Adjust code to review findings

test/libsolidity/syntaxTests/operators/custom/operator_bound_to_enum.sol

@@ -10,4 +10,4 @@ function add(E, E) pure returns (E) {
 }
 
 // ----
-// TypeError 9921: (0-23): The "using" directive cannot be used to attach functions to the enum type.
+// TypeError 9921: (0-23): The "using" directive cannot be used to attach functions to enum types.

test/libsolidity/syntaxTests/operators/custom/operator_consecutive_calls.sol

@@ -0,0 +1,14 @@
+struct S { uint v; }
+
+using {add as +} for S;
+
+function add(S memory _a, S memory) pure returns (S memory) {
+    return _a;
+}
+
+function g() pure returns (S memory) {
+    S memory a = S(0);
+    S memory b = S(1);
+    S memory c = S(2);
+    return a + b + c;
+}

test/libsolidity/syntaxTests/operators/custom/two_functions_bound_to_operator.sol

@@ -14,4 +14,4 @@ function test() {
     Int.wrap(0) + Int.wrap(1);
 }
 // ----
-// TypeError 2271: (213-238): Operator + not compatible with types Int and Int. A user-defined operator not unique.
+// TypeError 2271: (213-238): Operator + not compatible with types Int and Int. Multiple user-defined functions provided for this operator.