Irrelevant `Cannot compile modules using option 'outFile'` error when using `emitDeclarationOnly`

[mattmccutchen]

Noticed via https://stackoverflow.com/q/52326378 .

TypeScript Version: master (e471856)

Search Terms: cannot compile modules outFile emitDeclarationOnly

Code

// tsconfig.json
{
  "compilerOptions": {
    "declaration": true,
    "emitDeclarationOnly": true,
    "outFile": "out.d.ts"
  }
}
// a.ts
export default 42;

Expected behavior: Successful declaration file generation.

Actual behavior: a.ts:1:1 - error TS6131: Cannot compile modules using option 'outFile' unless the '--module' flag is 'amd' or 'system'.

Playground Link: N/A

Related Issues: None found