From: "zverok (Victor Shepelev) via ruby-core" <ruby-core@...>
Date: 2025-01-11T19:49:56+00:00
Subject: [ruby-core:120612] [Ruby master Bug#21026] `def __FILE__.a; end` should be a syntax error

Issue #21026 has been updated by zverok (Victor Shepelev).


> It already doesn't seem to actually do anything

This is a bunch of technicalities... But I don���t think it doesn���t do anyting :)

As far as I understand, every `__FILE__` invocation in the source code produces a _new_ instance of a string with the contents of the current file name:

```ruby
p __FILE__.object_id #=> 16
p __FILE__.object_id #=> 24
p __FILE__.frozen? #=> false
```

So, this code:
```ruby
def  __FILE__.a
  puts "works!"
end
```
can be treated as this:
```ruby
o = __FILE__

def  o.a
  puts "works!"
end

o.a
# prints "works!"
```
(The subsequent calls of `__FILE__` doesn���t have `a` method because they all return different objects.)

So, the code ���works��� (even if it is not of much utility). But the example with parentheses is interesting: it is not about __FILE__, any attempt to define a method on literal works this way:
 
```ruby
def ("file").a
end
```
results in...
```
test.rb:1: syntax error found (SyntaxError)
> 1 | def ("file").a
    |      ^~~~~~ cannot define singleton method for literals
  2 |   puts "works!"
  3 | end
```
...so I guess `__FILE__ ` is mostly treated as literal... Except when it does not :)

----------------------------------------
Bug #21026: `def __FILE__.a; end` should be a syntax error
https://bugs.ruby-lang.org/issues/21026#change-111442

* Author: Earlopain (Earlopain _)
* Status: Open
* ruby -v: ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
Constants like `__FILE__`, `__LINE__` and `__ENCODING__` are literals and as such you shouldn't be able to defined singleton methods on them.

It already doesn't seem to actually do anything:
```rb
def __FILE__.a
end
__FILE__.a #=>  undefined method 'a' for an instance of String (NoMethodError)
```

Wrapping it in brackets correctly reports a syntax error:
```rb
code.rb:1: syntax error found (SyntaxError)
> 1 | def (__FILE__).a
    |      ^~~~~~~~ cannot define singleton method for literals
  2 | end
```

The behavior is consistent between prism and parse.y

`__ENCODING__` is frozen and so will result in a runtime error. Same for `__LINE__`, and also `__FILE__` with frozen string literals.



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