Keep island arrays of intersection types as handles

`Choice[] & GenChoice[]`, an island array extended by the caller's own choice
type (pi's openrouter images: `imageResponse.choices`), was refused with
SC8010. The intersection has no native IR type of its own — mapTypeOf
returns null, or its arms map to different array types — so the crossing
failed before the container rules that keep an array the island does not
declare a handle.

An intersection of containers is a container: it is a value of each arm, and
where the program only uses it through island operations (member reads, an
island call, an island identity test, a handle binding) it now stays a
handle, exactly as an undeclared array does. Elements and fields the
declaration does name cross back out on their own terms, so an element's
`images` array copies, and any other use still refuses rather than inventing
a native layout.

Test: tests/corpus/6901-node-island-array-intersection (both shapes, the
island-declared arm and the caller's own arm, differential to Node, plain
and sanitized).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/compiler/src/frontend/lowering/lower-node-island.ts

@@ -681,12 +681,19 @@ function fromIsland(lowerer: Lowerer, raw: IrExpr, node: ts.Expression): IrExpr
   if ((tsType.flags & (ts.TypeFlags.Void | ts.TypeFlags.Undefined)) !== 0 && isDiscarded(node))
     return raw;
   const mapped = lowerer.mapTypeOf(tsType);
-  if (mapped === null)
+  if (mapped === null) {
+    // An intersection of containers (`Choice[] & GenChoice[]`: the
+    // island's array extended by the caller's own type) has no native
+    // representation of its own. Where the program only uses it through
+    // island operations it stays a handle, like an array the island does
+    // not declare.
+    if (containerType(lowerer, tsType) && handleUseOk(lowerer, node)) return raw;
     boundaryFail(
       lowerer,
       `an island value of type '${lowerer.checker.typeToString(tsType)}'`,
       node,
     );
+  }
   if (mapped.kind === "dyn") return raw;
   if (mapped.kind === "void") return raw;
   // An island promise is the same promise on both sides (the runtime keeps
@@ -887,7 +894,8 @@ function builtinHandleClass(type: ts.Type): boolean {
 
 /** Arrays, tuples, Maps and Sets (every arm of a union, nullish arms
  * aside), and the builtin classes of builtinHandleClass: the object types
- * an island hands back without declaring them. */
+ * an island hands back without declaring them. An intersection of those
+ * (`Choice[] & GenChoice[]`) is a container too: it is a value of each. */
 function containerType(lowerer: Lowerer, type: ts.Type): boolean {
   if (type.isUnionType())
     return ts
@@ -897,6 +905,14 @@ function containerType(lowerer: Lowerer, type: ts.Type): boolean {
           (t.flags & (ts.TypeFlags.Null | ts.TypeFlags.Undefined | ts.TypeFlags.Void)) !== 0 ||
           containerType(lowerer, t),
       );
+  if ((type.flags & ts.TypeFlags.Intersection) !== 0)
+    return ts
+      .constituentTypes(type)
+      .every(
+        (t) =>
+          (t.flags & (ts.TypeFlags.Null | ts.TypeFlags.Undefined | ts.TypeFlags.Void)) !== 0 ||
+          containerType(lowerer, t),
+      );
   if ((type.flags & ts.TypeFlags.Object) === 0) return false;
   if (lowerer.checker.isArrayType(type) || lowerer.checker.isTupleType(type)) return true;
   const name = type.getSymbol()?.escapedName;

packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6901-node-island-array-intersection/main.ts.json

@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/6901-node-island-array-intersection/main.ts"
+ ],
+ "diags": []
+}

tests/corpus/6901-node-island-array-intersection/main.ts

@@ -0,0 +1,24 @@
+// @requires-libnode
+// Reading an element of an island array whose declared type is an
+// INTERSECTION of two array types — `Choice[] & GenChoice[]`, the island's
+// choices extended by the caller's own choice type — has no native
+// representation of its own, so the array stays a handle and the read runs
+// in the island; the fields the declaration names cross back out.
+import { extendedImageResponse, imageResponse, unsafe } from "choicekit";
+import type { Choice } from "choicekit";
+
+const extended = extendedImageResponse();
+const first = extended.choices[0];
+console.log(first.index, first.text);
+console.log(first.images === undefined ? "no-images" : first.images.join("+"));
+
+const second = extended.choices[1];
+console.log(second.index, second.text, second.images === undefined ? "no-images" : second.images.join("+"));
+
+/** The caller's own choice type over the island's array. */
+interface NativeChoice extends Choice {
+  readonly images?: string[];
+}
+
+const third = unsafe<Choice[] & NativeChoice[]>(imageResponse().choices)[1];
+console.log(third.text, third.images === undefined ? "no-images" : third.images.join("+"));

tests/corpus/6901-node-island-array-intersection/node_modules/choicekit/index.d.ts

@@ -0,0 +1,18 @@
+export interface Choice {
+  readonly index: number;
+  readonly text: string;
+}
+export interface GenChoice extends Choice {
+  readonly images?: string[];
+}
+export interface ImageResponse {
+  readonly choices: Choice[];
+}
+export interface ExtendedImageResponse {
+  /** The island's choices, extended by the island's own choice type. */
+  readonly choices: Choice[] & GenChoice[];
+}
+export declare function imageResponse(): ImageResponse;
+export declare function extendedImageResponse(): ExtendedImageResponse;
+/** Declares the type of the value it is given, like `Type.Unsafe<T>`. */
+export declare function unsafe<T>(value: T): T;

tests/corpus/6901-node-island-array-intersection/node_modules/choicekit/index.js

@@ -0,0 +1,10 @@
+"use strict";
+
+const choices = () => [
+  { index: 0, text: "first", images: ["a.png", "b.png"] },
+  { index: 1, text: "second" },
+];
+
+exports.imageResponse = () => ({ choices: choices() });
+exports.extendedImageResponse = () => ({ choices: choices() });
+exports.unsafe = (value) => value;

tests/corpus/6901-node-island-array-intersection/node_modules/choicekit/package.json

@@ -0,0 +1 @@
+{ "name": "choicekit", "version": "1.0.0", "main": "index.js", "types": "index.d.ts" }

tests/corpus/6901-node-island-array-intersection/package.json

@@ -0,0 +1 @@
+{ "name": "island-array-intersection", "private": true, "type": "module", "scriptc": { "packages": { "choicekit": "node" } } }

tests/harness/corpus-costs/6901-node-island-array-intersection/main.ts.json

@@ -0,0 +1 @@
+{ "plain": 3000, "san": 42000 }