More details `Set.new`

[ksss]

Since the implementation of Set is Hash, the key must have a hash method.

$ ruby -e 'Set.new([BasicObject.new])'
/Users/ksss/.rbenv/versions/3.3.0/lib/ruby/3.3.0/set.rb:509:in `add': undefined method `hash' for an instance of BasicObject (NoMethodError)

    @hash[o] = true
         ^^^^^

The type argument A for Set should strictly be A < Hash::_Key. However, making this change would break compatibility, so it cannot be done. In practice, using BasicObject is rare, so there is no need to make this change.

[soutaro]

@ksss

The type argument A for Set should strictly be A < Hash::_Key. However, making this change would break compatibility, so it cannot be done.

I'm fan of making the type parameter bounded, A < Hash::_Key. Did you see any specific compatibility problem with it?

[ksss]

For Set only json has been found, but I have the following concern.

• Only ruby/rbs and ruby/gem_rbs_collection were checked, and I am not sure if they are also completely absent in the sig directory distributed in the rubygems.
• The same issue exists with Hash, which is used in activesupport.

[ksss]

This PR has resolved by https://github.com/ruby/rbs/pull/1810