[ruby-core:98691] [Ruby master Bug#16942] instance_method causes an infinite loop with prepend, include and private

Issue #16942 has been reported by pocke (Masataka Kuwabara).

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Bug #16942: instance_method causes an infinite loop with prepend, include and private
https://bugs.ruby-lang.org/issues/16942

* Author: pocke (Masataka Kuwabara)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.8.0dev (2020-06-09T13:51:54Z master 7f3efee102) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
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`instance_method` causes an infinite loop with the following code.


```ruby
module M
  def x
  end
end

module M2
  include M
  private :x
end

::Object.prepend(M2)

# infinite loop
p Object.instance_method(:x)
```


I've confirmed it on the master branch. The commit is 7f3efee102.
But `instance_method` works correctly with Ruby 2.7.1.



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