test: indexed (compound) assignment evaluation order and single-evaluation

test/libsolidity/semanticTests/viaYul/ir_lvalue_compound_assignment.sol

@@ -0,0 +1,31 @@
+// Pins down evaluation order and single-evaluation for indexed
+// (compound) assignment:
+//   - In `a[f()] = g()`, the RHS `g()` is evaluated before the LHS
+//     index `f()`, so RHS side effects are observable when the index
+//     is computed.
+//   - In `a[f()] += v`, the LHS index `f()` is evaluated exactly once;
+//     the slot computation is not re-evaluated for the write side.
+contract C {
+    uint[4] public a;
+    uint public counter;
+
+    function bumpIdx() internal returns (uint) {
+        counter += 1;
+        return counter - 1;
+    }
+
+    function bumpVal() internal returns (uint) {
+        return 10 * (counter + 100);
+    }
+
+    function run() public returns (uint c, uint a0, uint a1) {
+        a[bumpIdx()] = bumpVal();
+        a[bumpIdx()] = bumpVal();
+        a[bumpIdx()] += 5;
+        c = counter;
+        a0 = a[0];
+        a1 = a[1];
+    }
+}
+// ----
+// run() -> 3, 1000, 1010