[ruby-core:26307] Re: IO.foreach (and friends) effect on $< and $.

$. is on my "list of bugs to file". I believe there are bugs on both
1.9 and 1.8, but different ones. Or at least the documentation is
wrong. Here's what I had written so far, without finishing looking
into it:
----------
I believe there is a bug in trunk with $. but I am not certain how to
resolve it.

Is there an official documentation for $. (and all other global
variables) in rdoc?

The Ruby Prog Language (Flanagan & Matz) states:

The number of the last line read from the current input file.
Equivalent to ARGF.lineno.

I find "current input file" to be very imprecise. Is that the file
currently being read from ARGF? or the last IO read from?

Currently, it's a bit of both. It's usually the last read io:

$ rubydev -e "ARGF.gets; f=File.new '/etc/passwd'; 4.times{f.gets}; p
$., ARGF.lineno" /etc/hosts
4
1

Clearly, it is not equivalent to ARGF.lineno in this case.

On the other hand, it's affected by ARGF.lineno= (contrary to any
other IO#lineno=), and vice-versa:
$ rubydev -e "ARGF.lineno = 42; p $.; $. = 56; p ARGF.lineno"
42
56