Improve literal dataclass field conversion to argparse argument (#17391)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

tests/test_config.py

@@ -1,14 +1,47 @@
 # SPDX-License-Identifier: Apache-2.0
 
 from dataclasses import MISSING, Field, asdict, dataclass, field
+from typing import Literal, Union
 
 import pytest
 
-from vllm.config import ModelConfig, PoolerConfig, get_field
+from vllm.config import ModelConfig, PoolerConfig, config, get_field
 from vllm.model_executor.layers.pooler import PoolingType
 from vllm.platforms import current_platform
 
 
+class TestConfig1:
+    pass
+
+
+@dataclass
+class TestConfig2:
+    a: int
+    """docstring"""
+
+
+@dataclass
+class TestConfig3:
+    a: int = 1
+
+
+@dataclass
+class TestConfig4:
+    a: Union[Literal[1], Literal[2]] = 1
+    """docstring"""
+
+
+@pytest.mark.parametrize(("test_config", "expected_error"), [
+    (TestConfig1, "must be a dataclass"),
+    (TestConfig2, "must have a default"),
+    (TestConfig3, "must have a docstring"),
+    (TestConfig4, "must use a single Literal"),
+])
+def test_config(test_config, expected_error):
+    with pytest.raises(Exception, match=expected_error):
+        config(test_config)
+
+
 def test_get_field():
 
     @dataclass