ActiveSupport::Dependencies and const_get(const, false)

a.rb

class A
  def self.foo
    puts "::A#foo"
  end
end

b.rb

require "active_support/dependencies"

ActiveSupport::Dependencies.autoload_paths << File.dirname(__FILE__)

class B
end

B.const_get(:A, false).foo rescue p $!  # => ::A#foo
A.foo rescue p $!                   # => ::A#foo
B.const_get(:A, false).foo rescue p $!  # => #<NameError: uninitialized constant B::A>

Gemfile

source "https://rubygems.org"

#gem "activesupport", "~> 3.2.18"
gem "activesupport", "~> 4.1.1"