There was a blog post on August 11th claiming that all maintained Symfony branches were 100% compatible with PHP 7. At the time, (and as of right now), 2.3 is a maintained branch.

However, if you view the code under the Debug section, you'll see that there is no support for the new Error type. Function typing should be using \Throwable, but instead are still using \Exception.

Which leads to errors like:

Whoops, looks like something went wrong.
1/1FatalErrorException: Error: Uncaught TypeError: Argument 1 passed to Symfony\Component\Debug\ExceptionHandler::handle() must be an instance of Exception, instance of Error given in ...\vendor\symfony\symfony\src\Symfony\Component\Debug\ExceptionHandler.php:67
Stack trace:

0 [internal function]: Symfony\Component\Debug\ExceptionHandler->handle(Object(Error))

1 {main}

thrown in ...\vendor\symfony\symfony\src\Symfony\Component\Debug\ExceptionHandler.php line 67
in ...\vendor\symfony\symfony\src\Symfony\Component\Debug\ExceptionHandler.php line 67

So... is 2.3 fully compatible with PHP 7 or not?