From e3477fc1e3ea3619a4b24c5ed1c3f6f9dd9c060a Mon Sep 17 00:00:00 2001
From: Daniel Olano <daniel@olanod.com>
Date: Tue, 5 Dec 2023 16:26:38 +0100
Subject: [PATCH] Change utility::as_derivative index to u32

---
 substrate/frame/utility/src/lib.rs   | 11 ++++++++---
 substrate/frame/utility/src/tests.rs |  9 +++++++++
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/substrate/frame/utility/src/lib.rs b/substrate/frame/utility/src/lib.rs
index 7f963e3637d6f..6e5d925810d4b 100644
--- a/substrate/frame/utility/src/lib.rs
+++ b/substrate/frame/utility/src/lib.rs
@@ -266,7 +266,7 @@ pub mod pallet {
 		})]
 		pub fn as_derivative(
 			origin: OriginFor<T>,
-			index: u16,
+			index: u32,
 			call: Box<<T as Config>::RuntimeCall>,
 		) -> DispatchResultWithPostInfo {
 			let mut origin = origin;
@@ -504,8 +504,13 @@ impl TypeId for IndexedUtilityPalletId {
 
 impl<T: Config> Pallet<T> {
 	/// Derive a derivative account ID from the owner account and the sub-account index.
-	pub fn derivative_account_id(who: T::AccountId, index: u16) -> T::AccountId {
-		let entropy = (b"modlpy/utilisuba", who, index).using_encoded(blake2_256);
+	pub fn derivative_account_id(who: T::AccountId, index: u32) -> T::AccountId {
+		// Derive the same addresses as when index was u16
+		let entropy = if index <= u16::MAX as u32 {
+			(b"modlpy/utilisuba", who, index as u16).using_encoded(blake2_256)
+		} else {
+			(b"modlpy/utilisuba", who, index).using_encoded(blake2_256)
+		};
 		Decode::decode(&mut TrailingZeroInput::new(entropy.as_ref()))
 			.expect("infinite length input; no invalid inputs for type; qed")
 	}
diff --git a/substrate/frame/utility/src/tests.rs b/substrate/frame/utility/src/tests.rs
index 7b42fa511d100..8f3e2c97268d7 100644
--- a/substrate/frame/utility/src/tests.rs
+++ b/substrate/frame/utility/src/tests.rs
@@ -398,6 +398,15 @@ fn as_derivative_filters() {
 	});
 }
 
+#[test]
+fn as_derivative_u32_compat_with_old_u16() {
+	new_test_ext().execute_with(|| {
+		let sub_1_0 = Utility::derivative_account_id(1, 0);
+		const SAME_DERIVATIVE_AS_WITH_OLD_U16_INDEX: u64 = 13950085325555297514;
+		assert_eq!(sub_1_0, SAME_DERIVATIVE_AS_WITH_OLD_U16_INDEX);
+	})
+}
+
 #[test]
 fn batch_with_root_works() {
 	new_test_ext().execute_with(|| {