🐛 Fix parameterless `Depends()` with generics (#9479)

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>

fastapi/dependencies/utils.py

@@ -1,6 +1,6 @@
 import inspect
 from contextlib import AsyncExitStack, contextmanager
-from copy import deepcopy
+from copy import copy, deepcopy
 from typing import (
     Any,
     Callable,
@@ -384,6 +384,8 @@ def analyze_param(
             field_info.annotation = type_annotation
 
     if depends is not None and depends.dependency is None:
+        # Copy `depends` before mutating it
+        depends = copy(depends)
         depends.dependency = type_annotation
 
     if lenient_issubclass(

tests/test_generic_parameterless_depends.py

@@ -0,0 +1,77 @@
+from typing import TypeVar
+
+from fastapi import Depends, FastAPI
+from fastapi.testclient import TestClient
+from typing_extensions import Annotated
+
+app = FastAPI()
+
+T = TypeVar("T")
+
+Dep = Annotated[T, Depends()]
+
+
+class A:
+    pass
+
+
+class B:
+    pass
+
+
+@app.get("/a")
+async def a(dep: Dep[A]):
+    return {"cls": dep.__class__.__name__}
+
+
+@app.get("/b")
+async def b(dep: Dep[B]):
+    return {"cls": dep.__class__.__name__}
+
+
+client = TestClient(app)
+
+
+def test_generic_parameterless_depends():
+    response = client.get("/a")
+    assert response.status_code == 200, response.text
+    assert response.json() == {"cls": "A"}
+
+    response = client.get("/b")
+    assert response.status_code == 200, response.text
+    assert response.json() == {"cls": "B"}
+
+
+def test_openapi_schema():
+    response = client.get("/openapi.json")
+    assert response.status_code == 200, response.text
+    assert response.json() == {
+        "info": {"title": "FastAPI", "version": "0.1.0"},
+        "openapi": "3.1.0",
+        "paths": {
+            "/a": {
+                "get": {
+                    "operationId": "a_a_get",
+                    "responses": {
+                        "200": {
+                            "content": {"application/json": {"schema": {}}},
+                            "description": "Successful " "Response",
+                        }
+                    },
+                    "summary": "A",
+                }
+            },
+            "/b": {
+                "get": {
+                    "operationId": "b_b_get",
+                    "responses": {
+                        "200": {
+                            "content": {"application/json": {"schema": {}}},
+                            "description": "Successful " "Response",
+                        }
+                    },
+                    "summary": "B",
+                }
+            },
+        },
+    }