[ruby-core:122873] [Ruby Bug#21139] Prism and parse.y parses `it = it` differently

Issue #21139 has been updated by vinistock (Vinicius Stock).


No, I don't mean all variables. However, the `it` case feels unintuitive because `it` refers to the first argument of the block. Take this other example here:

```ruby
42.tap { it = it; p it }
42.tap { |x| x = x; p x }
```

To me, these two are exact equivalents. You are storing the first argument of the block in a local variable that shadows the name of that argument. Right now, `parse.y` returns different results for these two cases, which to me is quite surprising. If you run that snippet with `parse.y`, you get `nil` and `42`.

The way I reason about this code is as follows:

```ruby
42.tap do
  it = it
 #     ^ First step: read the value of `it`, which is the block's argument and therefore is equal to 42
 # ^     Second step: declare a local variable called `it` and store the value of the right hand side (which is 42)
  p it
 #  ^    Third step: at this point `it` no longer refers to the special variable `it`, but to a local variable `it`, for which the value is currently 42
end
```

> I think it would be far simpler to forbid it as a local variable name, though the compatibility implications would need to be carefully considered.

Maybe this is a better path forward as these edge cases can be confusing. That said, I still think that the current behaviour in Prism is more intuitive and consistent.

----------------------------------------
Bug #21139: Prism and parse.y parses `it = it` differently
https://bugs.ruby-lang.org/issues/21139#change-114176

* Author: tompng (tomoya ishida)
* Status: Feedback
* Assignee: prism
* ruby -v: ruby 3.5.0dev (2025-02-14T16:49:52Z master ee181d1bb7) +PRISM [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
~~~
# ruby --parser=parse.y -e "42.tap { it = it; p it }"
nil
# ruby --parser=prism -e "42.tap { it = it; p it }"
42
~~~


---Files--------------------------------
clipboard-202503081702-idzz2.png (22.6 KB)


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