What does it mean to “poison a function” in C++?

Question

At the very end of Scott Schurr\'s talk \"Introducing constexpr\" at CppCon, he asks \"Is there a way to poison a function\"? He then explains that this can be done (albeit

Answer 1

'Poisoning' an identifier means that any reference to the identifier after the 'poisoning' is a hard compiler error. This technique may be used, for instance, for hard deprecation (function IS deprecated, never use it!).

In GCC traditionally there was a pragma for this: #pragma GCC poison.