Unable to stub without overriding function

[d0x2f]

The online documentation states

Note: When andReturn()/andRun() is not applied, toReceive() simply act as a "spy" and let the code execution flow to be unchanged.

But I can't quite get this to work as I expect, I've made a minimal reproduction repo here: d0x2f/kahlan-stub-repro.

To be clear, what I'm trying to do is spy on a function and check that it was called n times without changing the behaviour of the function.

Thanks for your help!

[d0x2f]

Relevant snippets:

    class MyClass
    {
        private function getString(): string
        {
            return "the string";
        }

        public function useString(): string
        {
            return $this->getString();
        }
    }

    describe('without explicit return value', function () {
        beforeEach(function () {
            allow(MyClass::class)->toReceive("getString");
            $myClass = new MyClass();
            $this->result = $myClass->useString();
        });

        it('returns the string', function () {
            expect(MyClass::class)->toReceive("getString")->once();
        });

        it('returns the string', function () {
            expect($this->result)->toBe("the string");
        });
    });

MyClass->getString()
  without explicit return value
    ✖ it returns the string
      an uncaught exception has been thrown in `src/MyClass.php` line 7
      
      message:`TypeError` Code(0) with message "KahlanStub\\MyClass::getString(): Return value must be of type string, null returned"
      
        [NA] - src/MyClass.php, line  to 7
        KahlanStub\MyClass::getString() - src/MyClass.php, line 13
        KahlanStub\MyClass::useString() - spec/MyClass.spec.php, line 10

[samsonasik]

you need to set ->andReturn('the string')

-            allow(MyClass::class)->toReceive("getString");
+            allow(MyClass::class)->toReceive("getString")->andReturn('the string');

[d0x2f]

Thanks @samsonasik, but this was just a minimal example to demonstrate that I can't stub a function without overriding it.
In reality the function is much too complicated to reimplement in the tests using andReturn or andRun.

I want to just spy on the function and assert that it was called, as the docs say is possible.

[samsonasik]

I see, in that case, you need to use expect() instead of allow():

-            allow(MyClass::class)->toReceive("getString");
+            expect(MyClass::class)->toReceive("getString");

[d0x2f]

Brilliant, that works.
Thank you!