Function checkForConflicts

packages/jest-core/src/watch.ts:474–515

(
  watchPluginKeys: WatchPluginKeysMap,
  plugin: WatchPlugin,
  globalConfig: Config.GlobalConfig,
)

}

const checkForConflicts = (
  watchPluginKeys: WatchPluginKeysMap,
  plugin: WatchPlugin,
  globalConfig: Config.GlobalConfig,
) => {
  const key = getPluginKey(plugin, globalConfig);
  if (!key) {
    return;
  }

  const conflictor = watchPluginKeys.get(key);
  if (!conflictor || conflictor.overwritable) {
    watchPluginKeys.set(key, {
      overwritable: false,
      plugin,
    });
    return;
  }

  let error;
  if (conflictor.forbiddenOverwriteMessage) {
    error = `
  Watch plugin ${chalk.bold.red(
    getPluginIdentifier(plugin),
  )} attempted to register key ${chalk.bold.red(`<${key}>`)},
  that is reserved internally for ${chalk.bold.red(
    conflictor.forbiddenOverwriteMessage,
  )}.
  Please change the configuration key for this plugin.`.trim();
  } else {
    const plugins = [conflictor.plugin, plugin]
      .map(p => chalk.bold.red(getPluginIdentifier(p)))
      .join(' and ');
    error = `
  Watch plugins ${plugins} both attempted to register key ${chalk.bold.red(
    `<${key}>`,
  )}.
  Please change the key configuration for one of the conflicting plugins to avoid overlap.`.trim();
  }

  throw new ValidationError('Watch plugin configuration error', error);
};

const getPluginIdentifier = (plugin: WatchPlugin) =>
  // This breaks as `displayName` is not defined as a static, but since