Extra requirements in `union __lazy_ctor_storage::setup()` method

[SergeyKopienko]

oneDPL/include/oneapi/dpl/pstl/utils.h

Line 1124 in 3e0e05b

new (&__v) _Tp(std::forward<_U>(init));

The unqualified &__v selects a user-defined operator& if _Tp has one.
For a type with a deleted operator& this is a compile error.
std::addressof(__v) should be used instead.

[SergeyKopienko]

What I would like to propose:

template <typename _T, typename _U>
#if _ONEDPL___cplusplus >= 202002L
    requires std::constructible_from<_T, _U&&> || (std::default_initializable<_T> && std::is_assignable_v<_T&, _U &&>)
#endif
void
__construct_from(_T* __dst, _U&& __src)
{
#if _ONEDPL___cplusplus < 202002L
    ::new (__dst) _T(std::forward<_U>(__src));
#else
    if constexpr (std::constructible_from<_T, _U&&>)
    {
        std::construct_at(__dst, std::forward<_U>(__src));
    }
    else if constexpr (std::default_initializable<_T> && std::is_assignable_v<_T&, _U&&>)
    {
        std::construct_at(__dst);
        *__dst = std::forward<_U>(__src);
    }
    else
    {
        static_assert(std::constructible_from<_T, _U&&> ||
                          (std::default_initializable<_T> && std::is_assignable_v<_T&, _U&&>),
                      "_T cannot be initialized from _U");
    }
#endif
}

[danhoeflinger]

The static assert and the requires are probably redundant here, I'd lose one.

This is a nit pick, but on the host, one could also possibly leak memory if an exception is thrown during assignment (successfully default constructed, but __constructed is never set to true).

[SergeyKopienko]

Disagree about memory leak in case of exception in this code
it's not memory allocation code, it's just object construction in already allocated memory.
Correct?

[danhoeflinger]

yep. my mistake. its not a leak but just a constructed object which is not destroyed.