This behavior is documented here in section 4.1.2: http://guides.rubyonrails.org/i18n.html#looking-up-translations

Thanks. I believe it is related with #18308 or with the fix of one regression that this PR caused #19102.

Yeah, I think I broke it in 90fcfea and I'll try to fix/test this case.

After doing some bisecting, this error was introduced by 362557e.

I'm wondering why this issue was closed. Is it still considered a regression bug? Was it fixed in a separate PR?

I have a problem that looks to be related to this. We had some code that uses default: [[]] because we want the default to be [] if nothing is found. This broke in 4.2.1 because 4.2.1 made changes to the way that default: [[]] is "unwrapped" to [] (because it uses options.delete and then re-adds it with options[:delete]=).

class NestedArrayTest
  include ActionView::Helpers::TranslationHelper

  def translate(key, options = {})
    default = super

    super(key, options.merge(scope: "this.does.not.exist", default: [[]]))
  end
  alias_method :t, :translate
end

class SimpleArrayTest
  include ActionView::Helpers::TranslationHelper

  def translate(key, options = {})
    default = super

    super(key, options.merge(scope: "this.does.not.exist", default: []))
  end
  alias_method :t, :translate
end

# pre-4.2.1
NestedArrayTest.new.translate("lol") #=> []
SimpleArrayTest.new.translate("lol") #=> translation missing <span>

# 4.2.1
NestedArrayTest.new.translate("lol") #=> translation missing <span>
SimpleArrayTest.new.translate("lol") #=> translation missing <span>

@aprescott see #19421

Ah, thanks! It looks like my problem still exists. I'll open a PR/issue if it's still broken on 4-2-stable/master.