Bug with subgroups and None

[tachyonicClock]

Describe the bug
When using the simple_parsing.subgroups function inside an optional child
data class, the default None value is not respected.

To Reproduce

import simple_parsing
from dataclasses import dataclass
from typing import Optional


@dataclass
class SubgroupA:
    pass

@dataclass
class SubgroupB:
    pass

@dataclass
class Child:
    subgroups: SubgroupA | SubgroupB = simple_parsing.subgroups(
        {
            "a": SubgroupA,
            "b": SubgroupB,
        },
        default="a"
    )

@dataclass
class Options:
    child: Optional[Child] = None

options: Options = simple_parsing.parse(Options, args=[])

print(options)

Expected behavior

The child field should be None, as it is the default value.

Options(child=None)

Actual behavior

The child field is erroneously set to Child(subgroups=SubgroupA()).

Options(child=Child(subgroups=SubgroupA()))

Desktop (please complete the following information):

• Version 0.1.4
• Python version: 3.10.13

Additional context
There is no additional context.

[lebrice]

Thanks for posting @tachyonicClock!
I'll take a look, but don't expect a quick fix, I'm planning a major refactor of the implementation for subgroups at the moment, which should hopefully make it a lot easier to fix these kinds of issues.