[ruby-core:69108] [Ruby trunk - Bug #11120] Unexpected behavior when mixing Module#prepend with method aliasing

Issue #11120 has been updated by Pablo Herrero.


I gave some more thought to this but I can't really find a way to improve the migration path from aliases to prepend without creating new problems.
I think this issue should be closed.

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Bug #11120: Unexpected behavior when mixing Module#prepend with method aliasing
https://bugs.ruby-lang.org/issues/11120#change-52353

* Author: Pablo Herrero
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-linux]
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN
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I'm not completely sure myself if this should be considered a bug, but at least it should be up for discussion.

I stumbled upon this behavior when migrating some code using alias chains to Module#prepend.
Consider the following code:

```ruby
# thingy.rb
class Thingy
  def thingy
    puts "thingy"
  end
end

# thingy_with_foo.rb
module ThingyWithFoo
  def thingy
    puts "thingy with foo"
    super
  end
end

Thingy.prepend(ThingyWithFoo)

# thingy_with_bar.rb
class Thingy
  alias_method :thingy_without_bar, :thingy # Wont't alias create an alias for Thingy#thingy but ThingyWithFoo#thingy instead

  def thingy_with_bar
    puts "thingy with bar"
    thingy_without_bar # Expected to call original Thingy#thingy method but will call prepended method instead
  end

  alias_method :thingy, :thingy_with_bar
end

# some_file.rb
Thingy.new.thingy # raises: stack level too deep (SystemStackError))
```

In a nutshell when calling `super` from `ThingyWithFoo#foo` it will call `thingy_with_bar` method, and this method will call back to `ThingyWithFoo#foo` by invoking `thingy_without_bar`, thus producing an endless loop.

This situation arises because `alias_method` is producing an alias not for the Thingy#thingy method the but for the upper method from `ThingyWithFoo` instead. May be this behavior could be considered correct, I'm still not sure, but it will probably became a problem for source code migrating from alias chains to use `Modue#prepend`, specially when other active gems could potentially still be using alias chains themselves without the user knowledge.



-- 
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