[ruby-core:109477] [Ruby master Bug#18435] Calling `protected` on ancestor method changes result of `instance_methods(false)`

Issue #18435 has been updated by nagachika (Tomoyuki Chikanaga).


I agree to backport the revert to ruby_3_1, but I think removing the existing methods in stable releases could be a severe damage  to the depending applications/libraries.
I'd like to add dummy methods {Method,UnboundMethod}#{public?,protected?,private?} with warnings in 3.1.3.
@matz @jeremyevans0 I propose to the dummy implementation of {Method,UnboundMethod}#public? return true constantly, and the others return false. Do you have any objection for adding dummy methods?

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Bug #18435: Calling `protected` on ancestor method changes result of `instance_methods(false)`
https://bugs.ruby-lang.org/issues/18435#change-98644

* Author: ufuk (Ufuk Kayserilioglu)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 2.7.5p203 (2021-11-24 revision f69aeb8314) [x86_64-darwin20]
* Backport: 2.6: DONTNEED, 2.7: DONTNEED, 3.0: DONTNEED, 3.1: REQUIRED
----------------------------------------
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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