subscribe

# File activesupport/lib/active_support/testing/error_reporter_assertions.rb, line 38
            def subscribe
              return if @subscribed
              @mutex.synchronize do
                return if @subscribed

                if ActiveSupport.error_reporter
                  ActiveSupport.error_reporter.subscribe(self)
                  @subscribed = true
                else
                  raise Minitest::Assertion, "No error reporter is configured"
                end
              end
            end