https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95349
--- Comment #52 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Christopher Nerz from comment #45)
> This is a critical bug which renders gcc unusable for safety relevant
> systems using expected/variant or simple ipc.
I don't think your example demonstrates that.
> alignas(8) std::byte buffer[8]; // some buffer
> new (buffer) double{1}; // some completely trivial data
> // reuse memory -> double ends lifetime, uint64 starts lifetime
> std::uint64_t * res = new (buffer) std::uint64_t;
This starts the lifetime of a new object, but it has indeterminate value.
> // *res is allowed to be used as it is the correct pointer returned by
> new
The pointer does point to the new object, but derefencing it causes a read of
an indeterminate value, which is undefined behaviour.
> // *res == 0x3ff0000000000000 // and gives correct value
> // The very definition of std::launder says that it is suppose to be
> used as:
> return (*res == *std::launder(reinterpret_cast<std::uint64_t*>(buffer)));
It looks like what you're actually trying to do is:
alignas(8) std::byte buffer[8];
new (buffer) double{1};
std::uint64_t* res = std::start_lifetime_as<std::uint64_t>(buffer);
return *res == 0x3ff0000000000000;
This is not what std::launder is for.
