Reproduction link or steps

Given a source file with function overloads:

// src/useConfig.ts
interface Config {
  name: string;
  version: number;
}

export function useConfig(): Config;
export function useConfig<T>(selector: (config: Config) => T): T;
export function useConfig<T>(selector?: (config: Config) => T) {
  const config: Config = { name: 'app', version: 1 };
  return selector ? selector(config) : config;
}

tsc emits the following valid .d.ts:

// src/useConfig.d.ts
interface Config {
  name: string;
  version: number;
}
export declare function useConfig(): Config;
export declare function useConfig<T>(selector: (config: Config) => T): T;

When rolldown-plugin-dts tries to bundle this, it fails with:

[PARSE_ERROR] Error: Duplicated export 'useConfig'
   ╭─[ src/useConfig.d.ts:2:12 ]
   │
 2 │ export var useConfig = [
   │            ────┬────  
   │                ╰────── Export has already been declared here
   │ 
 7 │ export var useConfig = [
   │            ────┬────  
   │                ╰────── It cannot be redeclared here
───╯

Describe the bug

When using rolldown-plugin-dts in tsc mode (i.e. isolatedDeclarations is not enabled), function overloads in the generated .d.ts files cause a PARSE_ERROR: Duplicated export error during bundling.

TypeScript's tsc correctly emits overloaded function signatures as multiple export declare function declarations with the same name — this is valid .d.ts syntax. However, Rolldown's parser rejects these as duplicate exports.

Expected behaviour

rolldown-plugin-dts should correctly handle function overloads in .d.ts files, merging the overload signatures into the bundled output — matching the behaviour of rollup-plugin-dts.

Workaround

Replace function overloads with a single generic signature using rest args and a default type parameter:

export function useConfig<T = Config>(
  ...args: [selector: (config: Config) => T] | []
): T {
  const selector = args[0];
  const config: Config = { name: 'app', version: 1 };
  return (selector ? selector(config) : config) as T;
}

This avoids multiple export declare function lines in the .d.ts output, but requires a cast and is less idiomatic than overloads.

System Info

System:
    OS: Linux 6.6 Ubuntu 22.04.5 LTS 22.04.5 LTS (Jammy Jellyfish)
    CPU: (16) x64 AMD Ryzen 7 PRO 7840U w/ Radeon 780M Graphics
    Memory: 19.78 GB / 50.98 GB
    Container: Yes
    Shell: 5.1.16 - /bin/bash
  Browsers:
    Chrome: 146.0.7680.80


---

## Project environment

- `rolldown-plugin-dts`: `0.22.5`
- `vite`: `8.0.0`
- `typescript`: `5.9.3`
- `isolatedDeclarations`: **not enabled** (using `tsc` mode for `.d.ts` generation)

Validations

• Follow our Code of Conduct
• Read the Contributing Guide.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A, please open a GitHub Discussion instead.
• The provided reproduction is a minimal reproducible of the bug.

Contributions

• I am willing to submit a PR to fix this issue
• I am willing to submit a PR with failing tests (actually just go ahead and do it, thanks!)

Compensating engineering work will speed up resolution and support the project

• I'm willing to offer $16 for financial support