withDefault does not seem to be called on morphTo relationships.

[vpratfr]

• Laravel Version: 5.6.18
• PHP Version: 7.1

Description:

The method withDefault does not seem to be called on morphTo relationships.

Steps To Reproduce:

I have a table Mission which relates to other tables to store mission specific data, depending on the mission template. For example, when mission template is 'create-company', we use a CreateCompanyData model.

The Mission model has a mission_data() relationship described as:

public function mission_data() : MorphTo { 
    return $this->morphTo('mission_data')
                ->withDefault(function() {
                    if ($this->template==='') {
                        return new CreateCompanyData();
                    }
                    // Handle other template values + throw when template unknown
                    // ...
                });
}

When debugging my unit tests, it appears that breakpoints never get hit when placed inside the withDefault callback, even when no relation has been provided yet (function simply returns null, as if there was no withDefault call defined.

[vpratfr]

Thanks :)

[vpratfr]

@taylorotwell @staudenmeir Re-opened in #24725 after some field testing