[ruby-core:108833] [Ruby master Bug#18435] Calling `protected` on ancestor method changes result of `instance_methods(false)`

Issue #18435 has been updated by Eregon (Benoit Daloze).


I believe the only way that makes sense here is to remove "ZSUPER methods" altogether.
Other Ruby implementations do not have this needless complexity and near-impossible-to-understand semantics.
https://bugs.ruby-lang.org/issues/18751#note-11 might help to be closer, but IMHO we should remove "ZSUPER methods" altogether.

`public/private/protected` should shallow-copy a method entry (but still change `Method#owner` of course), just like `alias_method` already behaves.
This is what TruffleRuby and JRuby do, it's simpler, it is what Ruby users expect (Method is a Ruby object that captures a method entry, at the time it was requested), and it is consistent (`.owner` is always the module which has that method entry in its method table: #18729).

For instance this should be `:p1\n:orig1` but currently it's `:p2\n:orig1` on CRuby.
I claim no Ruby user expects that, because `Method` should capture a specific method entry, that's why we have bind/call and that's how Method objects are used.
```ruby
class P
  private
  def m
    :p1
  end
  
  public
  def orig
    :orig1
  end
end

class C < P
  public :m
  alias_method :alias, :orig
end

class P
  private
  def m
    :p2
  end
  
  public
  def orig
    :orig2
  end
end

p C.new.m
p C.new.alias
```

@matz OK to remove "ZSUPER methods" and make public/protected/private much simpler by having them shallow copy method entries, just like alias_method already does it?
This will solve a lot of confusion and inconsistency for Method objects of methods defined by public/protected/private.

----------------------------------------
Bug #18435: Calling `protected` on ancestor method changes result of `instance_methods(false)`
https://bugs.ruby-lang.org/issues/18435#change-97909

* Author: ufuk (Ufuk Kayserilioglu)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.7.5p203 (2021-11-24 revision f69aeb8314) [x86_64-darwin20]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
As documented `instance_methods(false)` works as follows:

```ruby
module A
  def method1()  end
end

class B
  include A

  def method2()  end
end

p B.instance_methods(false) #=> [:method2]
```

However, calling `protected` on the method defined by `A`, unexpectedly changes the result of `instance_methods(false)` on `B`, even though the owner of the method is still `A`:
```ruby
module A
  def method1()  end
end

class B
  include A

  protected :method1

  def method2()  end
end

p B.instance_methods(false) #=> [:method1, :method2]
p B.instance_method(:method1).owner #=> A
```

In contrast, calling `private` or `public` on the same method does not cause any changes on the result of `B.instance_methods(false)`.

This feels like a bug in the implementation of `instance_methods(false)`, but, if it is by design, it should at least be documented on `Module#instance_methods`.

This reproduction script gives the same output all the way from Ruby 2.0 up to Ruby-HEAD:
https://wandbox.org/permlink/LqbXMBTYxURRZmDz



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