I mean, you can just throw an exception or leave it undefined when the state isn't what you expect... isn't that a normal thing with any nontrivial class? e.g.,

I mean, you can just throw an exception or leave it undefined when the state isn't what you expect... isn't that a normal thing with any nontrivial class? e.g., what should unique_lock do if you call lock() when you passed adopt_lock_t to the constructor? What should unique_ptr do if you call operator->() when you passed null to the constructor? What should fstream do if you call open() when you already passed a file name to the constructor? etc.