DictConfig merge throws away data when element_type is structured

[Jasha10]

$ cat tmp.py
from dataclasses import dataclass
from omegaconf import DictConfig, OmegaConf


@dataclass
class User:
    name: str
    age: int


cfg1 = DictConfig({"user007": User("bond", 7)}, element_type=User)
cfg2 = {"user007": {"age": 99}}
print(OmegaConf.merge(cfg1, cfg2))

$ python tmp.py
{'user007': {'name': '???', 'age': 99}}

Desired output is

{'user007': {'name': 'bond', 'age': 99}}

[Jasha10]

This is related to the following lines in BaseContainer._map_merge, where dest[key] gets overwritten with an empty element_type instance before merging src_value with dest_node:

            if (
                is_structured_config(dest._metadata.element_type)
                and not missing_src_value
            ):
                dest[key] = DictConfig(content=dest._metadata.element_type, parent=dest)
                dest_node = dest._get_node(key)

[omry]

Thanks for reporting and for the PR! will take a look soon.