Keep a spread tuple alive until all its fields are packed into the rest

A spread tuple's first field carries the statements that evaluate the
tuple into a hidden local, and its later fields read that local. When
lowerArraySpreadItems stored the first field as one item's expression,
the local's frame ended with that store, so a tuple returned by a call
was released before its later fields were read (a use-after-free under
SCRIPTC_SAN=1). The field's statements now join the packing sequence at
the item's position, whose frame lasts until every item is stored.

packages/compiler/src/frontend/lowering/containers/array-construction.ts

@@ -174,7 +174,15 @@ export function lowerArraySpreadItems(
   ];
   for (const node of nodes) {
     if (!(node instanceof AstNode)) {
-      const value = undefinedForNullish(lowerer, node, arrType);
+      // A spread tuple's first field declares the tuple's storage, which
+      // the later fields read: its statements join this sequence, whose
+      // frame lasts until every item is stored.
+      let item = node;
+      if (item.kind === "seqExpr") {
+        body.push(...item.stmts);
+        item = item.result;
+      }
+      const value = undefinedForNullish(lowerer, item, arrType);
       const stored =
         lowerer.runtimeOptionalWidening(value.type, elem) !== null ||
         optionalElementValue(lowerer, value, elem)

tests/corpus/8004-optional-element-conversions.ts

@@ -58,6 +58,14 @@ const mixedChanging: [boolean, boolean | undefined] = [true, undefined];
 console.log(flags(...mixedChanging, ...mixedMore, flip(mixedChanging, true), ...mixedChanging));
 const mixedWords: [string, string | undefined] = ["a", undefined];
 console.log(label("z", ...["b", undefined, "c"], ...mixedWords), label("y", ...mixedWords, ...new Set(["s", undefined])));
+// A tuple returned by a call keeps its string fields alive until every
+// field is packed beside the other spreads.
+function makeTuple(n: number): [string, string | undefined, string] {
+  return ["p" + n.toString(16).repeat(4), n % 2 ? undefined : "q" + n, "r".repeat((n % 7) + 3) + n];
+}
+for (let i = 0; i < 3; i++) {
+  console.log(label("z", ...["m", undefined], ...makeTuple(i), ...mixedWords), label("y", ...makeTuple(i + 10), ...new Set(["s" + i])));
+}
 
 function pick(start: (number | undefined)[] | string[], other: (number | undefined)[] | string[]): string {
   let a = start;