I'm new at this, but it's my understanding that with classes: the 'name of the class' (Name), 'self' and simply typing 'new' garner the same result. Name and 'self' are receivers and they invoke #new. In the absence of a receiver and simply typing 'new', the method calls whatever the current object is, which happens to be 'self' and or Name. Why…

self implicitly?

class Array  

  def pad!(min_size, value = nil)
    if self.length < min_size
      self.fill(value, self.length...min_size)
    else
      return self
    end
  end

  def pad(min_size, value = nil)
    self.clone.pad!(min_size, value)
  end
end

##########################################

class Array    

  def pad!(min_size, value = nil)
    if Array.length < min_size                    # NoMethodError 
      Array.fill(value, Array.length...min_size)   
    else
      return Array
    end
  end

  def pad(min_size, value = nil)
    Array.clone.pad!(min_size, value)
  end
end

You're confusing the class Array with an instance of an Array. They are two different things. Inside an instance method like pad or pad!, the keyword self refers to the current instance, which is not the same as the class Array.

Try to think of the class as a blueprint, while an instance is one of many different things the blueprint describes...

There's only one Array class, but there are can be a ton of instances of Array...

a = [1, 3, 5]
b = (53..80).to_a
c = "foo bar".split

a, b, and c are all instances of Array.