Merge fails with pair of nested dataclasses

[Jasha10]

This bug report is motivated by facebookresearch/hydra#2349 .

The below example demonstrates two merge operations involving nested dataclasses.
The first merge succeeds, the second one raises ConfigKeyError. I think raising here is undesirable.

from dataclasses import dataclass
from omegaconf import OmegaConf, MISSING

@dataclass
class X:
    d: int
    p: int

@dataclass
class Y(X):
    q: int

@dataclass
class A:
    x: X

@dataclass
class B(A):
    x: Y

OmegaConf.merge(  # succeeds
    A(x=X(d=10, p=2)),
    B(x=Y(d=MISSING, p=MISSING, q=10)),
)

OmegaConf.merge(  # raises ConfigKeyError: Key 'q' not in 'X'
    A(x=X(d=10, p=2)),
    B(x=MISSING),
)

I feel that if the first merge succeeds then the second one should succeed too.