PhpStorm with PHPUnit 8.4 gives exception Uncaught PHPUnit\Runner\Exception class ... could not be found

Same thing happened to me. All of the sudden I started getting the following error:

PHP Fatal error:  Uncaught PHPUnit\Runner\Exception: Class 'Tests\\Feature\\ExampleTest' could not be found 

And after I have read @frank-vue's comment I noticed the same thing and he did: If I run tests on the entire folder it runs normally, but if I run test on a specific class/method I get that error.

I tried earlier version of PHPStorm, downgraded PHP plugin etc... and nothing worked.

In my case, when I checked the stacktrace looks like:

#0 /var/www/vendor/phpunit/phpunit/src/Runner/BaseTestRunner.php(145): PHPUnit\Runner\StandardTestSuiteLoader->load('Tests\\\\Unit\\\\Ex...', '/var/www/tests/...')
#1 /var/www/vendor/phpunit/phpunit/src/Runner/BaseTestRunner.php(105): PHPUnit\Runner\BaseTestRunner->loadSuiteClass('Tests\\\\Unit\\\\Ex...', '/var/www/tests/...')
#2 /var/www/vendor/phpunit/phpunit/src/TextUI/Command.php(177): PHPUnit\Runner\BaseTestRunner->getTest('Tests\\\\Unit\\\\Ex...', '/var/www/tests/...', Array)
#3 /var/www/vendor/phpunit/phpunit/src/TextUI/Command.php(159): PHPUnit\TextUI\Command->run(Array, true)
#4 /var/www/vendor/phpunit/phpunit/phpunit(61): PHPUnit\TextUI\Command::main()
#5 {main}
  thrown in /var/www/vendor/phpunit/phpunit/src/Runner/StandardTestSuiteLoader.php on line 69

Notice Tests\\\\Unit\\\\Ex... instead of Tests\\Unit\\Ex....

So in the end I broke the rule and I've modified vendor file, which should be avoided at any cost, but as a temporary solution it solves my problem.

So I added 1 line to the vendor/phpunit/phpunit/src/Runner/BaseTestRunner.php on line 98 (PHPUnit version 8.4.1), which replaces unnecessary '\'s.

if (empty($suiteClassFile) && \is_dir($suiteClassName) && !\is_file($suiteClassName . '.php')) {
    /** @var string[] $files */
    $files = (new FileIteratorFacade)->getFilesAsArray(
        $suiteClassName,
        $suffixes
    );

    $suite = new TestSuite($suiteClassName);
    $suite->addTestFiles($files);

    return $suite;
}
$suiteClassName = str_replace('\\\\', '\\', $suiteClassName); // THIS IS THE LINE I ADDED
try {
    $testClass = $this->loadSuiteClass(
        $suiteClassName,
        $suiteClassFile
    );
} catch (Exception $e) {
    $this->runFailed($e->getMessage());
    return null;
}