Save the receiver and key locals of a literal-union keyed write

packages/compiler/src/frontend/lowering/expressions/element-access.ts

@@ -2610,14 +2610,14 @@ export function lowerElementWrite(lowerer: Lowerer, expr: ts.BinaryExpression):
       // A key whose literal type names only declared fields of one type
       // writes the field it names. JavaScript evaluates the receiver, then
       // the key, then the value; the dispatch reads the saved receiver and
-      // key once per named field.
+      // key once per named field. Locals are saved too, because the key or
+      // the value can reassign them.
       const named =
         litKey === null ? sameTypedFieldKeys(lowerer, shape, target.argumentExpression) : null;
       if (named && !shape.fields.every((f) => typeEquals(f.type, named.fieldType))) {
         const loc = locOf(expr);
         const body: IrStmt[] = [];
         const save = (value: IrExpr, name: string): IrExpr => {
-          if (value.kind === "varRef") return value;
           const temp = lowerer.declareHiddenLocal(name, value.type);
           body.push({ kind: "varDecl", localId: temp.id, init: value, loc });
           return varRef(temp.id, temp.type, loc);

tests/corpus/record-literal-key-unions.ts

@@ -76,3 +76,23 @@ function only(s: Stats, key: "bytes"): number {
   return s[key];
 }
 console.log(only(stats[3]!, "bytes"));
+
+// A key or value that reassigns the receiver or key local: the write goes
+// to the receiver and key evaluated before them.
+let target = stats[0]!;
+let slot: Counter = "hits";
+target[slot] = ((slot = "misses"), 500);
+console.log(stats[0]!.hits, stats[0]!.misses, slot);
+target[slot] = ((target = stats[2]!), 600);
+console.log(stats[0]!.misses, stats[2]!.misses, target === stats[2]);
+function retarget(): Counter {
+  target = stats[3]!;
+  return "bytes";
+}
+target = stats[1]!;
+target[retarget()] = 700;
+console.log(stats[1]!.bytes, stats[3]!.bytes === 700);
+target = stats[0]!;
+slot = "hits";
+target[slot] += ((slot = "bytes"), (target = stats[1]!), 1);
+console.log(stats[0]!.hits, stats[0]!.bytes, stats[1]!.hits);