Add regression test for Annotated ForwardRef OpenAPI (issue #13056)

tests/test_issue_13056_openapi_annotated_forwardref.py

@@ -0,0 +1,29 @@
+from typing import Annotated, Union
+
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+from pydantic import BaseModel
+
+app = FastAPI()
+
+
+class Item(BaseModel):
+    name: str
+    # This is the ForwardRef issue context
+    next_item: Annotated[Union["Item", None], None] = None
+
+
+@app.get("/", response_model=Item)
+def read_root():
+    return Item(name="root")
+
+
+client = TestClient(app)
+
+
+def test_issue_13056_openapi_annotated_forwardref():
+    # This triggers the schema generation where the crash usually happens
+    response = client.get("/openapi.json")
+    assert response.status_code == 200, response.text
+    data = response.json()
+    assert data["components"]["schemas"]["Item"]