Show real LoadError on helpers require

When helper try to require missing file rails will throw exception about
missing helper.

  # app/helpers/my_helper.rb

  require 'missing'

  module MyHelper
  end

And when we try do load helper

  class ApplicationController
    helper :my
  end

Rails will throw exception. This is wrong because there is a helper
file.

  Missing helper file helpers/my_helper.rb

Now when helper try to require non-existed file rails will throw proper
exception.

  No such file to load -- missing

5 files changed, 38 insertions, 1 deletions

diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index ea1d090bc2..3105dbcc42 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Fix an issue where rails raise exception about missing helper where it
+    should throw `LoadError`. When helper file exists and only loaded file from
+    this helper does not exist rails should throw LoadError instead of
+    `MissingHelperError`.
+
+    *Piotr Niełacny*
+
 *   Fix `ActionDispatch::ParamsParser#parse_formatted_parameters` to rewind body input stream on
     parsing json params.
 
diff --git a/actionpack/lib/abstract_controller/helpers.rb b/actionpack/lib/abstract_controller/helpers.rb
index 5ae8c6c3b0..a928205963 100644
--- a/actionpack/lib/abstract_controller/helpers.rb
+++ b/actionpack/lib/abstract_controller/helpers.rb
@@ -150,7 +150,12 @@ module AbstractController
           @error = error
           @path  = "helpers/#{path}.rb"
           set_backtrace error.backtrace
-          super("Missing helper file helpers/%s.rb" % path)
+
+          if error.path =~ /^#{path}(\.rb)?$/
+            super("Missing helper file helpers/%s.rb" % path)
+          else
+            raise error
+          end
         end
       end
 
diff --git a/actionpack/test/abstract/.helper_test.rb.swp b/actionpack/test/abstract/.helper_test.rb.swp
new file mode 100644
index 0000000000..d029f89288
--- /dev/null
+++ b/actionpack/test/abstract/.helper_test.rb.swp
diff --git a/actionpack/test/abstract/helper_test.rb b/actionpack/test/abstract/helper_test.rb
index 7960e5b55b..2cc27fbecd 100644
--- a/actionpack/test/abstract/helper_test.rb
+++ b/actionpack/test/abstract/helper_test.rb
@@ -48,6 +48,14 @@ module AbstractController
       end
     end
 
+    class AbstractInvalidHelpers < AbstractHelpers
+      include ActionController::Helpers
+
+      path = File.join(File.expand_path('../../fixtures', __FILE__), "helpers_missing")
+      $:.unshift(path)
+      self.helpers_path = path
+    end
+
     class TestHelpers < ActiveSupport::TestCase
       def setup
         @controller = AbstractHelpers.new
@@ -97,5 +105,17 @@ module AbstractController
         assert_equal "Hello Default", @controller.response_body
       end
     end
+
+    class InvalidHelpersTest < ActiveSupport::TestCase
+      def test_controller_raise_error_about_real_require_problem
+        e = assert_raise(LoadError) { AbstractInvalidHelpers.helper(:invalid_require) }
+        assert_equal "No such file to load -- very_invalid_file_name", e.message
+      end
+
+      def test_controller_raise_error_about_missing_helper
+        e = assert_raise(Helpers::ClassMethods::MissingHelperError) { AbstractInvalidHelpers.helper(:missing) }
+        assert_equal "Missing helper file helpers/missing_helper.rb", e.message
+      end
+    end
   end
 end
diff --git a/actionpack/test/fixtures/helpers_missing/invalid_require_helper.rb b/actionpack/test/fixtures/helpers_missing/invalid_require_helper.rb
new file mode 100644
index 0000000000..d8801e54d5
--- /dev/null
+++ b/actionpack/test/fixtures/helpers_missing/invalid_require_helper.rb
@@ -0,0 +1,5 @@
+require 'very_invalid_file_name'
+
+module InvalidRequireHelper
+end
+