ivar initialized error with inherited double nested generic class

[maiha]

I expect instance variable @a is initialized, but got not initialized error in following code.

Real code

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

Minimum reproducible code

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

versions

% crystal -v
Crystal 0.20.0 [b0cc6f7] (2016-11-22)

Got same error on master branch: Crystal 0.20.0+35 [eff31e5] (2016-12-04)

Best wishes,
maiha