ruby 3.0: test/unit/suite.rb fails due to change in flatten behavior for subclass of Array

[mtasaka]

Test suite of coderay executes test/unit/suite.rb, whic fails with ruby 3.0:

[mockbuild@8eabb111bdc5457cb9de184d095055ec coderay]$ ruby -v
ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-linux]
[mockbuild@8eabb111bdc5457cb9de184d095055ec coderay]$ ruby ./test/unit/suite.rb
Running CodeRay unit tests: comment_filter, count, debug, duo, file_type, filter, html, json_encoder, lines_of_code, null, plugin, statistic, text, token_kind_filter, tokens, word_list
Loaded suite ./test/unit/suite
Started
....E
===========================================================================================================================================================================
Error: test_filtering_text_tokens(DebugEncoderTest): NoMethodError: undefined method `debug' for ["10", :integer, "(\\)", :operator, :begin_group, :string, "test", :content, :end_group, :string, :begin_line, :head, "\n", :space, "\n  \t", :space, "   \n", :space, "[]", :method, :end_line, :head]:Array
/builddir/build/BUILD/GIT/coderay/test/unit/debug.rb:37:in `test_filtering_text_tokens'
     34:   
     35:   def test_filtering_text_tokens
     36:     assert_equal TEST_OUTPUT, CodeRay::Encoders::Debug.new.encode_tokens(TEST_INPUT)
  => 37:     assert_equal TEST_OUTPUT, TEST_INPUT.debug
     38:   end
     39:   
     40: end
===========================================================================================================================================================================
.................................E
===========================================================================================================================================================================
Error: test_filtering_text_tokens(StatisticEncoderTest): NoMethodError: undefined method `statistic' for ["10", :integer, "(\\)", :operator, :begin_group, :string, "test", :content, :end_group, :string, :begin_line, :test, "\n", :space, "\n  \t", :space, "   \n", :space, "[]", :method, :end_line, :test]:Array
/builddir/build/BUILD/GIT/coderay/test/unit/statistic.rb:56:in `test_filtering_text_tokens'
     53:   
     54:   def test_filtering_text_tokens
     55:     assert_equal TEST_OUTPUT, CodeRay::Encoders::Statistic.new.encode_tokens(TEST_INPUT)
  => 56:     assert_equal TEST_OUTPUT, TEST_INPUT.statistic
     57:   end
     58:   
     59: end
===========================================================================================================================================================================
..............
Finished in 0.07171167 seconds.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
53 tests, 161 assertions, 0 failures, 2 errors, 0 pendings, 0 omissions, 0 notifications
96.2264% passed

Note that the above output (error ) says:

NoMethodError: undefined method `debug' for ["10", :integer, "(\\)", :operator, :begin_group, :string, "test", :content, :end_group, :string, :begin_line, :head, "\n", :space, "\n  \t", :space, "   \n", :space, "[]", :method, :end_line, :head]:Array

despite TEST_INPUT is expected to be class of CodeRay::Tokens.

As far as I examined this, this is due to this change:
https://bugs.ruby-lang.org/issues/6087
https://github.com/ruby/ruby/pull/3690

i.e. with ruby 3.0, for object of subclass of Array, the result of flatten method now becomes class of Array, not class of the subclass.