Bug #21098: rb_alias: confusing error messages when the original method cannot be found and ZSUPER methods are involved

• Status: Closed
• Assignee: -
• Target version: -
• 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

[sanjioh (Fabio Sangiovanni)]

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:

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

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:

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

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!

[alanwu (Alan Wu)]

• Status changed from Open to Closed

https://github.com/ruby/ruby/pull/12665 landed in f0dc9dcdc7a3b15b3192b1503a3c3d9eec3ada06.