Fix support for unevaluated stringified annotations (Python 3.14)

PR #14485 fixed support for if TYPE_CHECKING, non-evaluated stringified
annotations, when using `from __future__ import annotations`.

This change adds the same support when using deferred evaluation of
annotations introduced in Python 3.14

fastapi/dependencies/utils.py

@@ -204,6 +204,11 @@ def _get_signature(call: Callable[..., Any]) -> inspect.Signature:
         except NameError:
             # Handle type annotations with if TYPE_CHECKING, not used by FastAPI
             # e.g. dependency return types
+            if sys.version_info >= (3, 14):
+                from annotationlib import Format
+
+                signature = inspect.signature(call, annotation_format=Format.FORWARDREF)
+            else:
                 signature = inspect.signature(call)
     else:
         signature = inspect.signature(call)

tests/test_stringified_annotation_dependency_py314.py

@@ -0,0 +1,30 @@
+from typing import TYPE_CHECKING, Annotated
+
+from fastapi import Depends, FastAPI
+from fastapi.testclient import TestClient
+
+from .utils import needs_py314
+
+if TYPE_CHECKING:  # pragma: no cover
+
+    class DummyUser: ...
+
+
+@needs_py314
+def test_stringified_annotation():
+    # python3.14: Use forward reference without "from __future__ import annotations"
+    async def get_current_user() -> DummyUser | None:
+        return None
+
+    app = FastAPI()
+
+    client = TestClient(app)
+
+    @app.get("/")
+    async def get(
+        current_user: Annotated[DummyUser | None, Depends(get_current_user)],
+    ) -> str:
+        return "hello world"
+
+    response = client.get("/")
+    assert response.status_code == 200

tests/utils.py

@@ -6,8 +6,8 @@ needs_py39 = pytest.mark.skipif(sys.version_info < (3, 9), reason="requires pyth
 needs_py310 = pytest.mark.skipif(
     sys.version_info < (3, 10), reason="requires python3.10+"
 )
-needs_py_lt_314 = pytest.mark.skipif(
+needs_py314 = pytest.mark.skipif(
-    sys.version_info >= (3, 14), reason="requires python3.13-"
+    sys.version_info < (3, 14), reason="requires python3.14+"
 )