[ruby-core:115551] [Ruby master Bug#19392] Endless method and parsing priorities

Issue #19392 has been updated by zverok (Victor Shepelev).

Subject changed from Endless method vs and/or to Endless method and parsing priorities
Description updated

Updated the description to underline the nature of the problem better.

----------------------------------------
Bug #19392: Endless method and parsing priorities
https://bugs.ruby-lang.org/issues/19392#change-105483

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
* Backport: 2.7: DONTNEED, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
**Initial description**

[Discovered](https://twitter.com/lucianghinda/status/1617783952353406977) by Lucian Ghinda: 

```ruby
def test = puts("foo") and puts("bar")
# prints "bar" immediately
test
# prints "foo"
```

It seems that it is a parser error, right?..
```ruby
RubyVM::AbstractSyntaxTree.parse('def test = puts("foo") and puts("bar")')
#  => 
# (SCOPE@1:0-1:38                                                         
#  tbl: []                                                                
#  args: nil                                                              
#  body:                                                                  
#    (AND@1:0-1:38                                                        
#       (DEFN@1:0-1:22                                                    
#        mid: :test                                                       
#        body:                                                            
#          (SCOPE@1:0-1:22                                                
#           tbl: []                                                       
#           args:                                                         
#             (ARGS@1:0-1:8 pre_num: 0 pre_init: nil opt: nil first_post: nil post_num: 0 post_init: nil rest: nil kw: nil kwrest: nil block: nil)
#           body: (FCALL@1:11-1:22 :puts (LIST@1:16-1:21 (STR@1:16-1:21 "foo") nil))))
#       (FCALL@1:27-1:38 :puts (LIST@1:32-1:37 (STR@1:32-1:37 "bar") nil)))) 
```

E.g. it is parsed as 
```ruby
(def test = puts("foo")) and (puts("bar"))
```
...which is hardly intentional or have any practical use. The rightly parsed code in this case _can_ have practical use, like
```ruby
def write(data) = File.write(@filename, data) == data.size or raise "Something went wrong"
```

**Additional cases of what seems to be the same problem**

```ruby
def save = File.write(name, self.to_yaml) unless invalid?
# Parsed as:
(def save = File.write(name, self.to_yaml)) unless invalid?
```
...which makes it very hard for the users to diagnose the real reason, see #19731

```ruby
def initialize(a, b) = @a, b = a, b
# syntax error, unexpected ',', expecting end-of-input (SyntaxError)                                                 
# def initialize(a, b) = @a, b = a, b                            
#                          ^         

# Again, parsed as
(def initialize(a, b) = @a), b = a, b
```
While this one is at least diagnosed early, in pathological cases, it might lead to very subtle bugs:
```ruby
private def start = @operation, @conversion = :print, :to_s
```
This code doesn't throw a syntax error, but its effect is very far from expected. Again, it is parsed as
```ruby
private( (def start = @operation), @conversion = :print, :to_s )
```
...and ends up in:
* defining a private method `start`
* making private methods `:print` and `:to_s`




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