replace_gem

# File lib/bundler/rubygems_integration.rb, line 370
    def replace_gem(specs, specs_by_name)
      reverse_rubygems_kernel_mixin

      executables = nil

      kernel = (class << ::Kernel; self; end)
      [kernel, ::Kernel].each do |kernel_class|
        redefine_method(kernel_class, :gem) do |dep, *reqs|
          executables ||= specs.map(&:executables).flatten if ::Bundler.rubygems.binstubs_call_gem?
          if executables && executables.include?(File.basename(caller.first.split(":").first))
            break
          end

          reqs.pop if reqs.last.is_a?(Hash)

          unless dep.respond_to?(:name) && dep.respond_to?(:requirement)
            dep = Gem::Dependency.new(dep, reqs)
          end

          if spec = specs_by_name[dep.name]
            return true if dep.matches_spec?(spec)
          end

          message = if spec.nil?
            "#{dep.name} is not part of the bundle."              " Add it to your #{Bundler.default_gemfile.basename}."
          else
            "can't activate #{dep}, already activated #{spec.full_name}. "              "Make sure all dependencies are added to Gemfile."
          end

          e = Gem::LoadError.new(message)
          e.name = dep.name
          if e.respond_to?(:requirement=)
            e.requirement = dep.requirement
          elsif e.respond_to?(:version_requirement=)
            e.version_requirement = dep.requirement
          end
          raise e
        end

        # backwards compatibility shim, see https://github.com/bundler/bundler/issues/5102
        kernel_class.send(:public, :gem) if Bundler.feature_flag.setup_makes_kernel_gem_public?
      end
    end