private_spec.rb
index 78c5d287d4..cac0645b40 100644
--- a/spec/ruby/core/main/private_spec.rb
+++ b/spec/ruby/core/main/private_spec.rb
@@ -32,8 +32,16 @@ describe "main#private" do
     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
diff --git a/spec/ruby/core/main/public_spec.rb b/spec/ruby/core/main/public_spec.rb
index bfc27a9e80..91f045dbab 100644
--- a/spec/ruby/core/main/public_spec.rb
+++ b/spec/ruby/core/main/public_spec.rb
@@ -32,10 +32,19 @@ describe "main#public" do
     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