bad.rb

class Foo
  def self.bar(&block)
    instance_exec(&block)
  end
  
  attr_accessor :x
end

Foo.bar do
  x = 5
end

good.rb

class Foo
  def self.bar(&block)
    yield(self)
  end
  
  attr_accessor :x
end

Foo.bar do |foo|
  foo.x = 5
end