[Bug c++/95349] Using std::launder(p) produces unexpected behavior where (p) produces expected behavior

["richard-gccbugzilla at metafoo dot co.uk" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95349

--- Comment #18 from Richard Smith <richard-gccbugzilla at metafoo dot co.uk> ---
(In reply to Andrew Downing from comment #17)
> Also none of the behavior described in p0593 is required for this C++
> program to be well defined. All objects that are required to exists here are
> created explicitly. It's not relying on the implicit creation of any
> objects. This is valid C++17 code.

I agree, for what it's worth. I think the only thing that might suggest
otherwise is the wording in the C standard that says that memcpy copies the
effective type, but that doesn't mean anything in C++ (and it's also specified
in the language section of C, not the library section, so isn't part of the
wording that C++ incorporates by reference).

C++ doesn't have any wording that says what value an object has after you
memcpy the representation of a value of a different type over it, but there
isn't any provision for memcpy to change the dynamic type of the object prior
to P0593 (and after P0593, memcpy is only allowed to change the dynamic type if
doing so makes the program's behavior more defined).