[committed, wwwdocs] Document some GCC 17 C++ header dependency changes

[Jonathan Wakely]

---

Pushed to wwwdocs.

 htdocs/gcc-17/porting_to.html | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/htdocs/gcc-17/porting_to.html b/htdocs/gcc-17/porting_to.html
index 715a79911..187388c1c 100644
--- a/htdocs/gcc-17/porting_to.html
+++ b/htdocs/gcc-17/porting_to.html
@@ -24,6 +24,32 @@ porting to GCC 17. This document is an effort to identify common issues
 and provide solutions. Let us know if you have suggestions for improvements!
 </p>
 
+<h2 id="cxx">C++ language issues</h2>
+
+<p>
+Note that all GCC releases make
+<a href="../bugs/#upgrading">improvements to conformance</a>
+which may reject non-conforming, legacy codebases.
+</p>
+
+<h3 id="header-dep-changes">Header dependency changes</h3>
+<p>Some C++ Standard Library headers have been changed to no longer include
+other headers that were being used internally by the library.
+As such, C++ programs that used standard library components without
+including the right headers will no longer compile.
+</p>
+<p>
+In particular, the following headers are used less widely within libstdc++ and
+may need to be included explicitly when compiling with GCC 17:
+</p>
+<ul>
+<li> <code><iosfwd.h></code>
+  (for forward declarations of iostreams types)
+  and <code><ios></code>
+  (for <code>std::streamsize</code>, <code>std::ios_base</code> etc.)
+</li>
+</ul>
+
 <h2 id="x86">IA-32/x86-64 issues</h2>
 
 <h3 id="x86_avx10_2_scalar_satcvt">Renamed AVX10.2 scalar saturating conversion intrinsics</h3>
-- 
2.55.0