As I promised in <https://gcc.gnu.org/pipermail/gcc-patches/2022-October/603189.html>, I'd like to update our GCC 13 porting_to.html with the following note.
Does this look OK to commit? Thanks, diff --git a/htdocs/gcc-13/porting_to.html b/htdocs/gcc-13/porting_to.html index 84a00f21..243ed29d 100644 --- a/htdocs/gcc-13/porting_to.html +++ b/htdocs/gcc-13/porting_to.html @@ -42,5 +42,57 @@ be included explicitly when compiled with GCC 13: </li> </ul> +<h3 id="two-stage-or">Two-stage overload resolution for implicit move removed</h3> +<p> +GCC 13 removed the two-stage overload resolution when performing +implicit move, whereby the compiler does two separate overload resolutions: +one treating the operand as an rvalue, and then (if that resolution fails) +another one treating the operand as an lvalue. In the standard this was +introduced in C++11 and implemented in gcc in +<a href="https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=4ce8c5dea53d80736b9c0ba6faa7430ed65ed365";> +r251035</a>. In +<a href="https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=1722e2013f05f1f1f99379dbaa0c0df356da731f";> +r11-2412</a>, the fallback overload resolution was disabled in C++20 (but +not in C++17). Then C++23 <a href="https://wg21.link/p2266";>P2266</a> +removed the fallback overload resolution, and changed the implicit move +rules once again. +</p> +<p> +The two overload resolutions approach was complicated and quirky, so users +should transition to the newer model. This change means that code that +previously didn't compile in C++17 will now compile, for example:</p> + +<pre><code> + struct S1 { S1(S1 &&); }; + struct S2 : S1 {}; + + S1 + f (S2 s) + { + return s; // OK, derived-to-base, use S1::S1(S1&&) + } +</code></pre> + +<p> +And conversely, code that used to work in C++17 may not compile anymore: +</p> + +<pre><code> + struct W { + W(); + }; + + struct F { + F(W&); + F(W&&) = delete; + }; + + F fn () + { + W w; + return w; // use w as rvalue -> use of deleted function F::F(W&&) + } +</code></pre> + </body> </html>
