Re: Should reinterpret_cast be constexpr?

[Josh Marshall <joshua.r.marshall.1991@gmail.com>]

Is this in master?  If so, I'll just merge it in.

On Mon, Dec 28, 2020 at 3:47 PM Jonathan Wakely <jwakely.gcc@gmail.com>
wrote:

> On Mon, 28 Dec 2020 at 20:33, Jonathan Wakely <jwakely.gcc@gmail.com>
> wrote:
> >
> > On Mon, 28 Dec 2020 at 19:51, Josh Marshall via Libstdc++
> > <libstdc++@gcc.gnu.org> wrote:
> > >
> > > Hello all,
> > >
> > > From the test error `libstdc++-v3/include/bits/stl_vector.h:1837:
> error:
> > > 'reinterpret_cast' is not a constant expression` and the documentation
> at
> > > https://en.cppreference.com/w/cpp/language/reinterpret_cast , is
> > > `reinterpret_cast()` not constexpr in error?
> >
> > No, it's not an error. reinterpret_cast is not allowed in constant
> expressions.
>
> This patch replaces the use of reinterpret_cast with an alternative
> using C++20 features:
>
> diff --git a/libstdc++-v3/include/bits/vector.tcc
> b/libstdc++-v3/include/bits/vector.tcc
> index 27e63388feb..93b2ccc02e3 100644
> --- a/libstdc++-v3/include/bits/vector.tcc
> +++ b/libstdc++-v3/include/bits/vector.tcc
> @@ -516,9 +516,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>            {
> #if __cplusplus < 201103L
>              value_type __x_copy = __x;
> -#else
> +#elif ! __cpp_lib_make_obj_using_allocator
>              _Temporary_value __tmp(this, __x);
>              value_type& __x_copy = __tmp._M_val();
> +#else
> +             value_type __x_copy = std::make_from_tuple<value_type>(
> +                 std::uses_allocator_construction_args<value_type>(
> +                   _M_get_Tp_allocator(), __x));
> #endif
>              const size_type __elems_after = end() - __position;
>              pointer __old_finish(this->_M_impl._M_finish);
>