[ruby-core:120207] [Ruby master Bug#20930] Different semantics for nested `it` and `_1`

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


Discussed at the dev meeting, and matz confirmed that the current master's behavior is good.

----------------------------------------
Bug #20930: Different semantics for nested `it` and `_1`
https://bugs.ruby-lang.org/issues/20930#change-110970

* 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
----------------------------------------
With --parser=parse.y:

```
$ ruby --parser=parse.y -ve '[1].each { p it; [5].each { p it } }'
ruby 3.4.0dev (2024-12-04T19:29:24Z master 3c91a1e5fd) [x86_64-linux]
1
5
$ ruby --parser=parse.y -ve '[1].each { p _1; [5].each { p _1 } }'
ruby 3.4.0dev (2024-12-04T19:29:24Z master 3c91a1e5fd) [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)
```

The behavior is inconsistent between it and _1.

---

<details>
<summary>Side note about mixing `_1` and `it`, which seems good</summary>

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
```

</details>

---

<details>
<summary>Prism's bug, moved to: https://github.com/ruby/prism/issues/3291</summary>

```
$ 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)
```

</details>



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