[intro.object] Example for p10

[eisenwave]

I think p10 could need an example, ideally on which highlights the idiom of using std::launder to force the creation of one particular type of object.

int x;
new (storage) std::byte[sizeof(x)];              // implicitly begin the lifetime of objects in x
*reinterpret_cast<char*>(&x);                    // unspecified whether placement new created
                                                 // an int object inside x, float, char[], or any other objects

std::memmove(&x, &x, sizeof(x));                 // transparently replace ([basic.life]) x with another object
x = 0;                                           // the new object must be of type (no cv) int because 
                                                 // only that may give the program well-defined behavior

std::memmove(&x, &x, sizeof(x));                 // begin the lifetime of one or multiple objects in the storage of x
std::launder(reinterpret_cast<short*>(&x));      // created object at &x must be of type short int
x = 0;                                           // undefined behavior

[frederick-vs-ja]

I'm afraid that the currently proposed example might be an anti-pattern. Note that it's almost always (if not always) a bad practice to ignore the return of std::launder, and std::launder is marked with [[nodiscard]] (since C++20) as a result.

Perhaps it would be better to write something like *std::launder(reinterpret_cast<short*>(&x)) = 0 instead of a single std::launder call.

On ther other hand, as said in [intro.object]/10, the std::memmove call may recreate or merely rewrite the int object. I'm not sure whether it would be helpful to mention such indeterminism in comments.