Real code: https://github.com/maiha/rocksdb.cr/blob/iteration/src/rocksdb/iterator.cr#L73

generics-problem-min.cr

module Core(T)
  @a : Bool
end

class Base(T)
  include Core(Int32)

  def initialize
    @a = true
  end
end

Base(String).new  # OK                                                          

# This code fails
class Foo < Base(String)
end
# Error in test.cr:2: instance variable '@a' of Base(String) was not initialized in all of the 'initialize' methods, rendering it nilable