Problem: Existing interfaces with generic types may be only partially supportable with nullable types

[natebosch]

Concrete example: List<int?>(1); is safe, but List<int>(1) will most likely need to throw at runtime because [null] is not a List<int> with NNBD. List<int>.filled(1, 0); would be safe.

[leafpetersen]

Yes. The length setter is another known case.