Fix Annotated forward ref resolution with from __future__ import annotations

When using rom __future__ import annotations, annotations are stored as
strings. If Annotated[SomeClass, Depends()] references a class defined
after the route decorator, the forward reference could not be resolved at
decoration time.

FastAPI's evaluate_forwardref() does not raise on unresolvable references
- it returns the raw ForwardRef object. The existing code only handled the
string-to-ForwardRef conversion but did not check whether resolution
actually succeeded, allowing an unresolvable ForwardRef to leak into the
type annotation. This caused Pydantic to fail with
'class-not-fully-defined' errors and the Depends metadata to be lost.

The fix adds a fallback: when evaluate_forwardref() returns a ForwardRef
(rather than the resolved type) and the annotation string is Annotated-
shaped, a lenient resolution evaluates the annotation with a namespace
that maps undefined names to Any. This preserves the Annotated structure
so FastAPI can extract the Depends metadata.

fastapi/dependencies/utils.py

@@ -246,11 +246,42 @@ def get_typed_annotation(annotation: Any, globalns: dict[str, Any]) -> Any:
     if isinstance(annotation, str):
         annotation = ForwardRef(annotation)
         annotation = evaluate_forwardref(annotation, globalns, globalns)
-        if annotation is type(None):
+        if isinstance(annotation, ForwardRef):
+            if "Annotated[" in annotation.__forward_arg__:
+                annotation = _resolve_forward_ref_lenient(
+                    annotation, globalns
+                )
+        elif annotation is type(None):
             return None
     return annotation
 
 
+def _resolve_forward_ref_lenient(
+    fwd_ref: ForwardRef,
+    globalns: dict[str, Any],
+) -> Any:
+    class _LenientNamespace(dict):
+        def __missing__(self, key: str) -> Any:
+            return Any
+
+    localns = _LenientNamespace(globalns)
+    result = fwd_ref._evaluate(globalns, localns, set())
+    return result
+
+
+def _resolve_forward_ref_lenient(
+    fwd_ref: ForwardRef,
+    globalns: dict[str, Any],
+) -> Any:
+    class _LenientNamespace(dict):
+        def __missing__(self, key: str) -> Any:
+            return Any
+
+    localns = _LenientNamespace(globalns)
+    result = fwd_ref._evaluate(globalns, localns, set())
+    return result
+
+
 def get_typed_return_annotation(call: Callable[..., Any]) -> Any:
     signature = _get_signature(call)
     unwrapped = inspect.unwrap(call)

tests/test_annotated_forward_ref.py

@@ -0,0 +1,67 @@
+from __future__ import annotations
+
+from typing import Annotated
+
+from inline_snapshot import snapshot
+
+from fastapi import Depends, FastAPI
+
+# Simulate the real-world bug: Potato is defined AFTER the route decorator
+# under `from __future__ import annotations`.
+
+app = FastAPI()
+
+
+def get_potato():
+    return Potato(color="red", size=10)
+
+
+@app.get("/")
+async def read_root(potato: Annotated[Potato, Depends(get_potato)]):
+    return {"color": potato.color, "size": potato.size}
+
+
+from dataclasses import dataclass
+
+
+@dataclass
+class Potato:
+    color: str
+    size: int
+
+
+def test_forward_ref_annotated_depends():
+    from fastapi.testclient import TestClient
+
+    client = TestClient(app)
+    resp = client.get("/")
+    assert resp.status_code == 200, resp.text
+    assert resp.json() == {"color": "red", "size": 10}
+
+
+def test_forward_ref_annotated_depends_openapi():
+    from fastapi.testclient import TestClient
+
+    client = TestClient(app)
+    resp = client.get("/openapi.json")
+    assert resp.status_code == 200, resp.text
+    assert resp.json() == snapshot(
+        {
+            "openapi": "3.1.0",
+            "info": {"title": "FastAPI", "version": "0.1.0"},
+            "paths": {
+                "/": {
+                    "get": {
+                        "summary": "Read Root",
+                        "operationId": "read_root__get",
+                        "responses": {
+                            "200": {
+                                "description": "Successful Response",
+                                "content": {"application/json": {"schema": {}}},
+                            }
+                        },
+                    }
+                }
+            },
+        }
+    )