jest: `jest` fails to generate coverage when using the `testRegex` config option

🐛 Bug Report

jest@24.0.0-alpha.1 fails to generate coverage when using the testRegex config param. This issue has been introduced recently by https://github.com/facebook/jest/pull/7209.

To Reproduce

A very simple end to end test can be created in e2e/__tests__/coverage_report.test.js to reproduce the issue:

test('generates coverage when using the testRegex config param ', () => {
  const {stdout, status} = runJest(DIR, [
    '--no-cache',
    '--testRegex=__tests__',
    '--coverage',
  ]);
  expect(status).toBe(0);
});

Expected behavior

jest should execute correctly when generating coverage with the testRegex config param.

Additional information

This is the stack trace from the error:

ERROR: regex.test is not a function
      STACK: TypeError: regex.test is not a function
          at config.testRegex.config.testRegex.some.regex (~/jest/packages/jest-runtime/build/should_instrument.js:64:42)
          at Array.some (<anonymous>)
          at shouldInstrument (~/packages/jest-runtime/build/should_instrument.js:64:22)
          at Function.shouldInstrument (~/packages/jest-runtime/build/index.js:241:74)
          at exports.default (~/packages/jest-cli/build/generateEmptyCoverage.js:14:51)
          at Object.worker (~/packages/jest-cli/build/reporters/coverage_worker.js:52:80)
          at execFunction (~/packages/jest-worker/build/child.js:146:17)
          at execHelper (~/packages/jest-worker/build/child.js:128:5)
          at execMethod (~/packages/jest-worker/build/child.js:132:5)
          at process.on (~/packages/jest-worker/build/child.js:43:7)

The issue seems to be caused by the fact that the coverage is generated from a worker, so the config object gets serializer when it’s sent to the worker by jest-worker.

jest-worker does not have any special logic to serialize regular expressions, so it defers it to Node’s child_process logic, which converts a RegExp into an empty object when serializing and deserializing it.

Potential solutions

A couple of potential solutions that occur to me:

1. Make the serialization of objects a bit smarter in jest-worker: it could even use jest-serializer for it, which handles RegExps (this may affect the performance though).
2. Do not normalize the testRegex config param to an array of RegExp objects, but to an array of strings, and generate the RegExp object on demand when needed (similarly than how it was done before https://github.com/facebook/jest/pull/7209).