[wwwdocs] porting_to: Two-stage overload resolution for implicit move removed

[Marek Polacek <polacek@redhat.com>]

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>
+
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