[TS 5.4.0-beta] .d.ts emit adding an inline type import instead of utilising existing import

🔎 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

// @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

// 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

// 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.