Right now inheriting from generic types, and even inheriting a non-generic type that's not Reference is broken. There are many issues related to this, which I'll link here:

• Compiler problem in 0.16.0 with generics #2558 bug with generics inheritance
• class method returns one value, but is_a?(<this value>) contradicts. Can't debug #2376 mentions generics, the bug might be related to that
• Return type as generic base class is not allowed #2283 Return type as generic base class is not allowed
• How does macro inherited work with generics? #1793 How does macro inherited work with generics
• Bug with macro def and generic class inheriting non-generic class #846 Bug with macro def and generic class inheriting non-generic class
• Typing an instance variable as a generic module with specific type #2459 Can't type an instance variable as a generic module type
• Recursive alias does not match non generic subclass of generic type #1473 Recursive alias does not match non generic subclass of generic type
• Variance with generic parent class #1943 Variance with generic parent class
• Inheriting from generic class/struct ignoring the type causes segmentation fault #1944 Inheriting from generic class/struct ignoring the type causes segmentation fault
• Compiler error when subclassing HTTP::Handler with a generic class #2051 Compiler error when subclassing HTTP::Handler with a generic class

And one pending decision:

• Covariance and contravariance in generic types #1297 Covariance and contravariance in generic types

All of these should be fixed. The main issue is that the current implementation is very weak and needs a redesign. This is only a matter of time.

This issue will be used to mark all bugs related to generics as a duplicate of this one, because it's essentially one problem.