From: "rwstauner (Randy Stauner) via ruby-core" <ruby-core@...>
Date: 2026-10-28T18:35:56+00:00
Subject: [ruby-core:127264] [Ruby Bug#21142] Lazy enumerator `.each_with_index` ignores `.take(0)` before it

Issue #21142 has been updated by rwstauner (Randy Stauner).


@nobu
The above commit changed the behavior such that `lazy#each_with_index { ... }` is no longer running the block and just returning the enumerator.

4.0:
```
$ ruby -ve 'arr = []; p 3.times.lazy.each { |*a| arr << a }; p arr;'
ruby 4.0.7 (2026-09-15 revision 229531a6cf) +PRISM [x86_64-linux]
3
[[0], [1], [2]]

$ ruby -ve 'arr = []; p 3.times.lazy.each_with_index { |*a| arr << a }; p arr;'
ruby 4.0.7 (2026-09-15 revision 229531a6cf) +PRISM [x86_64-linux]
3
[[0, 0], [1, 1], [2, 2]]
```

4.1.0dev:
```
$ ruby -ve 'arr = []; p 3.times.lazy.each { |*a| arr << a }; p arr;'
ruby 4.1.0dev (2026-10-28T17:51:51Z master 5127bd0dbc) +PRISM [x86_64-linux]
3
[[0], [1], [2]]

$ ruby -ve 'arr = []; p 3.times.lazy.each_with_index { |*a| arr << a }; p arr;'
ruby 4.1.0dev (2026-10-28T17:51:51Z master 5127bd0dbc) +PRISM [x86_64-linux]
#<Enumerator::Lazy: #<Enumerator::Lazy: #<Enumerator: 3:times>>:each_with_index(0)>
[]
```

This seems inconsistent and likely unintentional.
We must be missing tests for this case.

----------------------------------------
Bug #21142: Lazy enumerator `.each_with_index` ignores `.take(0)` before it
https://bugs.ruby-lang.org/issues/21142#change-119686

* Author: aaronkison (Aaron Kison)
* Status: Closed
* ruby -v: 3.2.7 
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
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.



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