fix (rsc): Fix unsafe {} type in application code for StreamableValue (#1786)

Co-authored-by: Lars Grammel <lars.grammel@gmail.com>

Author: gclark-eightfold

Diff for: .changeset/nice-fishes-hear.md

@@ -0,0 +1,5 @@
+---
+'ai': patch
+---
+
+fix(ai/rsc): Fix unsafe {} type in application code for StreamableValue

Diff for: packages/core/rsc/types.test-d.ts

@@ -0,0 +1,25 @@
+import { expectTypeOf } from 'vitest';
+
+import type { StreamableValue } from './dist';
+
+describe('StreamableValue type', () => {
+  it('should not contain types marked with @internal after compilation', () => {
+    expectTypeOf<StreamableValue>().not.toHaveProperty('type');
+    expectTypeOf<StreamableValue>().not.toHaveProperty('curr');
+    expectTypeOf<StreamableValue>().not.toHaveProperty('error');
+    expectTypeOf<StreamableValue>().not.toHaveProperty('diff');
+    expectTypeOf<StreamableValue>().not.toHaveProperty('next');
+  });
+
+  it('should yield a type error when assigning a wrong value', () => {
+    expectTypeOf<StreamableValue<string>>().not.toEqualTypeOf<
+      StreamableValue<boolean>
+    >();
+
+    expectTypeOf<StreamableValue<string>>().not.toEqualTypeOf<string>();
+
+    expectTypeOf<
+      StreamableValue<string>
+    >().not.toEqualTypeOf<'THIS IS NOT A STREAMABLE VALUE'>();
+  });
+});

Diff for: packages/core/rsc/types.ts

@@ -89,6 +89,9 @@ export type MutableAIState<AIState> = {
 
 export type StreamablePatch = undefined | [0, string]; // Append string.
 
+declare const __internal_curr: unique symbol;
+declare const __internal_error: unique symbol;
+
 /**
  * StreamableValue is a value that can be streamed over the network via AI Actions.
  * To read the streamed values, use the `readStreamableValue` or `useStreamableValue` APIs.
@@ -114,4 +117,8 @@ export type StreamableValue<T = any, E = any> = {
    * @internal Use `readStreamableValue` to read the values.
    */
   next?: Promise<StreamableValue<T, E>>;
+
+  // branded types to maintain type signature after internal properties are stripped.
+  [__internal_curr]?: T;
+  [__internal_error]?: E;
 };

Diff for: packages/core/vitest.edge.config.js

@@ -7,5 +7,8 @@ export default defineConfig({
     globals: true,
     include: ['**/*.test.ts', '**/*.test.tsx'],
     exclude: ['**/*.ui.test.ts', '**/*.ui.test.tsx', 'node_modules/**'],
+    typecheck: {
+      enabled: true,
+    },
   },
 });

Diff for: packages/core/vitest.node.config.js

@@ -7,5 +7,8 @@ export default defineConfig({
     globals: true,
     include: ['**/*.test.ts', '**/*.test.tsx'],
     exclude: ['**/*.ui.test.ts', '**/*.ui.test.tsx', 'node_modules/**'],
+    typecheck: {
+      enabled: true,
+    },
   },
 });