Reuse original type nodes even when part of the node is not usable

[dragomirtitian]

🔍 Search Terms

node reuse declarations

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

When tryReuseExistingTypeNodeHelper encounters an node it can't use, it will throw away the whole original node. We can instead fall back on type printing for only the subnode that is not reusable.

📃 Motivating Example

// @filename: a.ts
export const nImported = "nImported"
export const nNotImported = "nNotImported"
const nPrivate = "private"
export const o = (p1: typeof nImported, p2: typeof nNotImported, p3: typeof nPrivate) => null! as { foo: typeof nImported, bar: typeof nPrivate, baz: typeof nNotImported }

// @filename: b.ts
import { o, nImported } from "./a";
export const g = o
console.log(nImported);

// @filename: c.ts
import * as a from "./a";
export const g = a.o

Playgorund link

// currently generates
// @filename:c.d.ts
import { nImported } from "./a";
export declare const g: (p1: typeof nImported, p2: "nNotImported", p3: "private") => {
    foo: "nImported";
    bar: "private";
    baz: "nNotImported";
};

/// proposed
// @filename: c.d.ts
export declare const g: (p1: typeof a.nImported, p2: typeof a.nNotImported, p3: "private") => {
    foo: typeof a.nImported;
    bar: "private";
    baz: typeof a.nNotImported;
};

💻 Use Cases

1. What do you want to use this for? - Improve declaration fidelity and speed
2. What shortcomings exist with current approaches? - Throwing away the type node can happen half way through resulting in duplicate work.
3. What workarounds are you using in the meantime?