Respect Ruby semantics for bare visibility with singleton defs

[unurgunite]

Description

YARD incorrectly marked singleton method definitions (def self.x) as private when a bare private appeared earlier in the class body. This problem was originally described here (#1496). Ruby semantics say a bare access modifier affects only the current def-target:

• class body: affects instance methods only
• inside class << self: affects class methods only
• def self.x in a class body remains public unless:
  • private_class_method is used, or
  • private appears inside class << self

This PR brings YARD in line with Ruby.

Behavior change:

• Bare private/public/protected now only affect methods whose scope matches the current def-target.
• @!visibility directive continues to apply regardless of method scope (as before), since it’s a documentation control.
• Explicit statements private :foo/public :foo are unaffected.
• private_class_method/public_class_method handlers continue to set exact visibilities.

So, the code from the issue now outputs correctly:

require 'yard'

code = <<~CODE
  class T
    def m1
      T.m2
    end

    private

    def self.m2
      print 123
    end

    class << self
      private

      def m3; end
    end
  end
CODE

def detect_private_methods(code)
  YARD.parse_string(code)
  YARD::Registry.all(:class).map do |class_obj|
    class_obj.meths(inherited: false).map do |method_obj|
      if method_obj.scope == :class && method_obj.visibility == :private
        puts "Method #{method_obj.name} is a #{method_obj.visibility} #{method_obj.scope} method!"
      end
    end
  end
end

detect_private_methods(code)
# Method m3 is a private class method!
# => [[nil, nil, nil]]

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).