FEATURE: unlisted *only* means not listed in topic lists

Remove security by obscurity feature that tries for exact slug match

If you need to hide a topic from users either move to a secure category
or convert to a PM

app/controllers/topics_controller.rb

@@ -67,7 +67,7 @@ class TopicsController < ApplicationController
     # up that particular number
     if params[:id] && params[:id] =~ /^\d+[^\d\\]+$/
       topic = Topic.find_by(slug: params[:id].downcase)
-      return redirect_to_correct_topic(topic, opts[:post_number]) if topic && topic.visible
+      return redirect_to_correct_topic(topic, opts[:post_number]) if topic
     end
 
     if opts[:print]
@@ -84,7 +84,7 @@ class TopicsController < ApplicationController
     rescue Discourse::NotFound
       if params[:id]
         topic = Topic.find_by(slug: params[:id].downcase)
-        return redirect_to_correct_topic(topic, opts[:post_number]) if topic && topic.visible
+        return redirect_to_correct_topic(topic, opts[:post_number]) if topic
       end
       raise Discourse::NotFound
     end
@@ -96,10 +96,6 @@ class TopicsController < ApplicationController
 
     discourse_expires_in 1.minute
 
-    if !@topic_view.topic.visible && @topic_view.topic.slug != params[:slug] && !request.format.json?
-      raise Discourse::NotFound
-    end
-
     if slugs_do_not_match || (!request.format.json? && params[:slug].nil?)
       redirect_to_correct_topic(@topic_view.topic, opts[:post_number])
       return

spec/controllers/topics_controller_spec.rb

@@ -568,7 +568,14 @@ describe TopicsController do
   end
 
   describe 'show unlisted' do
-    it 'returns 404 unless exact correct URL' do
+    it 'returns 301 even if slug does not match URL' do
+      # in the past we had special logic for unlisted topics
+      # we would require slug unless you made a json call
+      # this was not really providing any security
+      #
+      # we no longer require a topic be visible to perform url correction
+      # if you need to properly hide a topic for users use a secure category
+      # or a PM
       topic = Fabricate(:topic, visible: false)
       Fabricate(:post, topic: topic)
 
@@ -576,10 +583,10 @@ describe TopicsController do
       expect(response).to be_success
 
       xhr :get, :show, topic_id: topic.id, slug: "just-guessing"
-      expect(response.code).to eq("404")
+      expect(response.code).to eq("301")
 
       xhr :get, :show, id: topic.slug
-      expect(response.code).to eq("404")
+      expect(response.code).to eq("301")
     end
   end