MethodProphecy::willThrow() doesn't accept (an extension of) Throwable

[timoschinkel]

Working on implementing unit tests for a piece of code that uses a PSR-18 client I was unable to have a \Psr\Http\Client\ClientInterface::sendRequest() MethodProphecy to throw one of the expected exception interface. To reproduce:

<?php

class ReproductionTest extends \PHPUnit\Framework\TestCase
{
    public function testThrowable()
    {
        $client = $this->prophesize(\Psr\Http\Client\ClientInterface::class);
        $client->sendRequest(Argument::type(\Psr\Http\Message\RequestInterface::class))
            ->willThrow(\Psr\Http\Client\ClientExceptionInterface::class);
        
        // do assertions
    }
}

Since \Psr\Http\Client\ClientExceptionInterface extends \Throwable this should work. But it doesn't:

Prophecy\Exception\InvalidArgumentException : Exception / Throwable class or instance expected as argument to ThrowPromise, but got Psr\Http\Client\ClientExceptionInterface.

This scenario is reproducible on the latest stable version 1.8.0 and on the master branch.