Hello,

I recently noticed something unexpected: it is possible to construct an object over constant storage with `std::construct_at`. At least it works in gcc 13.2.0 and clang 18.

Note that this is not possible with placement new.

I tried playing with the following code:

template <class T>

void swap_const_references(T const& a, T const& b)

{

    T c{a};

    a.~T();

    std::construct_at(&a, b);

    b.~T();

    std::construct_at(&b, c);

}

And it compiles fine with both compilers. If I replace the std::construct_at with placement new, the code fails to compile (cannot convert 'const void*' to 'void*').

This swap_const_references is definitely something we don't want to be able to write! At least not without relying on UB. I can even make it constexpr and it is still compiling >< (and working as designed...)

I don't know who is in the wrong here, between the library implementers and the C++ standard. But here is a case where code with a non-constexpr placement new is actually safer than the shinny construct_at...

Should we open a defect to fix std::construct_at so it does not accept constructing over constant storage?

Best regards,

Sébastien BINI