Fix Array#concat signature on subclasses

[kachick]

$ ruby -v -e 'p Class.new(Array).new.concat.class'
ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin20]
#<Class:0x00007ff93307abf0>

$ ruby -v -e 'p Class.new(Array).new.concat.class'
ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-darwin20]
#<Class:0x00007faa0010d2e8>

[kachick]

I don’t know why even in without changing signature, this added test passed. But adding it makes better, I think.
Please tell me a way if rbs have something to ensure it. 🙏

[soutaro]

Good point. The test passes because Class.new(Array).new.is_a?(Array) holds. The runtime type checker can be improved...

[kachick]

😮 Thank you for letting me know the why!

[soutaro]

👍

[kachick]

Thank you!