Support checked closed generic portable bindings

test/emit-portable-reader.ts

@@ -51,3 +51,39 @@ for (const target of ["browser", "server"])
       `dist/portable-reader/composite/${name}-${target}.ts`,
       generatePortableTypeScriptBindings(composite.definition.bindingSchema, revision, { target }),
     );
+
+const { portableGenericPlan } = await import("./portable-generic-fixture.js");
+const generic = portableGenericPlan();
+const genericPackage = generic.definition.bindingSchema.packages[0]!;
+await mkdir("dist/portable-reader/generic", { recursive: true });
+for (const target of ["browser", "server"]) {
+  await writeFile(
+    `dist/portable-reader/generic/${target}.ts`,
+    generatePortableTypeScriptBindings(generic.definition.bindingSchema, genericPackage.revisionId, { target }),
+  );
+}
+const stringEntry = genericPackage.exports.find((e) => e.inputType.kind === "scalar")!;
+const listEntry = genericPackage.exports.find((e) => e.inputType.kind === "list")!;
+await writeFile(
+  "dist/portable-reader/generic/implementation.ts",
+  `
+import { createRegistry, type PortableImplementation } from "./gen/qx.js";
+export const registry = createRegistry({
+  ${JSON.stringify(stringEntry.displayName)}: context => context.input.toUpperCase(),
+  ${JSON.stringify(listEntry.displayName)}: context => context.input.map(value => value + 1n),
+});
+// The specialization must retain its actual contract, not become unknown or any.
+const negative: PortableImplementation = {
+  // @ts-expect-error a string specialization cannot return a number
+  ${JSON.stringify(stringEntry.displayName)}: context => context.input.length,
+  ${JSON.stringify(listEntry.displayName)}: context => {
+    // @ts-expect-error input collections are read-only
+    context.input.push(0n);
+    // @ts-expect-error integer values are exact bigint, not number
+    const lostPrecision: number = context.input[0]!;
+    return context.input;
+  },
+};
+void negative;
+`,
+);