Add check for ciruclar imports + test

fastapi/routing.py

@@ -1333,6 +1333,7 @@ class APIRouter(routing.Router):
         app.include_router(internal_router)
         ```
         """
+        assert self is not router, "Cannot include router into itself"
         if prefix:
             assert prefix.startswith("/"), "A path prefix must start with '/'"
             assert not prefix.endswith("/"), (

tests/test_router_circular_import.py

@@ -0,0 +1,11 @@
+import pytest
+from fastapi import APIRouter, FastAPI
+
+
+def test_router_circular_import():
+    app = FastAPI()
+    router = APIRouter()
+
+    app.include_router(router)
+    with pytest.raises(AssertionError, match="Router cannot be the same as parent"):
+        router.include_router(router)