[ruby-core:80243] [Ruby trunk Feature#13332] Forwardable#def_instance_delegator nil

Issue #13332 has been reported by eike.rb (Eike Dierks).

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Feature #13332: Forwardable#def_instance_delegator nil
https://bugs.ruby-lang.org/issues/13332

* Author: eike.rb (Eike Dierks)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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I'd like to suggest an extension to the signature of stdlib Forwardable#def_instance_delegator

currently (ruby 2.3.3) it is declared as:
  def_instance_delegator(accessor, method, ali = method)

I'd like to add a nil: option like this:
  def_instance_delegator(accessor, method, ali = method, nil_behaviour:NoMethodError)

Rationale:
As of ruby2.3, delegated methods fail with
  #<NoMethodError: undefined method `foo' for nil:NilClass>
when the declared accessor returns nil

aka, when the delegator may be nil,
the Forwardable module can not be used,
but one has to ressort to implement the forward manually.

Adding an option to the signature could solve this.
I believe that this can be implemented in a fully backward compatible way.

I named this option :nil in my example,
(some people will cry out loud)

Specification scribble:
~~~
if accessor.nil?
  case nil_behaviour
  when nil then nil    # return nil, when accessor was nil
  when Error             # raise that Error with proper args
  when Proc             # invoke that Proc with proper args
  when String           # return that string
  else                        # return that object
end
~~~































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