Overloading declaration droped

[imjuni]

Version: 3.1.1, 3.2.0 Rollup Version: 3.7.5 Operating System and version (if applicable): wsl2 Node Version (if applicable): 14.20.1 Does it work with tsc (if applicable): fine works in tsc

Reproduction

rollup-plugin-ts drop overloading declaration.

// index.ts
export { default as isError } from './isError/isError';

// isError.ts
function isError(err: unknown): Error | undefined;
function isError(err: unknown, defaultValue: Error): Error;
function isError(err?: unknown, defaultValue?: Error): Error | undefined {
  if (err instanceof Error) {
    return err;
  }

  if (
    err !== undefined &&
    err !== null &&
    typeof err === 'object' &&
    'message' in err &&
    'stack' in err
  ) {
    return err as Error;
  }

  if (defaultValue !== undefined && defaultValue !== null) {
    return defaultValue;
  }

  return undefined;
}

export default isError;

Expected Behavior

I think overloading function .d.ts file like that,

declare function isError(err: unknown): Error | undefined;
declare function isError(err: unknown, defaultValue: Error): Error;
declare function isError(err?: unknown, defaultValue?: Error): Error | undefined;
export {  isError };

But rollup-plugin-ts drop first declaration in generated .d.ts file.

Actual Behavior

// generated .d.ts
declare function isError(err: unknown): Error | undefined;
declare function isError(err: unknown, defaultValue: Error): Error;
export {  isError };

[runspired]

This would be correct behavior as the final "implementation" signature is merely the fall-through combination of all overloads and not a signature that can be itself matched: thus when using declare for the type the implementation signature is unnecessary and dropped.