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From: Marek Polacek <mpolacek@sourceware.org> To: gcc-cvs-wwwdocs@gcc.gnu.org Subject: gcc-wwwdocs branch master updated. 0f3b5000bce0237ddf2f852099fea998cf476267 Date: Wed, 12 Oct 2022 22:47:09 +0000 (GMT) [thread overview] Message-ID: <20221012224709.BB0ED3858D38@sourceware.org> (raw) This is an automated email from the git hooks/post-receive script. It was generated because a ref change was pushed to the repository containing the project "gcc-wwwdocs". The branch, master has been updated via 0f3b5000bce0237ddf2f852099fea998cf476267 (commit) from 205feff8608570c4be7306cbf05b53d0540905f8 (commit) Those revisions listed above that are new to this repository have not appeared on any other notification email; so we list those revisions in full, below. - Log ----------------------------------------------------------------- commit 0f3b5000bce0237ddf2f852099fea998cf476267 Author: Marek Polacek <polacek@redhat.com> Date: Wed Oct 12 18:46:30 2022 -0400 gcc-13/porting_to: Add notes wrt implicit move rules changes diff --git a/htdocs/gcc-13/porting_to.html b/htdocs/gcc-13/porting_to.html index 84a00f21..ccd3f08f 100644 --- a/htdocs/gcc-13/porting_to.html +++ b/htdocs/gcc-13/porting_to.html @@ -42,5 +42,69 @@ be included explicitly when compiled with GCC 13: </li> </ul> +<h3 id="P2266">Implicit move rules change</h3> +<p> +GCC 13 implements C++23 <a href="https://wg21.link/p2266">P2266</a> which +simplified the rules for implicit move. As a consequence, valid C++20 +code that relies on a returned <em>id-expression</em>'s being an lvalue +may change behavior or fail to compile in C++23. For example:</p> + +<pre><code> + decltype(auto) f(int&& x) { return (x); } // returns int&&; previously returned int& + int& g(int&& x) { return x; } // ill-formed; previously well-formed +</code></pre> + +<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>Conversely, code that used to work in C++17 may not compile anymore. +For example, the following example used to compile in C++11...17 because +we performed two separate overload resolutions: one treating the operand +as an rvalue, and then (if that resolution failed) another one treating +the operand as an lvalue.<br> + +<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> ----------------------------------------------------------------------- Summary of changes: htdocs/gcc-13/porting_to.html | 64 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 64 insertions(+) hooks/post-receive -- gcc-wwwdocs
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