Repository: TypeScript
Issue: [TS 5.4.0-beta] .d.ts emit adding an inline type import instead of utilising existing import
Labels: Help Wanted,Possible Improvement

### 🔎 Search Terms

declaration emit inline type import resolution

### 🕗 Version & Regression Information

- This changed between versions 5.3.3 and 5.4.0-beta
- This changed in PR #56857


### ⏯ Playground Link

https://www.typescriptlang.org/dev/bug-workbench/?emitDeclarationOnly=true&baseUrl=true&ts=5.4.0-beta#code/PTAEAEBMFMGMBsCGAnRAXAlgewHYCgQIBnACywHcBRAWwzQLHFIprrWkgDEN5oAuUIgAOQ4DkQA3DAHN0WZMHIZI06GgC0AM0Sw08gJ4A6SIbRE8DCJp7Rx1fqHgYARsAw4YAD2OnzGakLyaKAAVIJEoAAi0NoArvBolJ6ByMGayFjUoABEhm447Mji8MAwcQkA+tDJQUTZANx41SnBAN5RMYjxiTWp4aBlXQmgAL6NzUGh-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

### 💻 Code

```ts repro
// @declaration
// @showEmit
// @showEmittedFile: app/navigator/widget-factory.d.ts

// @filename: lib/index.d.ts
import * as DefaultExport from "./internal/default_exports";
export { DefaultExport as default };
export * as BaseWindow from "./internal/windows/basewindow.d.ts";

// @filename: lib/internal/default_exports.d.ts
export { default as BaseWindow } from "./windows/basewindow";

// @filename: lib/internal/windows/basewindow.d.ts
interface BaseWindow {}
export interface BaseWindowConstructor {}
declare const BaseWindow: BaseWindowConstructor;
export default BaseWindow;


// @filename: lib-utils/shared-types/widget-config.d.ts
import type UI from "../../lib/index";
export interface WidgetFactory {
    instantiator(parentWindow: UI.BaseWindow): void;
}

// @filename: app/navigator/widget-factory.ts
import {WidgetFactory} from "../../lib-utils/shared-types/widget-config";

declare const instantiator: WidgetFactory['instantiator'];

// 5.3: __synthetic_export_lib_1.BaseWindow.default
// 5.4: import("../../lib/internal/default_exports").BaseWindow
export default Object.freeze({
    instantiator
});

// Our tools inject this before we pass the file to TS.
import type * as __synthetic_export_lib_1 from "../../lib/index";
```



### 🙁 Actual behavior

```typescript
// app/navigator/widget-factory.d.ts
declare const _default: Readonly<{
    instantiator: (parentWindow: import("../../lib/internal/default_exports").BaseWindow) => void;
}>;
export default _default;
import type * as __synthetic_export_lib_1 from "../../lib/index";
```

### 🙂 Expected behavior

```typescript
// app/navigator/widget-factory.d.ts
declare const _default: Readonly<{
    instantiator: (parentWindow: __synthetic_export_lib_1.BaseWindow.default) => void;
}>;
export default _default;
import type * as __synthetic_export_lib_1 from "../../lib/index";
```

### Additional information about the issue

More background in previous conversation around this: #38111 and this public blog post: https://www.bloomberg.com/company/stories/10-insights-adopting-typescript-at-scale/

The JavaScript runtime we are using TypeScript on is not based on npm style packages (no `node_modules` resolution or `package.json` manifest). For the `.d.ts` that we publish to be able to work consistently across different versions it's important that the inter-package import paths only resolve to the 'public' files and not internal implementation `.d.ts` files.

To encourage TypeScript to pick these safer paths when emitting `.d.ts` we add unused imports to the bottom of each file before passing them to the TS compiler.

In general this is still working in 5.4.0-beta, however during our testing at least 8 packages's `.d.ts` emit regressed with the beta. The playground above re-creates one of these.