As describe in Crystal 1.2.0 is released! - The Crystal Programming Language.
It is now possible to assign a subclass of a generic class to an element of the parent class:
class Foo(T); end class Bar(T) < Foo(T); end x = Foo x = Bar
But, this example never work for me, whatever use 1.5.1 or 1.2.0.
Regarding PR is here