[ruby-core:119178] [Ruby master Bug#20736] prism emits wrong warnings in syntax-error code

Issue #20736 has been updated by kddnewton (Kevin Newton).


This is because Prism recovers from the syntax error. For example:

```ruby
eval("1; 2; 9a; 3; 4; 5; 9b")
```

with Prism this will give:

```
(eval at test.rb:1):1: warning: possibly useless use of a literal in void context
(eval at test.rb:1):1: warning: possibly useless use of a literal in void context
(eval at test.rb:1):1: warning: possibly useless use of a literal in void context
(eval at test.rb:1):1: warning: possibly useless use of a literal in void context
(eval at test.rb:1):1: warning: possibly useless use of a literal in void context
(eval at test.rb:1):1: warning: possibly useless use of a literal in void context
(eval at test.rb:1):1: warning: possibly useless use of a literal in void context
test.rb:1:in 'Kernel#eval': (eval at test.rb:1):1: syntax errors found (SyntaxError)
> 1 | 1; 2; 9a; 3; 4; 5; 9b
    |        ^ unexpected local variable or method, expecting end-of-input
    |                     ^ unexpected local variable or method, expecting end-of-input

	from test.rb:1:in '<main>'
```

with parse.y this will give:

```
(eval at test.rb:1):1: warning: possibly useless use of a literal in void context
(eval at test.rb:1):1: warning: possibly useless use of a literal in void context
test.rb:1:in 'Kernel#eval': (eval at test.rb:1):1: syntax error, unexpected local variable or method, expecting end-of-input (SyntaxError)
1; 2; 9a; 3; 4; 5; 9b
       ^

	from test.rb:1:in '<main>'
```

this is because parse.y is not recovering from this error. I would imagine when parse.y _does_ recover from this error, it would have the same behavior.

So I'm not sure if this is wrong or not. @mame what do you think?

----------------------------------------
Bug #20736: prism emits wrong warnings in syntax-error code
https://bugs.ruby-lang.org/issues/20736#change-109756

* Author: mame (Yusuke Endoh)
* Status: Open
* Assignee: kddnewton (Kevin Newton)
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
```
$ ./local/bin/ruby -w --parser=parse.y -e 'begin eval("0a"); rescue SyntaxError; end'
$ ./local/bin/ruby -w --parser=prism -e 'begin eval("0a"); rescue SyntaxError; end'
(eval at -e:1):1: warning: possibly useless use of a literal in void context
```

```
$ ./local/bin/ruby -w --parser=parse.y -e 'begin eval("+a.0"); rescue SyntaxError; end'
$ ./local/bin/ruby -w --parser=prism -e 'begin eval("+a.0"); rescue SyntaxError; end'
(eval at -e:1):1: warning: possibly useless use of +@ in void context
```




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