Reproduction link or steps

https://github.com/BlueGreenMagick/tsdown_typeof_undefined

Describe the bug

rolldown-plugin-dts(v0.21.5) does not compile typeof undefined usage as a type correctly.

export function typeof_undefined(): typeof undefined {
  return undefined;
}

This is transformed into below .d.ts code which the typescript library(v5.9.3) fails to parse.

declare function typeof_undefined(): typeof void 0;

Various error may appear depending on context:

TS1144: '{' or ';' expected.
TS1005: ';' expected.

Apparantly, typeof undefined is a valid type in typescript because undefined is a global object. (ECMA spec)
You can test this out by opening a browser and running window.undefined: you'll get the value undefined

Found this error when using tsdown to build the library acorn-typescript (problematic code).

I think it was probably a typo - I can't think of any valid reasons to use typeof undefined over undefined.

System Info

System:
    OS: macOS 15.6
    CPU: (8) arm64 Apple M1
    Memory: 351.30 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 25.2.1 - /opt/homebrew/bin/node
    npm: 11.6.2 - /opt/homebrew/bin/npm
    pnpm: 10.23.0 - /opt/homebrew/opt/node@22/bin/pnpm
  Browsers:
    Chrome: 144.0.7559.97
    Firefox: 147.0.1
    Safari: 18.6

Validations

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• 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

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