Ruby 4.0 incompatibility with subclasses of `Set` due to new `Set::SubclassCompatible` layer

[davidcelis]

Ruby 4.0 introduced a compatibility layer for classes that inherit from Set, the Set::SubclassCompatible module which currently breaks Sorbet's static checks against Tapioca-generated RBI files: https://docs.ruby-lang.org/en/master/Set.html#class-set-inheriting-from-set

As of Ruby 4.0, when a class inherits from Set, Ruby dynamically requires a file in its standard library that defines the Set::SubclassCompatible constant, which seems to mean that it's otherwise unavailable during static checks: https://github.com/ruby/ruby/blob/master/set.c#L422-L432

static VALUE
set_s_inherited(VALUE klass, VALUE subclass)
{
    if (klass == rb_cSet) {
        // When subclassing directly from Set, include the compatibility layer
        rb_require("set/subclass_compatible.rb");
        VALUE subclass_compatible = rb_const_get(klass, id_subclass_compatible);
        rb_include_module(subclass, subclass_compatible);
        rb_extend_object(subclass, rb_const_get(subclass_compatible, id_class_methods));
    }
    return Qnil;
}

We discovered this when trying to upgrade our application to Ruby 4. One of our gem dependencies defines a class that inherits from Set, and running tapioca gem results in an RBI file for that gem that triggers errors when running srb tc:

sorbet/rbi/gems/block-kit@1.0.7.rbi:4180: Unable to resolve constant SubclassCompatible https://srb.help/5002
    4180 |  include ::Set::SubclassCompatible
                    ^^^^^^^^^^^^^^^^^^^^^^^^^
  Did you mean Comparable? Use -a to autocorrect
    sorbet/rbi/gems/block-kit@1.0.7.rbi:4180: Replace with Comparable
    4180 |  include ::Set::SubclassCompatible
                    ^^^^^^^^^^^^^^^^^^^^^^^^^
    https://github.com/sorbet/sorbet/tree/3558afcc707c2790d81c4ffd54243a55c4435a89/rbi/core/comparable.rbi#L40: Comparable defined here
    40 |module Comparable
        ^^^^^^^^^^^^^^^^^

sorbet/rbi/gems/block-kit@1.0.7.rbi:4181: Unable to resolve constant SubclassCompatible https://srb.help/5002
    4181 |  extend ::Set::SubclassCompatible::ClassMethods
                   ^^^^^^^^^^^^^^^^^^^^^^^^^
  Did you mean Comparable? Use -a to autocorrect
    sorbet/rbi/gems/block-kit@1.0.7.rbi:4181: Replace with Comparable
    4181 |  extend ::Set::SubclassCompatible::ClassMethods
                   ^^^^^^^^^^^^^^^^^^^^^^^^^
    https://github.com/sorbet/sorbet/tree/3558afcc707c2790d81c4ffd54243a55c4435a89/rbi/core/comparable.rbi#L40: Comparable defined here
    40 |module Comparable
        ^^^^^^^^^^^^^^^^^

[KaanOzkan]

Tapioca is correctly identifying the dynamic mixin inclusion but the definition for Set::SubclassCompatible and Set::SubclassCompatible::ClassMethods is missing. Best way to resolve it would be to add it to the Ruby standard library RBIs that are part of Sorbet https://github.com/sorbet/sorbet/blob/master/rbi/stdlib/set.rbi. Afterwards type checking error should go away.