Default I18n.enforce_available_locales to true

We will default this option to true from now on to ensure users properly
handle their list of available locales whenever necessary. This option
was added as a security measure and thus Rails will follow it defaulting
to secure option.

Also improve the handling of I18n config options in its railtie, taking
the new enforce_available_locales option into account, by setting it as
the last one in the process. This ensures no other configuration will
trigger a deprecation warning due to that setting.

activesupport/lib/active_support/i18n_railtie.rb

@@ -8,6 +8,8 @@ class Railtie < Rails::Railtie
     config.i18n.railties_load_path = []
     config.i18n.load_path = []
     config.i18n.fallbacks = ActiveSupport::OrderedOptions.new
+    # Enforce I18n to check the available locales when setting a locale.
+    config.i18n.enforce_available_locales = true
 
     # Set the i18n configuration after initialization since a lot of
     # configuration is still usually done in application initializers.
@@ -31,10 +33,11 @@ def self.initialize_i18n(app)
 
       fallbacks = app.config.i18n.delete(:fallbacks)
 
-      if app.config.i18n.has_key?(:enforce_available_locales)
-        # this option needs to be set before `default_locale=` to work properly.
-        I18n.enforce_available_locales = app.config.i18n.delete(:enforce_available_locales)
-      end
+      # Avoid issues with setting the default_locale by disabling available locales
+      # check while configuring.
+      enforce_available_locales = app.config.i18n.delete(:enforce_available_locales)
+      enforce_available_locales = I18n.enforce_available_locales unless I18n.enforce_available_locales.nil?
+      I18n.enforce_available_locales = false
 
       app.config.i18n.each do |setting, value|
         case setting
@@ -49,6 +52,9 @@ def self.initialize_i18n(app)
 
       init_fallbacks(fallbacks) if fallbacks && validate_fallbacks(fallbacks)
 
+      # Restore avalable locales check so it will take place from now on.
+      I18n.enforce_available_locales = enforce_available_locales
+
       reloader = ActiveSupport::FileUpdateChecker.new(I18n.load_path.dup){ I18n.reload! }
       app.reloaders << reloader
       ActionDispatch::Reloader.to_prepare { reloader.execute_if_updated }

railties/test/application/initializers/i18n_test.rb

@@ -184,14 +184,48 @@ class Foo < ActiveRecord::Base
       assert_fallbacks ca: [:ca, :"es-ES", :es, :'en-US', :en]
     end
 
-    test "config.i18n.enforce_available_locales is set before config.i18n.default_locale is" do
+    test "config.i18n.enforce_available_locales is set to true by default and avoids I18n warnings" do
       add_to_config <<-RUBY
         config.i18n.default_locale = :it
-        config.i18n.enforce_available_locales = true
       RUBY
 
-      assert_raises(I18n::InvalidLocale) do
-        load_app
+      output = capture(:stderr) { load_app }
+      assert_no_match %r{deprecated.*enforce_available_locales}, output
+      assert_equal true, I18n.enforce_available_locales
+
+      assert_raise I18n::InvalidLocale do
+        I18n.locale = :es
+      end
+    end
+
+    test "disable config.i18n.enforce_available_locales" do
+      add_to_config <<-RUBY
+        config.i18n.enforce_available_locales = false
+        config.i18n.default_locale = :fr
+      RUBY
+
+      output = capture(:stderr) { load_app }
+      assert_no_match %r{deprecated.*enforce_available_locales}, output
+      assert_equal false, I18n.enforce_available_locales
+
+      assert_nothing_raised do
+        I18n.locale = :es
+      end
+    end
+
+    test "default config.i18n.enforce_available_locales does not override I18n.enforce_available_locales" do
+      I18n.enforce_available_locales = false
+
+      add_to_config <<-RUBY
+        config.i18n.default_locale = :fr
+      RUBY
+
+      output = capture(:stderr) { load_app }
+      assert_no_match %r{deprecated.*enforce_available_locales}, output
+      assert_equal false, I18n.enforce_available_locales
+
+      assert_nothing_raised do
+        I18n.locale = :es
       end
     end
   end