On Wed, 12 Oct 2022 at 23:24, Marek Polacek <pola...@redhat.com> wrote: > > On Wed, Oct 12, 2022 at 09:50:36PM +0100, Jonathan Wakely wrote: > > On Wed, 12 Oct 2022 at 20:39, Marek Polacek <pola...@redhat.com> wrote: > > > > > > 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&&) > > > > Deleted move constructors are an abomination, and should never occur > > in real code. I'm not sure using one even in an example like this > > should be encouraged. The example added by P2266 to Annex D is more > > realistic (and actually broke a libstdc++ test): > > > > X& foo(X&& x) { return x; } > > Right, but this code still compiles in C++17, it only fails to compile > in C++23. The previous example now doesn't compile even in C++17. So > how about this improved patch which makes it clear that code with > deleted move constructors should never occur in practice, and adds a new > note, specifically about P2266 and the code you showed?
Doh, I've just realised that F(W&&) isn't a move ctor at all. For some reason I read the example as F(F&&). I think your original example is fine, and the note would just be confusing (because it's not a deleted move ctor!) > > Thanks for taking a look, > > diff --git a/htdocs/gcc-13/porting_to.html b/htdocs/gcc-13/porting_to.html > index 84a00f21..a9991e8b 100644 > --- a/htdocs/gcc-13/porting_to.html > +++ b/htdocs/gcc-13/porting_to.html > @@ -42,5 +42,71 @@ 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> > +<strong>NB:</strong> this example is contrived because deleted move > +constructors should not occur in real code.</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> >
