[ruby-core:119756] [Ruby master Bug#20817] Ruby 3.4.0dev emits `warning: possibly useless use of + in void context` while Ruby 3.3.5 does not

["kddnewton (Kevin Newton) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20817 has been updated by kddnewton (Kevin Newton).

Assignee changed from kddnewton (Kevin Newton) to prism

----------------------------------------
Bug #20817: Ruby 3.4.0dev emits `warning: possibly useless use of + in void context` while Ruby 3.3.5 does not
https://bugs.ruby-lang.org/issues/20817#change-110421

* Author: yahonda (Yasuo Honda)
* Status: Open
* Assignee: prism
* ruby -v: ruby 3.4.0dev (2024-10-25T11:52:32Z master 9c8c140d73) +PRISM [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
Ruby 3.4.0dev emits `warning: possibly useless use of + in void context` while Ruby 3.3.5 does not

This issue is based on Rails Action Pack test case against Ruby 3.4.0dev.
Here is the minimum script to reproduce it.

### Steps to reproduce

- Create a sample foo.rb file
```
class C
  class_eval "def throw_syntax_error; eval %(
    'abc' + pluralize 'def'
  ); end", "lib/file.rb", 42
end
c = C.new
c.throw_syntax_error
```
- Run this foo.rb using Ruby 3.4.0dev

### Expected behavior
It should report SyntaxError, but it shoud not emit any warnings as Ruby 3.3.5 does not.

```
$ ruby -v
ruby 3.3.5 (2024-09-03 revision ef084cc8f4) [x86_64-linux]
$ ruby -w foo.rb
lib/file.rb:42:in `eval': (eval at lib/file.rb:42):2: syntax error, unexpected string literal, expecting `do' or '{' or '(' (SyntaxError)
    'abc' + pluralize 'def'
                      ^

	from lib/file.rb:42:in `throw_syntax_error'
	from foo.rb:7:in `<main>'
```


### Actual behavior
It reports It should SyntaxError and it emits the `warning: possibly useless use of + in void context`

```
$ ruby -v
ruby 3.4.0dev (2024-10-25T11:52:32Z master 9c8c140d73) +PRISM [x86_64-linux]
$ ruby -w foo.rb
(eval at lib/file.rb:42):2: warning: possibly useless use of + in void context
lib/file.rb:42:in 'Kernel#eval': (eval at lib/file.rb:42):2: syntax error found (SyntaxError)
  1 |
> 2 |     'abc' + pluralize 'def'
    |                       ^ unexpected string literal, expecting end-of-input
  3 |

	from lib/file.rb:42:in 'C#throw_syntax_error'
	from foo.rb:7:in '<main>'
$
```



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