h.rb

class Foo
  attr_reader :v
  def initialize(v)
    @v = v
  end
  def hash
    v.hash
  end
  def eql?(o)
    puts "Foo#eql? called"
    if o.is_a?(self.class)
      o.v.eql?(v)
    else
      o.eql?(v)
    end
  end
end

h = {}

f = Foo.new('hello')
h[f] = 'world'
h['hello'] = 'jupiter'

p h[f]
p h['hello']

p h