Demonstration of a bug in rollup-plugin-commonjs

main.js

export default (a, b) => a + b;

package.json

{
  "name": "rollup-input-object",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "devDependencies": {
    "rollup": "^0.63.4",
    "rollup-plugin-commonjs": "^9.1.4"
  },
  "scripts": {
    "build": "rollup -c"
  }
}

rollup.config.js

import commonjs from 'rollup-plugin-commonjs'

const input = { outputFileName: 'main.js' };
// const input = ['main.js']

export default {
  input,
  output: {
    dir: 'build',
    format: 'esm'
  },
  experimentalCodeSplitting: true,
  plugins: [
    commonjs()
  ]
}