[ruby-core:122450] [Ruby Bug#21337] Using `not` on the RHS of a logical operator becomes valid syntax with Prism

Issue #21337 has been updated by mame (Yusuke Endoh).


`p(not 1)` is also allowed in Ruby 3.4, but this should also be prohibited, @matz said.

@kddnewton @tenderlovemaking @eileencodes Can you fix this issue soon? We need to backport to 3.4 to stop Ruby 3.4 users writing such code. 

----------------------------------------
Bug #21337: Using `not` on the RHS of a logical operator becomes valid syntax with Prism
https://bugs.ruby-lang.org/issues/21337#change-113631

* Author: koic (Koichi ITO)
* Status: Assigned
* Assignee: prism
* ruby -v: ruby 3.4.3 (2025-04-14 revision d0b7e5b6a0) +PRISM [x86_64-darwin24]
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
The following syntax behavior differs between Ruby 3.3 (parse.y by default) and Ruby 3.4 (Prism by default).

```ruby
if true && not true; end
```

## Expected

In Ruby 3.3 and earlier, the following code results in a syntax error. The default parser is `parse.y`.

```console
$ ruby -vce 'if true && not true; end'
ruby 3.3.8 (2025-04-09 revision b200bad6cd) [x86_64-darwin24]
-e:1: syntax error, unexpected `true', expecting '('
if true && not true; end
ruby: compile error (SyntaxError)
```

The key point is that `not` is used on the right-hand side of a logical operator.

## Actual

When the default parser is switched to Prism, it is accepted as valid syntax with Ruby 3.4+ (Prism by default):

```console
$ ruby -vce 'if true && not true; end'
ruby 3.4.3 (2025-04-14 revision d0b7e5b6a0) +PRISM [x86_64-darwin24]
Syntax OK

$ ruby -vce 'if true && not true; end'
ruby 3.5.0dev (2025-05-13T02:05:19Z master 9b8c846bdf) +PRISM [x86_64-darwin24]
Syntax OK
```

Specifying parse.y causes a syntax error, as before 3.3 (parse.y by default):

```console
$ ruby -vce 'if true && not true; end'
ruby 3.3.8 (2025-04-09 revision b200bad6cd) [x86_64-darwin24]
-e:1: syntax error, unexpected `true', expecting '('
if true && not true; end
ruby: compile error (SyntaxError)

$ ruby --parser=parse.y -vce 'if true && not true; end'
ruby 3.5.0dev (2025-05-13T02:05:19Z master 9b8c846bdf) [x86_64-darwin24]
-e:1: syntax error, unexpected 'true', expecting '('
if true && not true; end
ruby: compile error (SyntaxError)
```

The behavior seen in the parse.y producing a syntax error might be probably the expected one.
Any case, it is likely that users would not expect such a discrepancy between parsers.

This issue was noticed in the context of the following RuboCop issue:
https://github.com/rubocop/rubocop/issues/14177



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