Incremental builds ignore missing ambient declarations

[fgallardograzio]

TypeScript Version: 3.8.0-dev.20200208

Search Terms: incremental build ambient declarations declare global dts

Code

// globals.d.ts
declare namespace Config {
  const value: string;
}

// index.ts
console.log(Config.value);

// tsconfig.json
{
  "compilerOptions": {
    "incremental": true
  }
}

Steps to reproduce:

• Run tsc. The project should be built successfully.
• Remove the contents of globals.d.ts or delete the file itself. (vscode immediately reports the "Cannot find name 'Config'." error in index.ts).
• Run tsc again. The build process still completes successfully.
• Run tsc --incremental false. Now it fails correctly.

Expected behavior:
The removal of an ambient declaration should be caught by TS when using --incremental or --build and the proper error should be reported.

Actual behavior:
When a complete ambient type declaration is removed, related types don't seem to be re-checked.

On the other hand, changing an ambient declaration works as expected.
If, e.g., the value constant is removed from the namespace declaration, the proper error is thrown.

// globals.d.ts
declare namespace Config {
//   const value: string;
}

Please note that this also affects project references' --builds.

Playground Link:
Not needed.

Related Issues:
Not an issue, but maybe #32849 is related?

Thanks