Return value generation interferes with proxying to original method

[sebastianbergmann]

<?php declare(strict_types=1);
final class Foo
{
    private $value;

    public function __construct($value)
    {
        $this->value = $value;
    }

    public function value()
    {
        return $this->value;
    }
}

class Bar
{
    public function doSomething(): Foo
    {
        return new Foo('value');
    }
}

use PHPUnit\Framework\TestCase;

final class Test extends TestCase
{
    public function testOne(): void
    {
        $proxy = $this->createTestProxy(Bar::class);

        $proxy->expects($this->once())
              ->method('doSomething');

        $this->assertSame(
            'value',
            $proxy->doSomething()->value()
        );
    }
}

$ phpunit Test    
PHPUnit 7.5.8 by Sebastian Bergmann and contributors.

W                                                                   1 / 1 (100%)

Time: 52 ms, Memory: 4.00 MB

There was 1 warning:

1) Test::testOne
Class "Foo" is declared "final" and cannot be mocked.

WARNINGS!
Tests: 1, Assertions: 0, Warnings: 1.

[sebastianbergmann]

Invocation::generateReturnValue() is wrongly called to generate a default return value for : Foo. This does not work as Foo is final. If Foo were not final the call to Invocation::generateReturnValue() works but would still be superfluous as we are proxying and use the return value of the original method.