Bug Report

🔎 Search Terms

inlining duplication declaration emit optional union parameters

🕗 Version & Regression Information

• This is not a regression.
• This is a stylistic issue in emitted DTS. The code is correct.
• This is the behavior in every version I tried (3.3 ... nightly)

⏯ Playground Link

Playground link with relevant code

💻 Code

type u = 1 | 2 | 3 | 4 | 5; 

export const o = {
    mandatory(arg: u): void {arg},  // DTS references union (good)
    optional(arg?: u): void {arg},  // DTS inlines union (bad)
}

🙁 Actual behavior

DTS emit expands the definition of the union for the optional parameter.

declare type u = 1 | 2 | 3 | 4 | 5;
export declare const o: {
    mandatory(arg: u): void;
    optional(arg?: 1 | 2 | 3 | 4 | 5 | undefined): void;
};

🙂 Expected behavior

DTS emit should treat it the same as a mandatory parameter and reference it by name.

declare type u = 1 | 2 | 3 | 4 | 5;
export declare const o: {
    mandatory(arg: u): void;
    optional(arg?: u): void;
};

Related Issues

This is a very specific instance of the issue of declarations inlining types. I'm raising it because it looks like a pure win to simplify emitted DTS files. Fixing this will make other userland tricks to simplify types more effective.

If fixing this is contentious, or requires a large overhaul, please feel free to immediately close. I will not be offend because I realize there is a long tail of issues in this space and do not wish to create noise.