[wwwdocs] Document libstdc++ changes in GCC 11

[Jonathan Wakely <jwakely@redhat.com>]

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Also add porting-to notes about tr1::bind.

Pushed to wwwdocs.



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commit 927e80dc01f505a625f1fcc4e1ca38aeb9f88e67
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Dec 8 13:05:42 2020 +0000

    Document libstdc++ changes in GCC 11
    
    Also add porting-to notes about tr1::bind.

diff --git a/htdocs/gcc-11/changes.html b/htdocs/gcc-11/changes.html
index 4d3efed5..50e35505 100644
--- a/htdocs/gcc-11/changes.html
+++ b/htdocs/gcc-11/changes.html
@@ -239,7 +239,28 @@ a work-in-progress.</p>
   </li>
 </ul>
 
-<!-- <h4 id="libstdcxx">Runtime Library (libstdc++)</h4> -->
+<h4 id="libstdcxx">Runtime Library (libstdc++)</h4>
+<ul>
+  <li>Improved C++17 support, including:
+    <ul>
+      <li><code>std::from_chars</code> for floating-point types.
+    <ul>
+  </li>
+  <li>Improved experimental C++2a support, including:
+    <ul>
+      <li>Calendar additions to <code>&lt;chrono&gt;</code>.</li>
+      <li><code>std::bit_cast</code></li>
+      <li><code>std::source_location</code></li>
+      <li>Atomic wait and notify operations.</li>
+      <li><code>&lt;latch&gt;</code> and <code>&lt;semaphore&gt;</code></li>
+      <li><code>&lt;syncstream&gt;</code></li>
+      <li>Efficient access to <code>basic_stringbuf</code>'s buffer.</li>
+    </ul>
+  </li>
+  <li>Faster <code>std::uniform_int_distribution</code>,
+      thanks to Daniel Lemire.
+  </li>
+</ul>
 
 <h3 id="fortran">Fortran</h3>
 
diff --git a/htdocs/gcc-11/porting_to.html b/htdocs/gcc-11/porting_to.html
index 41efc3b6..4187dd8e 100644
--- a/htdocs/gcc-11/porting_to.html
+++ b/htdocs/gcc-11/porting_to.html
@@ -114,6 +114,33 @@ be included explicitly when compiled with GCC 11:
 </li>
 </ul>
 
+<h3 id="Old-iostream-Members">Old iostream Members</h3>
+<p>
+The deprecated iostream members <code>ios_base::io_state</code>,
+<code>ios_base::open_mode</code>, <code>ios_base::seek_dir</code>, and
+<code>basic_streambuf::stossc</code> are not available in C++17 mode.
+References to those members should be replaced by <code>std::iostate</code>,
+<code>std::openmode</code>, <code>std::seekdir</code>, and
+<code>basic_streambuf::sbumpc</code> respectively.
+</p>
+
+<h3 id="tr1-bind">Call of overloaded <code>'bind(...)'</code> is ambiguous</h3>
+<p>
+The placeholders for <code>std::tr1::bind</code> have been changed to use
+the same placeholder objects as <code>std::bind</code>.  This means that
+following <code>using std::tr1::bind;</code> an unqualified call to
+<code>bind(f, std::tr1::placeholders::_1)</code> may be ambiguous.
+This happens because <code>std::tr1::bind</code> is brought into scope by
+the using-declaration and <code>std::bind</code> is found by
+Argument-Dependent Lookup due to the type of the <code>_1</code> placeholder.
+</p>
+<p>
+To resolve this ambiguity replace unqualified calls to <code>bind</code>
+with <code>std::tr1::bind</code> or <code>std::bind</code>. Alternatively,
+change the code to not include the <code>&lt;tr1/functional&gt;</code> header,
+so that only <code>std::bind</code> is declared.
+</p>
+
 <!--
 <h2 id="fortran">Fortran language issues</h2>
 -->