Cop idea: Not enough arguments for enum_for/to_enum

[pocke]

Is your feature request related to a problem? Please describe.

We have an idiom of enum_for/to_enum method for defining an iterator.

class C
  def each_item(&block)
    return enum_for(__method__) unless block_given?

    iterate_someting_with_block block
  end
end

c = C.new

# We can call C#each with and without a block.
c.each_item do |item|
end
c.each_item.map do |item|
end

It is a useful idiom! But we sometimes make a bug if the iterator receives arguments. For example:

class C
  def each_item(type, &block)
    return enum_for(__method__) unless block_given?

    iterate_something_with_block type, block
  end
end
c = C.new

# It works well.
c.each_item(:foo) do |item|
end

# But it does not work well because enum_for should receive `type` but actually doesn't.
c.each_item(:foo).map do |item|
end

It should be return enum_for(__method__, type) unless block_given?.

In this case, it is not a serious problem because c.each_item(:foo).map{} raises an ArgumentError.
But it will be serious when the argument is optional, like def each_item(type = :default, &block).
It ignores type argument silently.

Describe the solution you'd like

Add a new cop to detect this problem.

Describe alternatives you've considered

Nothing

Additional context

• to_enum is an alias of enum_for
• Probably we should focus the idiom, it means the cop should accept any enum_for without return enum_for(__method__) unless block_given? form.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contribution and understanding!

[marcandre]

I've made that mistake myself a few times.

Cop should only apply when: the receiver is nil or self.
Maybe it should check that either the method name (in your example :each_item) or __method__ is used as the first argument too.