class RSpec::Expectations::ExpectationTarget

def self.for(value, block)
  if UndefinedValue.equal?(value)
    unless block
      raise ArgumentError, "You must pass either an argument or a block to `expect`."
    end
    BlockExpectationTarget.new(block)
  elsif block
    raise ArgumentError, "You cannot pass both an argument and a block to `expect`."
  else
    new(value)
  end
end