[ruby-core:120110] [Ruby master Bug#20930] Different semantics for nested `it` and `_1`

["mame (Yusuke Endoh) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20930 has been updated by mame (Yusuke Endoh).


Good catch. I see two problems.

One is an incompatibility with Prism's handling of `_1`. I think it should be handled as an error like parse.y. Especially when parsed with the 3.3 version's syntax, there is no other choice but an error:

```ruby
Prism.parse("[1].each { _1; [2].each { _1 } }", version: "3.3.0")
```

The other problem is how to interpret `it`. I think Ruby master's it behavior is good. I have experienced mistakes of a name conflict issue of a local variable in a different scope, but not as often. However, just adding a read from `it` outside of a block only changes the meaning of inner `it`, which I think would increase the frequency of mistakes very much.

```ruby
[1].each {     [2].each { p it }} #=> 1
[1].each { it; [2].each { p it }} #=> 2 (!)
```

----------------------------------------
Bug #20930: Different semantics for nested `it` and `_1`
https://bugs.ruby-lang.org/issues/20930#change-110858

* Author: Eregon (Benoit Daloze)
* Status: Open
* ruby -v: ruby 3.4.0dev (2024-12-04T19:29:24Z master 3c91a1e5fd) +PRISM [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
```
$ ruby -ve '[1].each { p it; [5].each { p it } }'
ruby 3.4.0dev (2024-12-04T19:29:24Z master 3c91a1e5fd) +PRISM [x86_64-linux]
1
5
$ ruby -ve '[1].each { p _1; [5].each { p _1 } }'
ruby 3.4.0dev (2024-12-04T19:29:24Z master 3c91a1e5fd) +PRISM [x86_64-linux]
1
1
```
Notice the inconsistency, `it` uses the innermost block, `_1` uses the outermost block.
I think `_1` semantics are slightly better, at least `_1` behaves like a normal local variable declared in the outer block then.

Note that on 3.3.5 it was forbidden to nest `_1` which I think might be good for clarity/avoiding ambiguity:
```
$ ruby -ve '[1].each { p _1; [5].each { p _1 } }'
ruby 3.3.5 (2024-09-03 revision ef084cc8f4) [x86_64-linux]
-e:1: numbered parameter is already used in
-e:1: outer block here
[1].each { p _1; [5].each { p _1 } }
ruby: compile error (SyntaxError)
```

---

As an aside, mixing `_1` and `it` is allowed, I think this is [good](https://bsky.app/profile/eregon.bsky.social/post/3lcg4fjcf7225), they are different things so there is not much confusion there:
```
$ ruby -ve '[1].each { p _1; [5].each { p it } }'
ruby 3.4.0dev (2024-12-04T19:29:24Z master 3c91a1e5fd) +PRISM [x86_64-linux]
1
5
$ ruby -ve '[1].each { p it; [5].each { p _1 } }'
ruby 3.4.0dev (2024-12-04T19:29:24Z master 3c91a1e5fd) +PRISM [x86_64-linux]
1
5
```



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