Raise invalid files in test, ensure file only loads for one locale

lib/i18n/backend/lazy_loadable.rb

@@ -118,14 +118,17 @@ def available_locales
 
       # Load translations from files that belong to the current locale.
       def init_translations
+        invalid_files = []
+
         if lazy_load?
-          load_translations(filenames_for_current_locale)
+          load_translations(filenames_for_current_locale) { |file, translations| invalid_files << file unless file_named_correctly?(file, translations) }
           initialized_locales[I18n.locale] = true
         else
-          super
-          filenames_named_incorrectly = I18n.load_path.reject { |path| file_named_correctly?(path) }
-          raise InvalidFilenames.new(filenames_named_incorrectly) unless filenames_named_incorrectly.empty?
+          load_translations { |file, translations| invalid_files << file unless file_named_correctly?(file, translations) }
+          @initialized = true
         end
+
+        raise InvalidFilenames.new(invalid_files) unless invalid_files.empty?
       end
 
       def initialized_locales
@@ -144,9 +147,13 @@ def supported_extension?(path)
         path.end_with?(*SUPPORTED_EXTENSIONS)
       end
 
-      def file_named_correctly?(path)
-        extracted_locale = LocaleExtractor.locale_from_path(path)
-        available_locales.include?(extracted_locale)
+      # Checks if a filename is named in correspondence to the translations it loaded.
+      # The locale extracted from the path must be the single locale loaded in the translations.
+      def file_named_correctly?(path, translations)
+        locales = translations.keys.map(&:to_sym)
+        return false unless locales.one?
+
+        LocaleExtractor.locale_from_path(path) == locales.first
       end
     end
   end

test/backend/lazy_loadable_test.rb

@@ -111,6 +111,30 @@ def setup
     end
   end
 
+  test "lazy mode: raises error if translations loaded don't correspond to locale extracted from filename" do
+    filename = ["en_", ".yml"]
+    file_contents = { fr: { dog: "chien" } }.to_yaml
+
+    with_lazy_mode do
+      with_translation_file_in_load_path(filename, nil, file_contents) do |file_path|
+        exception = assert_raises(I18n::InvalidFilenames) { I18n.t("foo.bar") }
+        assert_equal "Locales cannot be extracted from the following paths: #{[file_path]}", exception.message
+      end
+    end
+  end
+
+  test "lazy mode: raises error if translations for more than one locale are loaded from a single file" do
+    filename = ["en_", ".yml"]
+    file_contents = { en: { alice: "bob" },  fr: { dog: "chien" } }.to_yaml
+
+    with_lazy_mode do
+      with_translation_file_in_load_path(filename, nil, file_contents) do |file_path|
+        exception = assert_raises(I18n::InvalidFilenames) { I18n.t("foo.bar") }
+        assert_equal "Locales cannot be extracted from the following paths: #{[file_path]}", exception.message
+      end
+    end
+  end
+
   test "eager mode: load all translations, irrespective of locale" do
     with_eager_mode do
       @backend.reload!