Assignment of primitive type to subscripted Dict modifying the value.

[pereman2]

Describe the bug
Assigning an int into a Subscripted Dict raises an error without a descriptive message also, it modifies the value after it to an empty dict.
To Reproduce

@dataclass
class DictClass:
    foo: Dict[str, int] = field(default_factory=lambda: {"a": 4})

@dataclass
class User:
    age: int = 4
    name: str = "foo"

cfg = OmegaConf.structured(DictClass)
cfg.foo = 10

Output

ValidationError: 
        full_key: foo
        reference_type=Optional[DictClass]
        object_type=DictClass

After the error cfg.foo is modified:
print(cfg.foo) # prints '{}'

Expected behaviour
cfg.foo = 1 should raise a descriptive error message.

Additional context

• OmegaConf version: 2.1.0
• Python version: 3.8
• Operating system : macOS
• Please provide a minimal repro