So - as others have written - for the first part you have to argue, why it is much different from declaring either as =delete or not at all. For the second part, how would a user implement it?  - Is the function defined in the same translation unit?  - Or only declared? This time without [[unimplemented]]?  - Or both defined and declared?  - Or nothing would change in the translation unit, but the user would turn off the unimplemented warnings? And the linker would do the rest? In the fourth option: If this is done a lot and the warnings are turned on again, one would not know, which functions are and which functions are not implemented.   Generally, you also should describe, why it is such a bad thing, if the linker finds out that symbols are missing. If it is just the error message, which often does not refer well to the line of code calling the function, perhaps that can be changed.       -----Ursprüngliche Nachricht----- Von:Hans Åberg via Std-Proposals <std-proposals_at_[hidden]> It means that it is not implemented but is called for by a specification such as the C++ standard. If you call such a function, you get a compiler diagnostic saying that it is not implemented. That is the first part. The second part is the ability to implement it on your own. This part is tricky, and it is unclear if it can be admitted.