Fix srml-balance burn implementation (#3443)

* tests

* fix

* bump version

substrate/srml/balances/src/lib.rs

@@ -824,12 +824,12 @@ where
 	}
 
 	fn burn(mut amount: Self::Balance) -> Self::PositiveImbalance {
-		<TotalIssuance<T, I>>::mutate(|issued|
-			issued.checked_sub(&amount).unwrap_or_else(|| {
+		<TotalIssuance<T, I>>::mutate(|issued| {
+			*issued = issued.checked_sub(&amount).unwrap_or_else(|| {
 				amount = *issued;
 				Zero::zero()
-			})
-		);
+			});
+		});
 		PositiveImbalance::new(amount)
 	}
 

substrate/srml/balances/src/tests.rs

@@ -789,3 +789,19 @@ fn signed_extension_take_fees_is_bounded() {
 		}
 	);
 }
+
+#[test]
+fn burn_must_work() {
+	with_externalities(
+		&mut ExtBuilder::default()
+			.monied(true)
+			.build(),
+		|| {
+			let init_total_issuance = Balances::total_issuance();
+			let imbalance = Balances::burn(10);
+			assert_eq!(Balances::total_issuance(), init_total_issuance - 10);
+			drop(imbalance);
+			assert_eq!(Balances::total_issuance(), init_total_issuance);
+		}
+	);
+}

substrate/srml/treasury/src/lib.rs

@@ -507,10 +507,13 @@ mod tests {
 	#[test]
 	fn unused_pot_should_diminish() {
 		with_externalities(&mut new_test_ext(), || {
+			let init_total_issuance = Balances::total_issuance();
 			Treasury::on_dilution(100, 100);
+			assert_eq!(Balances::total_issuance(), init_total_issuance + 100);
 
 			<Treasury as OnFinalize<u64>>::on_finalize(2);
 			assert_eq!(Treasury::pot(), 50);
+			assert_eq!(Balances::total_issuance(), init_total_issuance + 50);
 		});
 	}