Do not raise an exception if an invalid route was generated automatically.

2 files changed, 12 insertions, 3 deletions

diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 88e422c05d..4d83c6dee1 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -1286,7 +1286,7 @@ module ActionDispatch
             action = nil
           end
 
-          if !options.fetch(:as) { true }
+          if !options.fetch(:as, true)
             options.delete(:as)
           else
             options[:as] = name_for_action(options[:as], action)
@@ -1472,8 +1472,16 @@ module ActionDispatch
               [name_prefix, member_name, prefix]
             end
 
-            candidate = name.select(&:present?).join("_").presence
-            candidate unless as.nil? && @set.routes.find { |r| r.name == candidate }
+            if candidate = name.select(&:present?).join("_").presence
+              # If a name was not explicitly given, we check if it is valid
+              # and return nil in case it isn't. Otherwise, we pass the invalid name
+              # forward so the underlying router engine treats it and raises an exception.
+              if as.nil?
+                candidate unless @set.routes.find { |r| r.name == candidate } || candidate !~ /\A[_a-z]/i
+              else
+                candidate
+              end
+            end
           end
       end
 
diff --git a/actionpack/test/dispatch/routing_test.rb b/actionpack/test/dispatch/routing_test.rb
index bc4e2e31c8..5325f81364 100644
--- a/actionpack/test/dispatch/routing_test.rb
+++ b/actionpack/test/dispatch/routing_test.rb
@@ -91,6 +91,7 @@ class TestRoutingMapper < ActionDispatch::IntegrationTest
       match "/local/:action", :controller => "local"
 
       match "/projects/status(.:format)"
+      match "/404", :to => lambda { |env| [404, {"Content-Type" => "text/plain"}, ["NOT FOUND"]] }
 
       constraints(:ip => /192\.168\.1\.\d\d\d/) do
         get 'admin' => "queenbee#index"