Modify behaviour for Hash and (maybe nested) Array translations

lib/i18n.rb

@@ -251,12 +251,8 @@ def interpolation_keys(key, **options)
       return [] unless exists?(key, **options.slice(:locale, :scope))
 
       translation = translate(key, **options.slice(:locale, :scope))
-
-      raise I18n::NonStringTranslationError unless translation.is_a?(String)
-      translation
-        .scan(Regexp.union(I18n.config.interpolation_patterns))
-        .flatten
-        .compact
+      interpolation_keys_from_translation(translation)
+        .flatten.compact
     end
 
     # Returns true if a translation exists for a given key, otherwise returns false.
@@ -457,6 +453,17 @@ def normalize_key(key, separator)
           keys
         end
     end
+
+    def interpolation_keys_from_translation(translation)
+      case translation
+      when ::String
+        translation.scan(Regexp.union(I18n.config.interpolation_patterns))
+      when ::Array
+        translation.map { |element| interpolation_keys_from_translation(element) }
+      else
+        []
+      end
+    end
   end
 
   extend Base

lib/i18n/exceptions.rb

@@ -154,6 +154,4 @@ def initialize(file_errors)
       MSG
     end
   end
-
-  class NonStringTranslationError < ArgumentError; end
 end

test/i18n_test.rb

@@ -301,13 +301,18 @@ def setup
     assert_equal [], I18n.interpolation_keys("does-not-exist")
   end
 
-  test "interpolation_keys raises I18n::ArgumentError when non-string argument" do
-    assert_raises(I18n::ArgumentError) { I18n.interpolation_keys(["bad-argument"]) }
+  test "interpolation_keys returns an empty array when nested translation" do
+    store_translations(:en, "example_nested" => { "one" => "One %{foo}", "two" => "Two %{bar}" })
+    assert_equal [], I18n.interpolation_keys("example_nested")
+  end
+
+  test "interpolation_keys returns an array of keys when translation is an Array" do
+    store_translations(:en, "example_array" => ["One %{foo}", ["Two %{bar}", ["Three %{baz}"]]])
+    assert_equal ["foo", "bar", "baz"], I18n.interpolation_keys("example_array")
   end
 
-  test "interpolation_keys raises I18n::NonStringTranslationError when translation is not a String" do
-    store_translations(:en, "example_nested" => { "one" => "One", "two" => "Two" })
-    assert_raises(I18n::NonStringTranslationError) { I18n.interpolation_keys("example_nested") }
+  test "interpolation_keys raises I18n::ArgumentError when non-string argument" do
+    assert_raises(I18n::ArgumentError) { I18n.interpolation_keys(["bad-argument"]) }
   end
 
   test "exists? given an existing key will return true" do