[BUG] Empty validation error when request parameters are of Union type

Describe the bug

The Union type works as expected when response model is defined as such according to docs: https://fastapi.tiangolo.com/tutorial/extra-models/#union-or-anyof However when parameters (body payload) are defined as Union the code runs fine until executing method, and docs are picking up the types and generating schema correctly but the empty ValidationRequestError is thrown when route method is called even if there are all parameters sent are valid.

To Reproduce

Sample code to reproduce:

class SimpleData(BaseModel):
    foo: Optional[str] = None

class ExtendedData(SimpleData):
    bar: str # Note that this is required

PostData = Union[ExtendedData, SimpleData]

@router.post("/test")
async def post(data: PostData):
    return "OK"

Then the POST /test route is called with a body payload:

{
    "foo": "test1",
    "bar": "test2"
}

As a result the empty ValidationRequestError is thrown with value_error.missing message but no actual field assigned to it.

Expected behavior Parameters from the request are resolved and parsed against the types inside Union.

Environment:

• FastAPI Version: 0.29.0

[LKay]

This seems to be fixed, so closing to clean up backlog.

[nckswt]

Interesting! Yup, can confirm that

@app.post("/test")
def post(data: PostData = Body(...)):
    return "OK"

Does return successfully.