Some interesting behavior with PHPUnit mocks & return types that are final classes.

Test.php

<?php

// The return type of a method, imagine this is a
// nice value object.
final class Foo
{
}

// interface that returns a `Foo` value object
interface Bar
{
    public function stuff() : Foo;
}

class Qux
{
    private $bar;

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

    public function doAThing()
    {
        $this->bar->stuff();
    }
}

class Test extends \PHPUnit_Framework_TestCase
{
    private $bar, $qux;

    // this produces a warning, "Final Classes Can't be Mocked"
    public function testCannotMockCallsToMethodsWithFinalReturnTypes()
    {
        $this->bar->expects($this->once())
            ->method('stuff');

        $this->qux->doAThing();
    }

    // but this works just fine.
    public function testMockingCallsAndSpecifyingAReturnManuallWorksJustFine()
    {
        $this->bar->expects($this->once())
            ->method('stuff')
            ->willReturn(new Foo());

        $this->qux->doAThing();
    }

    protected function setUp()
    {
        $this->bar = $this->getMock(Bar::class);
        $this->qux = new Qux($this->bar);
    }
}