Updating values inside optional structured config container with default None is not possible

[sstieger]

Describe the bug When creating and extending a structured config with an attribute opt: Optional[T] = None (where T is a container type), it is not possible to set any keys in opt using OmegaConf.update(), as trying to do so raises the error "Unexpected type for root" here (OmegaConf expects an existing Container, but receives None, our default value).

To Reproduce

from dataclasses import dataclass
from omegaconf import OmegaConf
from typing import Optional

@dataclass
class SubConfig:
    val: int = 0

@dataclass
class Config:
    opt: Optional[SubConfig] = None

if __name__ == "__main__":
    conf = OmegaConf.structured(Config)
    OmegaConf.update(conf, "opt.val", 42, merge=True)

Expected behavior The structured config attribute opt should be initialized as a container, followed by the requested update.

Additional context

• [x] OmegaConf version: 2.2.3
• [x] Python version: 3.10
• [x] Operating system: Linux
• [x] Please provide a minimal repro

[sstieger]

@Jasha10 I think this is a duplicate of your #1035, sorry - I'll close this :)