Imo. we should make this the first check.

This targets 2.7, preventing the fatal error. Making this the first check would break for people which rely on the current first check, because it doesn't need $request. I would suggest to throw an exception here as it is and deprecate passing something else than a Request in 3.3, for adding the typehint in 4.0.

Good point 👍

But we never add any similar checks if the doctype restricts the argument to a particular type.

Problem is it also lacks a object typehint (Request $request). We should prefer that over docblocks of course.. however the current policy doesnt seem to allow it.. (?)

#19960 (comment) closed for the same reason. Allthough i still disagree ;-)

And i also prefer a cosmetic error over PHP just crashing.

I agree with @xabbuh. If we go down this path, we can potentially add such checks in a lot of places, which is a no go.

Fair enough :)) I guess the thing is we want to improve method signatures / type hints in a upcoming major release, but the policy makes it real hard. And personally i feel it's holding us back from improving (read: simplifying) the codebase.

Ie. this is not about adding safety guards, scalar type checks, etc. but about forcing integrity where possible.

Anyway, as any error will be probably clear enough i understand it's not worth the effort.