[akritid]

This came to mind while considering your interesting point: After such a change, wouldn’t you feel the urge to inspect all users of the stricter return type and remove unnecessary handling of a potential null return?

[cubefox]

I don't know about such urges. But sometimes there is no possibility to inspect all user code, e.g. when you are providing a library or API function.

[akritid]

Good point. In such case I would probably consider leaving the signature as is, even after tightening, and possibly offer a function with stricter signature for new code to use while deprecating the older variant. This would inform the users without rug pulling.

[cubefox]

But that's not necessary in a language with union types of this sort. No rugs being pulled.