Since ruby 2.3.0, you can call []= method on nil. I don't understand the purpose of this method.
For instance:
nil[1] = 1
# or
nil['foo'] = 'bar'
but [] method does not exist:
nil[1]
# => NoMethodError: undefined method `[]' for nil:NilClass
The ruby 2.3.0 changelog does not mention that changes, although it seems close to the safe navigation operator.
What is the purpose of this operator?
That method isn't documented in Ruby 2.3.0 and I cannot reproduce this behavior in Ruby 2.3.1 (both examples raise NoMethodError: undefined method '[]=' for nil:NilClass).
Furthermore I reinstalled 2.3.0 and was only partly able to reproduce your examples:
$ rbenv install 2.3.0
Downloading ruby-2.3.0.tar.bz2...
-> https://cache.ruby-lang.org/pub/ruby/2.3/ruby-2.3.0.tar.bz2
Installing ruby-2.3.0...
Installed ruby-2.3.0 to /Users/spickermann/.rbenv/versions/2.3.0
$ rbenv shell 2.3.0
$ ruby -v
ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin15]
$ irb
irb > RUBY_VERSION
irb => "2.3.0"
irb > nil[1] = 1
irb => nil
irb > nil['foo'] = 'bar'
NoMethodError: undefined method `[]=' for nil:NilClass
from (irb):3
from /Users/spickermann/.rbenv/versions/2.3.0/bin/irb:11:in `<main>'
It seems like NilClass#[]= doesn't work properly in Ruby 2.3.0. Since it was completely removed in 2.3.1, I guess that this method or this behavior was added by accident.
Update: Cary Swoveland pointed out in a comment on another question that this behavior was a bug and was fixed in later versions (see: https://bugs.ruby-lang.org/issues/11976).
That seems to be actually a bug in 2.3.0 - https://bugs.ruby-lang.org/issues/11976
It does not evaluate the arguments:
nil[undefined_index_variable] = raise "Fooo!" # => nil
