--incremental retains stale diagnostics when augmenting a re-exported interface #64386

🔎 Search Terms

incremental stale diagnostics module augmentation re-export interface tsbuildinfo

🕗 Version & Regression Information

Reproduces on TypeScript 6.0.3, 7.0.2, and 7.1.0-dev.20260922.1 (the next version when tested).

Tested on macOS arm64 with Node 24.14.1 and npm 11.18.0. I reviewed the FAQ and searched existing augmentation and incremental-build reports.

⏯ Playground Link

https://github.com/infomiho/typescript-incremental-augmentation-repro

💻 Code

// src/registry.ts
export interface Registry {}

// src/index.ts
export type { Registry } from "./registry.js"

// src/augment.ts
import "./index.js"

declare module "./index.js" {
  interface Registry {
    value: number
  }
}

// src/consumer.ts
import type { Registry } from "./registry.js"

const value: Registry["value"] = "hello"

package.json:

{
  "private": true,
  "type": "module",
  "scripts": {
    "check": "tsc"
  },
  "devDependencies": {
    "typescript": "7.0.2"
  }
}

tsconfig.json:

{
  "compilerOptions": {
    "incremental": true,
    "noEmit": true,
    "tsBuildInfoFile": ".cache/tsconfig.tsbuildinfo"
  },
  "include": ["src"]
}

1. Run npm run check. The initial TS2322 error is expected.
2. Change only value: number to value: string in src/augment.ts.
3. Run npm run check again, retaining the build cache.
4. Delete .cache/tsconfig.tsbuildinfo and run npm run check again, without changing any source.

🙁 Actual behavior

Step 3 still reports this error in src/consumer.ts:

error TS2322: Type 'string' is not assignable to type 'number'.

Step 4 succeeds. Incremental and clean builds disagree on identical source files.

🙂 Expected behavior

Step 3 should succeed, matching the clean build. The augmentation now declares value: string.

Additional information about the issue

Changing both ./index.js paths in src/augment.ts to ./registry.js avoids the stale error. Defining Registry in src/index.ts and re-exporting it from src/registry.ts also avoids it. Both controls were verified on all three compiler versions above.

Related: #44094, fixed by #45156, concerns missed augmentation updates during incremental compilation. #63960 also involves augmentation through a type-only re-export, but reports a file-order issue.