You can create auto-encoding-detection version with your favorite logic. i.e. super(rhs.force_encoding(mydetect(rhs)))

gistfile1.rb

require 'refinement'

module StringForceEncodingConcat
  refine String do
    def +(rhs)
      begin
        super
      rescue Encoding::CompatibilityError
        super(rhs.force_encoding(self.encoding))
      end
    end
  end
end

No "require 'refinement'" needed ATM.