From: "sanjioh (Fabio Sangiovanni) via ruby-core" <ruby-core@...>
Date: 2025-01-29T15:17:59+00:00
Subject: [ruby-core:120830] [Ruby master Bug#21098] rb_alias: confusing error messages when the original method cannot be found and ZSUPER methods are involved

Issue #21098 has been reported by sanjioh (Fabio Sangiovanni).

----------------------------------------
Bug #21098: rb_alias: confusing error messages when the original method cannot be found and ZSUPER methods are involved
https://bugs.ruby-lang.org/issues/21098

* Author: sanjioh (Fabio Sangiovanni)
* Status: Open
* ruby -v: ruby 3.5.0dev (2025-01-29T13:19:04Z master 63b6323e04) +PRISM [x86_64-darwin24]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
Hi all,

I've noticed that the current implementation of `rb_alias` can raise `NameError` exceptions with confusing error messages when the original method cannot be found and ZSUPER methods are involved in the inheritance chain traversal.
That's because `rb_print_undef` is invoked with the current value of `klass`, which is repeatedly reassigned whenever a ZSUPER method is encountered in the search, potentially referencing an ICLASS or FALSE.

The following are 2 examples of this kind of behavior:

``` ruby
module M
  private :class
  alias_method :xyz, :class  # raises 'Module#alias_method': undefined method 'class' for class 'false' (NameError)
end
```

``` ruby
module M
  private :class
end
module N
  prepend M
  alias_method :xyz, :class  # raises 'Module#alias_method': undefined method 'class' for class '#<N:0x0000000124ba7f00>' (NameError)
end
```

The second example also triggers an assertion failure when they are enabled with `-DRUBY_DEBUG`:
```
../vm_insnhelper.c:2192: Assertion Failed: rb_vm_search_method_slowpath:RB_TYPE_2_P(klass, RUBY_T_CLASS, RUBY_T_ICLASS): klass: T_MODULE
```

In the following PR I tried to fix the issue by invoking `rb_print_undef` with `target_klass` as argument, which holds the initial value of `klass`.

https://github.com/ruby/ruby/pull/12665

The previous examples become as follows:

``` ruby
module M
  private :class
  alias_method :xyz, :class  # raises 'Module#alias_method': undefined method 'class' for module 'M' (NameError)
end
```

``` ruby
module M
  private :class
end
module N
  prepend M
  alias_method :xyz, :class  # raises 'Module#alias_method': undefined method 'class' for module 'N' (NameError)
end
```

This is my first contribution to Ruby, so please let me know if I can improve it further.

Cheers!



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