Loading `debug` gem breaks `blankslate` gem

Reproduce the interaction using the Ruby 2.7.2 and debug 1.6.2 environment described, then inspect lib/debug/session.rb around lines 1686-1689. Compare behavior with and without requiring debug, and verify the BlankSlate example remains compatible while determining the purpose of the redefinition.

Your environment

• ruby -v: ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-linux]
• rdbg -v: rdbg 1.6.2

Describe the bug

The debug gem undefines and redefines singleton_method_added.

The blankslate gem provides a "abstract base class with no predefined methods" except for some core Ruby builtins. "BlankSlate is useful as a base class when writing classes that depend upon method_missing (e.g. dynamic proxies)", so it only works if singleton_method_added is available. However, it hides singleton_method_added because it's redefined by the debug gem.

This problem does not occur when the debug gem is not required.

To Reproduce

require 'debug'
require 'blankslate'

BlankSlate.new.singleton_method_added

Buggy output -- the method isn't defined:

Traceback (most recent call last):
repro.rb:4:in `<main>': undefined method `singleton_method_added' for #<BlankSlate:0x000055a654b12fb0> (NoMethodError)

Expected behavior

The method is defined, but private:

Traceback (most recent call last):
repro.rb:3:in `<main>': private method `singleton_method_added' called for #<BlankSlate:0x000055d758dcd540> (NoMethodError)

Workaround

We can work around the bug by explicilty unhiding this method, or by requiring blankslate before debug:

BlankSlate.reveal(:singleton_method_added)

Additional context

The problem is caused by this code in the debug gem:

https://github.com/ruby/debug/blob/19b4dde3308f532943e4234d1588d4fa26c52345/lib/debug/session.rb#L1686-L1689

Why does the debug gem redefine this method?