[8.x] Restore auto-complete functionality on when and unless

[deleugpn]

Looks like when the Conditionable trait was merged, we lost the ability to get autocompletion from PHPStorm when doing $eloquent->newQuery()->when(...)->chain()->anotherChain()->get() because of the mixed return type.

I have not verified, but as far as I remember the goal with a Conditionable trait was consistency across the framework, in which case they're likely returning $this for chain capabilities.

[inxilpro]

I think a return type of $this|mixed might be more appropriate, because when() isn't guaranteed to return $this — it depends on what you return from your callback.

[driesvints]

mixed is the correct return type here. The callbacks can return anything imaginable. You need to type-check in userland. $this|mixed is invalid as mixed already includes $this.