Based on the [examples in `test_acronyms`][1], it looks like we should expect the following assertion to pass as well:

```ruby
ActiveSupport::Inflector.inflections do |inflect|
  inflect.acronym("PhD")
end

assert_equal("namespaced/phd_required", ActiveSupport::Inflector.underscore("Namespaced::PhDRequired"))
```

I added that assertion to my local checkout of this Pull Request. That assertion currently fails:

```
InflectorTest#test_underscore_acronym_sequence [/Users/j/src/rails/activesupport/test/inflector_test.rb:178]:
Expected: "namespaced/phd_required"
  Actual: "namespaced/ph_d_required"
```

It would be awesome to add that assertion to this Pull Request and to update the implementation to satisfy this assertion.

[1]: https://github.com/fredwu/rails/blob/e93ed6e621809739fee460d487543d1dd6a34947/activesupport/test/inflector_test.rb#L122