Declaration emit inlines types incorrectly

🔎 Search Terms

declaration emit inline

🕗 Version & Regression Information

I was unable to test this prior to 4.4 since React types are no longer compatible with that version.

⏯ Playground Link

No response

💻 Code

import * as React from "react";

export class Component extends React.Component<{ children: React.ReactNode }> {
  render() {
    if (Math.random()) {
      return <div>{this.props.children}</div>;
    } else {
      return this.props.children;
    }
  }
}

Full repro: https://github.com/eps1lon/ts-module-auto-export/tree/bad-inline

🙁 Actual behavior

Return value from render() is not referencing React.ReactNode but inlines its (incomplete) union members:

import * as React from "react";
export declare class Component extends React.Component<{
    children: React.ReactNode;
}> {
    render(): string | number | boolean | Iterable<React.ReactNode> | import("react/jsx-runtime").JSX.Element;
}

🙂 Expected behavior

Return type references the type alias. Inlining union members is incorrect (we may add or remove members or treat members as internal, local types) generally and here specially not even using all members.

export declare class Component extends React.Component<{
  children: React.ReactNode;
}> {
  render(): React.ReactNode | import("react/jsx-runtime").JSX.Element;
}

Additional information about the issue

The fact that it inlines the type here is a special case of https://github.com/microsoft/TypeScript/issues/37151. https://github.com/microsoft/TypeScript/issues/37151 is a more general since it's not always clear when inlining is correct or not. I'd be curious to know what would break if TS would not stop inlining in this special case.

But this issue is also broader since the inline is incomplete.

Beginne mit der verlinkten bad-inline-Reproduktion von ts-module-auto-export und untersuche die Declaration-Emit-Ausgabe von TypeScript für Component.render. Vergleiche den emittierten Rückgabetyp mit der erwarteten React.ReactNode-Referenz und nutze dann issue #37151, um das damit verbundene Inlining-Verhalten zu verstehen; als erledigt gilt die Aufgabe, wenn die Union vollständig und wie gezeigt über den Type Alias emittiert wird.