Run with `rspec action_controller_gem.rb`. Fails as it is, but works with https://github.com/rspec/rspec-rails/pull/1568 applied.

action_controller_gem.rb

begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  gem 'rails', '5.0.0.beta3'
  gem 'rspec-rails', '3.5.0.beta1'
  gem 'rails-controller-testing', '0.1.1'
end

require 'rspec-rails'
require 'action_controller/railtie'

class TestApp < Rails::Application
  config.root = File.dirname(__FILE__)
  config.session_store :cookie_store, key: 'cookie_store_key'
  secrets.secret_token    = 'secret_token'
  secrets.secret_key_base = 'secret_key_base'

  config.logger = Logger.new($stdout)
  Rails.logger  = config.logger

  routes.draw do
    get '/' => 'test#preview'
  end
end

class TestController < ActionController::Base
  include Rails.application.routes.url_helpers

  def show
  end

  def preview
    prepend_view_path(MyResolver.new)

    render action: :show
  end

  class MyResolver < ActionView::Resolver
    def find_all(*args)
      find_templates(*args)
    end

    def find_all_anywhere(*args)
      find_templates(*args)
    end

    private

    def find_templates(name, prefix = nil, partial = false, details = {}, key = nil, locals = [])
      name.prepend("_") if partial
      path = [prefix, name].compact.join("/")
      template = find_template(name, path)

      [template]
    end

    def find_template(name, path)
      ActionView::Template.new(
        "",
        name,
        ->(_template) { %("") },
        virtual_path: path,
        format: :html
      )
    end
  end
end

# ========================

require 'rspec/rails'

RSpec.describe TestController, type: :controller do
  it "GET #preview renders the show action" do
    get :preview

    expect(response).to render_template(:show)
  end
end