Support for generic dataclasses

[bilelomrani1]

🚀 Feature request

Add support for generic dataclasses in ArgumentParser.

Motivation

I often use generic dataclasses as configuration objects in libraries code. When building a CLI application, I would like to reuse these configurations to build a specialized parser.
Currently if a generic dataclass is used as a subparser, the argument parsing complains that all parameters are not typed and crashes.

from dataclasses import dataclass
from typing import Generic, TypeVar

from jsonargparse import ActionConfigFile, ArgumentParser

T = TypeVar("T")

# This is generic library code
@dataclass
class SubConfig(Generic[T]):
    generic: T
    a: int = 1
    b: float = 2.0

# This is CLI-specific
@dataclass
class Config:
    subconfig: SubConfig[int]


if __name__ == "__main__":
    parser = ArgumentParser(print_config="--print-config")
    parser.add_argument("--load", action=ActionConfigFile, help="Path to a YAML configuration")
    parser.add_dataclass_arguments(Config, "config")
    parser.parse_args()

$ python mre_jsonargparse.py --print-config
Traceback (most recent call last):
  File "/Users/bilelomrani/Documents/ILLUIN.nosync/instructions-finetuning/mre_jsonargparse.py", line 24, in <module>
    parser.add_dataclass_arguments(Config, "config")
  File "/Users/bilelomrani/Documents/ILLUIN.nosync/instructions-finetuning/.venv/lib/python3.10/site-packages/jsonargparse/_signatures.py", line 435, in add_dataclass_arguments
    self._add_signature_parameter(
  File "/Users/bilelomrani/Documents/ILLUIN.nosync/instructions-finetuning/.venv/lib/python3.10/site-packages/jsonargparse/_signatures.py", line 381, in _add_signature_parameter
    raise ValueError(
ValueError: With fail_untyped=True, all mandatory parameters must have a supported type. Parameter 'subconfig' from '__main__.Config.__init__' does not specify a type.

With fail_untyped=False, the printed config is empty.

Pitch

Would be cool if parser.add_dataclass_arguments would work with generic dataclasses and would generate a specialized subparser (in the case of my example, adding an int validator to config.subconfig.generic).

[mauvilsa]

Support for generic types would be a good addition to jsonargparse.

[mauvilsa]

I have implemented support for generic types in #378. @bilelomrani1 please test it.