default: nil breaks fallbacks

[daniel-rikowski]

I've noticed that using default: nil with fallbacks leads to breaking the fallback chain.

Given these translations:

de:
  bar: 'Bar from de'
en:
  bar: 'Bar from en'

I18n.t('bar', locale: :'de-DE', default: '') returns 'Bar from de' 👍
I18n.t('bar', locale: :'de-DE', default: nil) returns nil 👎

This is caused by the return statement in this code from fallbacks.rb: (simplified for readability)

I18n.fallbacks[locale].each do |fallback|
  result = super(fallback, key, options)
  # PROBLEMATIC STATEMENT:
  return result if (result.nil? && options.key?(:default) && options[:default].nil?) || !result.nil?
end

In other words: If you have a default of nil and the first fallback locale doesn't return a value, the default is applied immediately. The loop is exited and no further fallbacks are tried.

The solution is to delay the default check until all fallbacks are tried.

I've prepared a PR with a failing test case for this problem. See #387