[Feature request] Report willReturn values incompatible with method signature

[Slamdunk]

Given

class MyClass
{
    public function foo(): string {}
}

class MyOtherClassTest extends \PHPUnit\Framework\TestCase
{
    public function testFooMock()
    {
        $class = $this->createMock(MyClass::class);
        $class
            ->expects(static::once())
            ->method('foo')
            ->willReturn(123)
        ;
    }
}

PHPUnit reports:

PHPUnit 8.2.0 by Sebastian Bergmann and contributors.

W                                                                  1 /  1 (100%)

Time: 173 ms, Memory: 16.00 MB

There was 1 warning:

1) MyOtherClassTest::testFooMock
Method foo may not return value of type int

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

It would be awesome if PHPStan could report this errors too.

[ondrejmirtes]

This is a good idea. Can you take a look at it?

[Slamdunk]

Here we would have 3 involved parts:

1. the $class object
2. the method method with its argument
3. the willReturn method with its argument.

I have no idea where to start a Rule like this. Anything similar you can point us out?

[ondrejmirtes]

After:

$class = $this->createMock(MyClass::class);

$class is MyClass&MockObject.

expects() returns InvocationMocker. You'd need to write InvocationMockerType (extends ObjectType) that contains the mocked class + dynamic return type extension that makes sure that $mockObject->expects() returns InvocationMockerType.

Then you'd need to write another dynamic return type extension that makes sure that methods called on InvocationMockerType return the same type so that InvocationMockerType preserved across multiple calls.

Also, InvocationMockerType should probably have a second constructor argument with a method name. Once you call ->method(), it should know about both the mocked class and the mocked method.

Finally, you'd be able to write a rule that checks that calling ->willReturn on InvocationMockerType(MyClass, foo) has the correct argument type.

You should check out phpstan-doctrine (https://github.com/phpstan/phpstan-doctrine/tree/master/src/Type/Doctrine/QueryBuilder), there's a lot of similar code that analyses DQLs.

[Slamdunk]

Great help 👍