template <class T> constexpr T* launder(T* p) noexcept;
Requires: p represents the address A of a byte in memory. An object X that is within its lifetime ([basic.life]) and whose type is similar ([conv.qual]) to T is located at the address A. All bytes of storage that would be reachable through the result are reachable through p (see below).
Returns: A value of type T * that points to X.
Remarks: An invocation of this function may be used in a core constant expression whenever the value of its argument may be used in a core constant expression. A byte of storage is reachable through a pointer value that points to an object Y if it is within the storage occupied by Y, an object that is pointer-interconvertible with Y, or the immediately-enclosing array object if Y is an array element. The program is ill-formed if T is a function type or (possibly cv-qualified) void.
Notes: If a new object is created in storage occupied by an existing object of the same type, a pointer to the original object can be used to refer to the new object unless the type contains const or reference members; in the latter cases, this function can be used to obtain a usable pointer to the new object. See [basic.life].
[ Example:
struct X { const int n; };
X *p = new X{3};
const int a = p->n;
new (p) X{5}; // p does not point to new object ([basic.life])
// because X::n is const
const int b = p->n; // undefined behavior
const int c = std::launder(p)->n; // OK
— end example ]