Re: [Bug libstdc++/29286] [4.0/4.1/4.2/4.3 Regression] placement new does not change the dynamic type as it should

[Gabriel Dos Reis <gdr at cs dot tamu dot edu>]

"ian at airs dot com" <gcc-bugzilla@gcc.gnu.org> writes:

| I'm not sure what to make of comment #84.  We don't determine aliasing by
| alignment or size.  We determine it by type.  We don't currently treat int and
| long as aliasing each other even if they happen to have the same alignment and
| size.

That is GOOD.  :-)

| I believe this is correct according to the C standard but I am less
| familiar with the C++ standard.

It is also correct semantics with respect to C++.

C++ goes further (I don't have C standard handy to check).  The memory
pointed to by p must have the "right" alignment requirements etc.

So if you grab memory into p and it does not have the right alignment,
then the new placement yields undefined behaviour.  That is not a
property we can check statically (until we get the alignment proposal
into C++).

| We could change the aliasing machinery in that
| way for C++ if it seems to be appropriate, but I would prefer to take that to a
| different PR.

What I was trying to say was that C++ provides the same "dynamic type"
(non-)aliasing guarantees, and goes even further.  

Did I manage to confuse you again?

-- Gaby