Bug #21142: Lazy enumerator `.each_with_index` ignores `.take(0)` before it

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: 3.2.7
• Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[aaronkison (Aaron Kison)]

Minimum code to produce problem:

class Numbers; def each; 100.times { yield _1 }; end; include Enumerable; end

Numbers.new.lazy.take(0).each_with_index.map { _1 }.to_a

Output (at ruby 3.2.7, and 3.3.0):

[0, 1, ..., 99]

Expected output (and was as at ruby 3.1.4):

[]

It works when it opposite ordering: Numbers.new.lazy.each_with_index.take(0).map { _1 }.to_a.
I suspect it may be related to the change here https://github.com/ruby/ruby/pull/11868/files but I'm not familiar with any of that code. It seems like it replaces an allocated index with a counting index, which my hunch is it works for every value except for 0.