[ruby-core:126295] [Ruby Bug#22197] Backtraces show methods which do not exist

6 Aug 2026

      Issue #22197 has been updated by matz (Yukihiro Matsumoto).

This is a bug and should be fixed.

The file and line we print are those of the original definition, so the module must come from that same definition. Mixing it with the run-time owner produces a name for a method that does not exist, which is worse than either choice alone.

Please fix it in 4.1. It changes backtrace output, so I lean against not backporting, but I leave that to the branch maintainers.

Matz.

----------------------------------------
Bug #22197: Backtraces show methods which do not exist
https://bugs.ruby-lang.org/issues/22197#change-118392

* Author: Eregon (Benoit Daloze)
* Status: Open
* ruby -v: ruby 4.0.3 (2026-04-21 revision 85ddef263a) +PRISM [arm64-darwin25]
* Backport: 3.3: DONTNEED, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
```ruby
class Parent
  def original
    puts caller_locations(0), nil
  end
end

class Child < Parent
  alias_method :alias, :original
end

Child.new.alias

module Original
  def original
    puts caller_locations(0)
  end
end

class A
  define_method(:a, Original.instance_method(:original))
end

A.new.a
```
gives:
```
$ ruby -v backtrace_alias.rb
ruby 4.0.3 (2026-04-21 revision 85ddef263a) +PRISM [arm64-darwin25]
backtrace_alias.rb:3:in 'Child#original'
backtrace_alias.rb:11:in '<main>'

backtrace_alias.rb:15:in 'A#original'
backtrace_alias.rb:23:in '<main>'
```

But this seems clearly incorrect:
* there is no `Child#original`, there is only `Parent#original` and `Child#alias`.
* there is no `A#original`, there is only `Original#original` and `A#a`.

This behavior exists since Ruby 3.4 which added the module name in backtraces (#19117).

So the issue is that backtraces show the "original method name from the definition" with the "method owner of the actually-called method".

It should be consistent to avoid showing methods which don't exist.
And it should be the original method from the definition, because we show the file and line of that, so we should also show the original name (already the case) and the original module (not the case yet, the bug).

FWIW TruffleRuby already behaves like that:
```
$ truffleruby 34.0.1 (2026-04-26), like ruby 3.4.9, Oracle GraalVM Native [arm64-darwin23]
backtrace_alias.rb:3:in 'Parent#original'
backtrace_alias.rb:11:in '<main>'

backtrace_alias.rb:15:in 'Original#original'
backtrace_alias.rb:23:in '<main>'
```

In #19117 there was some discussion about this, notably:
https://bugs.ruby-lang.org/issues/19117#note-17
@byroot
...
My suggestion is for the owner. Simply put because it matches the path and other existing method representations.
But actually the owner isn't that, what is described here is the original module of the method definition, same as the expected from this issue.

https://bugs.ruby-lang.org/issues/19117#note-18
@eregon
...
It must be the owner, anything else would be very confusing.
I believe nobody wants String#then (there is no such method), they want to see Kernel#then.
Where I was wrong about owner being the only thing. I was also expressing we should show where the method is defined, but I overlooked the "originally" part (aliases, define_method, etc).

Also CC @mame who implemented the change.

I think this is worth fixing for Ruby 4.1 (not sure if worth backporting).

PR: https://github.com/ruby/ruby/pull/17963

-- 
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