[PATCH] wwwdocs: Document user-visible changes in libstdc++ generate_canonical and variant implementations.

[Jonathan Wakely]

On Mon, 22 Dec 2025 at 18:54, Tomasz Kamiński <tkaminsk@redhat.com> wrote:
>
> ---
> v2 updates description per review suggestions.
>
>  htdocs/gcc-16/changes.html | 32 ++++++++++++++++++++++++++++++--
>  1 file changed, 30 insertions(+), 2 deletions(-)
>
> diff --git a/htdocs/gcc-16/changes.html b/htdocs/gcc-16/changes.html
> index 013282ec..c700d089 100644
> --- a/htdocs/gcc-16/changes.html
> +++ b/htdocs/gcc-16/changes.html
> @@ -101,6 +101,33 @@ for general information.</p>
>      with GNU dialects (<code>-std=gnu++17</code>, <code>-std=gnu++14</code>,
>      etc.) and not with strict dialects (<code>-std=c++17</code>, etc.)
>    </li>
> +
> +  <li>
> +    The <a

"The proposal <a"

OK for wwwdocs with that change, thanks.


> +    href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p0952r2.html"/>
> +    P0952R2: A new specification for <code>std::generate_canonical</code></a>
> +    was implemented in all affected modes (since C++11), impacting the observed
> +    output. The previous behavior can be restored by defining
> +    <code>_GLIBCXX_USE_OLD_GENERATE_CANONICAL</code>.
> +  </li>
> +  <li>
> +    The <code>std::variant</code> ABI was updated to make it conforming
> +    and consistent with C++20 and later modes. This impacts the layout
> +    of classes which have a <code>std::variant</code> as the first member
> +    and a base class of the same type as one of the <code>variant's</code>
> +    alternatives, if that type is an empty class and has a non-trivial
> +    destructor:
> +    <pre>struct E { ~E(); };
> +struct Affected : E
> +{
> +  std::variant<E, int> mem; // previously stored at offset zero,
> +                           // uses non-zero offset now
> +};</pre>
> +    The previous behavior can be restored by defining
> +    <code>_GLIBCXX_USE_VARIANT_CXX17_OLD_ABI</code>. This impacts only
> +    C++17 mode.
> +  </li>
> +
>    <li>Improved experimental support for C++20, including:
>      <ul>
>      <li>
> @@ -136,7 +163,7 @@ for general information.</p>
>        <li>
>          Representation of some range adaptors in <code><ranges></code>.
>        </li>
> -    </ul>.
> +    </ul>
>      This list is not necessarily complete. As with all experimental support,
>      programs using C++20 components should assume that those components
>      are not compatible between different major releases.
> @@ -165,7 +192,8 @@ for general information.</p>
>        thanks to Nathan Myers.
>      </li>
>      <li>
> -      Padded mdspan layouts, aligned accessor, and <code>std::dims</code>,
> +      Padded mdspan layouts, aligned accessor, <code>std::dims</code>,
> +      <code>std::constant_wrapper</code>, and <code>std::submdspan</code>
>        thanks to Luc Grosheintz.
>      </li>
>      </ul>
> --
> 2.52.0
>