Add tests and error handling for non-string translations

lib/i18n.rb

@@ -250,7 +250,10 @@ def interpolation_keys(key, **options)
 
       return [] unless exists?(key, **options.slice(:locale, :scope))
 
-      translate(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

lib/i18n/exceptions.rb

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

test/i18n_test.rb

@@ -287,6 +287,29 @@ def setup
     assert_equal I18n.config.available_locales.size * 2, I18n.config.available_locales_set.size
   end
 
+  test "interpolation_keys returns an array of keys" do
+    store_translations(:en, "example_two" => "Two interpolations %{foo} %{bar}")
+    assert_equal ["foo", "bar"], I18n.interpolation_keys("example_two")
+  end
+
+  test "interpolation_keys returns an empty array when no interpolations " do
+    store_translations(:en, "example_zero" => "Zero interpolations")
+    assert_equal [], I18n.interpolation_keys("example_zero")
+  end
+
+  test "interpolation_keys returns an empty array when missing translation " do
+    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"]) }
+  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") }
+  end
+
   test "exists? given an existing key will return true" do
     assert_equal true, I18n.exists?(:currency)
   end