Annotated dependencies are interpreted incorrectly when using PEP 695-style type alias.

[Rockmizu]

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Example Code

from __future__ import annotations

from typing import Annotated, TypeAlias

from fastapi import Depends, FastAPI

app = FastAPI()


# dependency
async def some_value() -> int:
    return 123

# This works.
DependedValue: TypeAlias = Annotated[int, Depends(some_value)]

# This won't work.
# type DependedValue = Annotated[int, Depends(some_value)]


@app.get('/')
async def get_with_dep(value: DependedValue) -> str:
    print(f'{type(value) = !r}')
    print(f'{value = !r}')

    assert isinstance(value, int), '`value` should be an integer.'
    assert value == 123, '`value` should be 123.'

    return f'value: {value}'

Description

When using the method in line 15:

DependedValue: TypeAlias = Annotated[int, Depends(some_value)]

to declare a type alias, FastAPI correctly interprets it and fills the value with 123 for get_with_dep.

However, when using the new type alias syntax introduced in Python 3.12 (line 18):

type DependedValue = Annotated[int, Depends(some_value)]

FastAPI interprets it as a URL query parameter and responds with HTTP 422 Unprocessable Entity when accessing /.

Expected Response

HTTP 200 OK

"value: 123"

Actual Response

HTTP 422 Unprocessable Entity

{"detail":[{"type":"missing","loc":["query","value"],"msg":"Field required","input":null,"url":"https://errors.pydantic.dev/2.5/v/missing"}]}

Operating System

Windows

Operating System Details

No response

FastAPI Version

0.104.1

Pydantic Version

2.5.1

Python Version

Python 3.12.0

Additional Context

No response

[9en9i]

I also encountered this problem 😥
Apparently the problem is that when using type the type becomes TypeAliasType.

[Rockmizu]

Yes, fortunately, it's possible to extract the aliased type from TypeAliasType both in static analysis and at runtime. We can hope that when FastAPI officially supports 3.12, this bug will be fixed.

[Kludex]

I've created #10719 to track this.