detect circular constant autoloading

Nowadays circular autoloads do not work, but the user gets a NameError
that says some constant is undefined. That's puzzling, because he is
normally trying to autoload a constant he knows can be autoloaded.
With this check we can give a better error message.

activesupport/lib/active_support/dependencies.rb

@@ -479,10 +479,17 @@ module ActiveSupport #:nodoc:
 
       file_path = search_for_file(path_suffix)
 
-      if file_path && ! loaded.include?(File.expand_path(file_path).sub(/\.rb\z/, '')) # We found a matching file to load
-        require_or_load file_path
-        raise LoadError, "Expected #{file_path} to define #{qualified_name}" unless from_mod.const_defined?(const_name, false)
-        return from_mod.const_get(const_name)
+      if file_path
+        expanded = File.expand_path(file_path)
+        expanded.sub!(/\.rb/, '')
+
+        if loaded.include?(expanded)
+          raise "Circular dependency detected while autoloading constant #{qualified_name}"
+        else
+          require_or_load(expanded)
+          raise LoadError, "Expected #{file_path} to define #{qualified_name}" unless from_mod.const_defined?(const_name, false)
+          return from_mod.const_get(const_name)
+        end
       elsif mod = autoload_module!(from_mod, const_name, qualified_name, path_suffix)
         return mod
       elsif (parent = from_mod.parent) && parent != from_mod &&

activesupport/test/autoloading_fixtures/circular1.rb

@@ -0,0 +1,6 @@
+silence_warnings do
+  Circular2
+end
+
+class Circular1
+end

activesupport/test/autoloading_fixtures/circular2.rb

@@ -0,0 +1,4 @@
+Circular1
+
+class Circular2
+end

activesupport/test/dependencies_test.rb

@@ -145,6 +145,12 @@ class DependenciesTest < ActiveSupport::TestCase
     end
   end
 
+  def test_circular_autoloading_detection
+    with_autoloading_fixtures do
+      assert_raise(RuntimeError, "Circular dependency detected while autoloading constant Circular1") { Circular1 }
+    end
+  end
+
   def test_module_loading
     with_autoloading_fixtures do
       assert_kind_of Module, A