> I mean the API should support "useless" code where it makes sense, not that you should manually write it.

There is the conflict - if I write useless code I want the compiler to stop me. This is very different from generic code where sometimes a part simplifies to useless code in some particular context even though the whole is useful. Anytime the system can tell the difference between these two I want them treated different.

Telling the difference between useless code and generic code that has a useless form in this context is often not a solvable problem. There are also cases where if we had known better 40 years ago we would have made a different choice, but it is too late now. However I think the rules stands: the system should reject useless code when that is what someone writes even if the code otherwise parses correctly.