From: "zverok (Victor Shepelev) via ruby-core" <ruby-core@...>
Date: 2025-01-12T08:20:20+00:00
Subject: [ruby-core:120621] [Ruby master Bug#21026] `def __FILE__.a; end` should be a syntax error

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


As far as I understand (though it is an intuitive understanding, not backed by looking into particular implementation), `__FILE__` and `__LINE__` are handled at the parsing stage, behaving in (almost) all situations like there was just a corresponding literal in the code.

Say, with `-W:deprecated`, this code:
```ruby
def __FILE__.a
end
```
will dutifully emit (as if it would be just a literal)
```
warning: literal string will be frozen in the future
```
...and with `# frozen_string_literal: true` pragma, it will fail with
```
can't define singleton (TypeError)
```

So, as I said, it is _almost_ like it would be a literal string... Except that with a literal string this code would be yelled at by the parser as an impossible (which doesn���t happen with `__FILE__` because, I think, of the order of substitution for it to the ���real��� literal):
```ruby
def 'test.rb'.a
end
```
```
test.rb:1: syntax errors found (SyntaxError)
> 1 | def 'test.rb'.a
    |    ^ expected a delimiter to close the parameters
    |     ^ unexpected string literal; expected a method name
```

----------------------------------------
Bug #21026: `def __FILE__.a; end` should be a syntax error
https://bugs.ruby-lang.org/issues/21026#change-111450

* 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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