[ruby-core:99099] [Ruby master Feature#16978] Ruby should not use realpath for __FILE__

Issue #16978 has been updated by jeremyevans0 (Jeremy Evans).

Backport deleted (2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN)
ruby -v deleted (ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-linux])
Tracker changed from Bug to Feature

I don't think this is a bug.  `__FILE__` is documented as follows: `The path to the current file.`  Which path (real, absolute, relative, expanded) is not specified.

Not using the real path would lead to behavior that depends on the first path used when requiring the file.

`a/test.rb` (`b` symlinked to `a`):

```ruby
def a
  __FILE__
end
```

```
ruby -Ia -Ib -rtest -e 'a'
# /path/to/a/test.rb
ruby -Ib -Ia -rtest -e 'a'
# Current: /path/to/a/test.rb
# Your proposed: /path/to/b/test.rb
```

What actually happens is not the file path being converted to a real path, but the include directory being converted to a real path before the file is required (in `rb_construct_expanded_load_path`).  Changing this to not use a real path would probably break the code that checks that a feature hasn't been require twice.  For example, this code would change behavior:

```
$: << 'a'
require 'test'
$: << 'b'
require 'test'
# Current: not loaded again
# Your proposed: loaded again
```

If you symlink the file itself and not the include directory, Ruby will attempt to require it as a separate feature.

Note that if you provide a path when requiring, Ruby already operates the way you want:

```
ruby -r./a/test -e 'a' # /path/to/a/test.rb
ruby -r./b/test -e 'a' # /path/to/b/test.rb
```

I can certainly see pros and cons from changing the behavior, but I would consider this a feature request and not a bug.

----------------------------------------
Feature #16978: Ruby should not use realpath for __FILE__
https://bugs.ruby-lang.org/issues/16978#change-86473

* Author: vo.x (Vit Ondruch)
* Status: Open
* Priority: Normal
----------------------------------------
This is the simplest test case:

~~~
$ mkdir a

$ echo "puts __FILE__" > a/test.rb

$ ln -s a b

$ ruby -Ib -e "require 'test'"
/builddir/a/test.rb
~~~

This behavior is problematic, because Ruby should not know nothing about the `a` directory. It was not instructed to use it. I should always refer to the file using the original path and do not dig into the underlying details, otherwise depending on file system setup, one might be forced to used `File.realpath` everywhere trying to use `__FILE__`.



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