[ruby/rdoc] Singleton method visibility should be isolated

Each singleton method definition of the form `def recv.method` has
visibility separate from the outer scope and is set to `public` by
default.

https://github.com/ruby/rdoc/commit/810913a7ea

lib/rdoc/parser/ruby.rb

@@ -1453,6 +1453,12 @@ class RDoc::Parser::Ruby < RDoc::Parser
     meth = RDoc::AnyMethod.new get_tkread, name
     look_for_directives_in meth, comment
     meth.singleton = single == SINGLE ? true : singleton
+    if singleton
+      # `current_line_visibility' is useless because it works against
+      # the normal method named as same as the singleton method, after
+      # the latter was defined.  Of course these are different things.
+      container.current_line_visibility = :public
+    end
 
     record_location meth
     meth.line   = line_no

test/rdoc/test_rdoc_context.rb

@@ -927,6 +927,8 @@ class TestRDocContext < XrefTestCase
     assert_equal :private, @c6.find_method_named('priv6').visibility
     assert_equal :protected, @c6.find_method_named('prot6').visibility
     assert_equal :public, @c6.find_method_named('pub6').visibility
+    assert_equal :public, @c6.find_method_named('s_pub1').visibility
+    assert_equal :public, @c6.find_method_named('s_pub3').visibility
   end
 
   def util_visibilities

test/rdoc/xref_data.rb

@@ -74,6 +74,7 @@ class C6
   def priv4() end
   public def pub5() end
   def priv5() end
+  def self.s_pub1() end
 
   protected
   private def priv6() end
@@ -82,6 +83,7 @@ class C6
   def prot5() end
   public def pub6() end
   def prot6() end
+  def self.s_pub3() end
 end
 
 class C7