Invalid DSL when passing a class with a `type_parameter` to `attribute`'s `cast_type`

Sorry if this belongs on Discourse – I was initially going to post this there but I believe this is a bug considering invalid DSL is being generated rather than an error popping up with the code I've written. I can move to Discourse if it makes more sense there.

Tapioca generates invalid DSL when I pass an instance of a class that has a type_member to attribute's cast_type (2nd argument). For example, given a class GenericValue which inherits from ActiveRecord::Type::Value and that has a ValueType = type_member:

• If I just do attribute :column, GenericValue.new, the methods in the generated DSL use just ValueType as a parameter/return value which doesn't exist outside GenericValue, so Sorbet raises Unable to resolve constant ValueType all over the place.
• If I explicitly pass a type attribute :column, GenericValue[SomeObject].new, the methods in the generated DSL fall back to T.untyped, even if the parameter/return value in the serialize/deserialize do not use ValueType.

Here's an example commit: https://github.com/ShopifyFRS/bourgeois/compare/do-not-merge-generic-attribute-cast-type

• non_generic_value uses ::Acceptance::SomeObject as expected.
• generic_value uses ValueType instead of ::Acceptance::SomeObject and which is undefined outside Acceptance::GenericValue.
• explicit_generic_value uses T.untyped instead of Acceptance::SomeObjectImpl (my expectation).

I've also tried reproducing the issue in a separate blank repo using the latest versions of Sorbet and Tapioca but the same thing happens. Unfortunately I cannot push this to GitHub since I cannot create repos in the Shopify organization.

Start by reproducing the issue with the GenericValue, non_generic_value, generic_value, and explicit_generic_value examples, then trace Tapioca's handling of attribute's cast_type and generated DSL types. Done means the generated DSL resolves the type_member correctly for implicit and explicit generic casts instead of emitting an undefined ValueType or T.untyped.