[ruby-core:115663] [Ruby master Bug#19392] Endless method and parsing priorities

Issue #19392 has been updated by Eregon (Benoit Daloze).


`def =` is clearly more about `def` than `=`.
It is not an assignment, as clearly said by other just before this message.

> Because `a = b and c` is interpreted as `(a = b) and c`, it is natural that `def a = b and c` is interpreted as `(def a = b) and c`, as Matz said in his comment #note-9.

By that reasoning, `def m; a = b and c; end` should be `(def m; a = b) and (c; end)` then? (if the most important thing is that `and` is lower precedence than everything else no matter the context)
`def` is stronger than `=` and `and`, that is very intuitive and simple. The same for `module`, `class`, they are scope-changing keywords.
It makes no sense that `=` and `and` would be stronger than a scope-changing keyword, from a usage point of view.

I think either this should be fixed or endless methods should be removed because hopelessly broken and uninintuitve.
Of course we cannot remove them for compatibility so let's fix this to be like the overwhelming majority of Rubyists expect?

----------------------------------------
Bug #19392: Endless method and parsing priorities
https://bugs.ruby-lang.org/issues/19392#change-105600

* Author: zverok (Victor Shepelev)
* Status: Closed
* 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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