False positive offense with Rails/UniqueValidationWithoutIndex using expression indexes

[ZilvinasKucinskas]

Rails 5 added support for expression indexes as follows:

def change
  add_index :emails,
            'lower(address)',
            name: "index_emails_on_address_unique",
            unique: true
end

However, it does not seem to be recognised by Rails/UniqueValidationWithoutIndex rule. I have added a test case below that fails, but should pass instead.

Expected behavior

No offense triggered.

Actual behavior

Offense is being triggered with Uniqueness validation should be with a unique index message.

Steps to reproduce the problem

Here is the test case we can put into spec/rubocop/cop/rails/unique_validation_without_index_spec.rb:

    context 'with expression indexes' do
      let(:schema) { <<~RUBY }
        ActiveRecord::Schema.define(version: 2020_02_02_075409) do
          create_table "emails", force: :cascade do |t|
            t.string "address", null: false
            t.index "lower(address)", name: "index_emails_on_LOWER_address", unique: true
          end
        end
      RUBY

      it 'registers no offense' do
        expect_no_offenses(<<~RUBY)
          class Email
            validates :address, presence: true, uniqueness: { case_sensitive: false }, email: true
          end
        RUBY
      end
    end

RuboCop version

$ [bundle exec] rubocop -V
0.80.1 (using Parser 2.7.0.5, running on ruby 2.6.5 x86_64-darwin18)