Tagged Union field validation results in the wrong error messages.

[ojii]

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Example Code

I've written a test which shows the problem:

https://github.com/ojii/fastapi/blob/tagged-union-validation/tests/test_tagged_union_field_validation_errors.py

Description

Given ```python
class A(BaseModel):
tag: Literal["a"]

class B(BaseModel):
tag: Literal["b"]
value: conint(ge=0, le=1)

class Item(BaseModel):
value: Union[A, B]

If you send `{value: {tag: 'b', value: 2}}` the error message is:

```python
{
    "detail": [
        {
            "loc": ["body", "value", "tag"],
            "msg": "unexpected value; permitted: 'a'",
            "type": "value_error.const",
            "ctx": {"given": "b", "permitted": ["a"]},
        },
        {
            "loc": ["body", "value", "value"],
            "msg": "ensure this value is less than or equal to 1",
            "type": "value_error.number.not_le",
            "ctx": {"limit_value": 1},
        },
    ]
}

I think it should be:

{
    "detail": [
        {
            "loc": ["body", "value", "value"],
            "msg": "ensure this value is less than or equal to 1",
            "type": "value_error.number.not_le",
            "ctx": {"limit_value": 1},
        },
    ]
}

Operating System

macOS

Operating System Details

(os should not matter)

FastAPI Version

0.68.0 (25a4828)

Python Version

3.8.3 and 3.9.1

Additional Context

No response

[ojii]

sorry, this is a bug in pydantic. will report it there.

[YuriiMotov]

This is not a bug, but just how it works.
Pydantic tries to validate data using all types in Union and collect all error messages.

You can make error message cleaner using discriminated unions: https://docs.pydantic.dev/latest/concepts/unions/#discriminated-unions