There was already a legal way to achieve this that everyone should already have been using (laundering a pointer through a no-op memmove). Using reinterpret_cast here is a bug.

The "start_lifetime_as" facility does one additional thing beyond providing a tidy standard name for the memory laundering incantation. Semantically it doesn't touch the memory whereas the no-op memmove intrinsically does. In practice, this makes little difference, since the compiler could see that the memmove was a no-op and optimized accordingly.