Hmm, the current behavior seems to comply CRuby behavior. Do we need to fix it?

I'm sorry, it may have been a confusing expression.
Can you see the example below?

• Ruby 3.0.1

  % ruby30 -e 'b1 = binding; b2 = b1.dup; b1.eval("x = 1"); p b2.eval("x")'
  (eval):1:in `<main>': undefined local variable or method `x' for main:Object (NameError)
          from -e:1:in `eval'
          from -e:1:in `<main>'

• current mruby head (1f1dca1)

  % bin/mruby -e 'b1 = binding; b2 = b1.dup; b1.eval("x = 1"); p b2.eval("x")'
  1

• this PR

  % bin/mruby -e 'b1 = binding; b2 = b1.dup; b1.eval("x = 1"); p b2.eval("x")'
  trace (most recent call last):
          [1] -e:1
  (eval):1:in eval: undefined method 'x' (NoMethodError)

OK, I understand the need for the change. But still, we need to check the following behavior:

x = 5
bind1 = binding
bind1.local_variable_set(:y, 10)
bind2 = bind1.dup
p bind2.local_variable_get(:x)      #=> 5
p bind2.local_variable_get(:y)      #=> 10
bind1.local_variable_set(:y, 20) 
p bind2.local_variable_get(:y)      #=> 20 (from CRuby, mruby, 10 from the PR)
p bind1.local_variable_get(:y)      #=> 20 
x = 50
p bind1.local_variable_get(:x)      #=> 50
p bind2.local_variable_get(:x)      #=> 50

I'm sorry again, I didn't confirm enough.
For now, the solution is to create a proc object for the origin binding object with the Binding#initialize_copy method.

But there is a concern, and if it repeats binding.dup.dup.dup.dup....., the proc gets deeper and deeper. Therefore, I think it is necessary to perform two more processes.

• (1) If there is no local variable, do not link to origin proc
• (2) If proc->upper seems to be too deep, raise an exception as "mruby limit"

Please give me some time...

OK.