Add regression test for #8395

Ref #8395
PR #8396

tests/test_client_functional.py

@@ -3644,3 +3644,24 @@ async def handler(_: web.Request) -> web.Response:
     session = await aiohttp_client(app, raise_for_status=None)  # type: ignore[arg-type]
 
     await session.get("/")
+
+
+@pytest.mark.xfail(
+    reason="#8395 Error message regression for large headers in 3.9.4",
+    raises=AssertionError,
+)
+async def test_header_too_large_error(aiohttp_client: Any) -> None:
+    """By default when not specifying `max_field_size` requests should fail with a 400 status code."""
+
+    async def handler(_: web.Request) -> web.Response:
+        return web.Response(headers={"VeryLargeHeader": "x" * 10000})
+
+    app = web.Application()
+    app.add_routes([web.get("/", handler)])
+    client = await aiohttp_client(app)
+
+    with pytest.raises(
+        aiohttp.ClientResponseError, match="Got more than 8190 bytes*"
+    ) as exc_info:
+        await client.get("/")
+    assert exc_info.value.status == 400