test.cr

abstract class Base
end

class Example1 < Base
end

class Example2 < Base
end

class Test
  setter :a
end

t1 = Test.new
t1.a = Example1
t2 = Test.new
t2.a = Example2

# crystal tool hierarchy test.cr

+- class Reference (4 bytes)
     |
     +- class Test (24 bytes)
     |      @a : Base+:Class? (16 bytes)
     
How can I specify an array of Base+:Class ?

klasses = [] of Base+
klasses << Example1

-- > no overload matches 'Array(Base+)#<<' with type Example1:Class

klasses = [] of Base+:Class

-- > unexpected token: Class