Behavior difference: Side-effect imports of custom file extension are emitted by tsgo but not tsc

[hkleungai]

Steps to reproduce

1. Start a plain new project
   • "@typescript/native-preview": "7.0.0-dev.20260525.1",
   • "typescript": "6.0.3"
2. Run npx tsgo --init to get the default tsconfig. Turn on these flags as well.
   • "allowJs": true,
   • "checkJs": true,
   • "stableTypeOrdering": true,
3. Add src/main.js & src/module.d.ts. Run npx tsc --noEmit & npx tsgo --noEmit to see the difference.

// src/main.js
import './foo.custom';

// src/module.d.ts
declare module '*.custom';

Behavior with typescript@6.0

export {};
//# sourceMappingURL=main.d.ts.map

Behavior with tsgo

import './foo.custom';
//# sourceMappingURL=main.d.ts.map