Re: [wwwdocs] Add a note about in-class initialization of static data member

[Marek Polacek <polacek at redhat dot com>]

On Thu, Feb 11, 2016 at 10:09:19AM -0700, Martin Sebor wrote:
> It''s interesting that when the example is modified to use a double
> initializer it is rejected with a hard error even in C++ 03 mode
> when -Wpedantic (but not -Werror) is used.  That seems like a bug.
> If it isn't, it might be worth mentioning the constraint that the
> initializer must be a integer in the text above.
> 
>   struct X {
>     const static double i = 3.14;
>   };
> 
>   error: floating-point literal cannot appear in a constant-expression
>     const static double i = 3.14;
>                            ^~~~

Hm, indeed; I hadn't notice that.  Dunno if is a bug (clang++ accepts this
with a warning).  I've added ", provided the initializer is an integer",
that should be enough.

> >+<h3>Stricter flexible array member rules</h3>
> >+
> >+<p>
> >+As of this release, the C++ compiler is now more strict about flexible array
> >+member rules.  As a consequence, the following code is no longer accepted:
> 
> In light of bug 69550 I think it might be useful to also mention
> (or show an example) that structs with a flexible array as the
> only member are rejected as well.

Somehow I knew you'd have something to add here ;).  How about this then?

Index: porting_to.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-6/porting_to.html,v
retrieving revision 1.9
diff -u -r1.9 porting_to.html
--- porting_to.html	10 Feb 2016 17:21:54 -0000	1.9
+++ porting_to.html	11 Feb 2016 17:58:43 -0000
@@ -269,6 +269,53 @@
 to port the code to use C++11's <code>std::unique_ptr</code> instead.
 </p>
 
+<h3>'constexpr' needed for in-class initialization of static data member</h3>
+
+<p>
+Since C++11, the <code>constexpr</code> keyword is needed when initializing a
+non-integral static data member in a class.  As a GNU extension, the following
+program is accepted in C++03 (albeit with a <tt>-Wpedantic</tt> warning),
+provided the initializer is an integer:
+</p>
+
+<pre><code>
+struct X {
+  const static double i = 10;
+};
+</pre></code>
+
+<p>
+The C++11 standard supports that in-class initialization using
+<code>constexpr</code> instead, so the GNU extension is no longer supported for
+C++11 or later.  Programs relying on the extension will be rejected with an
+error.  The fix is to use <code>constexpr</code> instead of <code>const</code>.
+</p>
+
+<h3>Stricter flexible array member rules</h3>
+
+<p>
+As of this release, the C++ compiler is now more strict about flexible array
+member rules.  As a consequence, the following code is no longer accepted:
+</p>
+
+<pre><code>
+union U {
+  int i;
+  char a[];
+};
+</pre></code>
+
+<p>
+Furthermore, the C++ compiler now rejects structures with a flexible array
+member as the only member:
+</p>
+
+<pre><code>
+struct S {
+  char a[];
+};
+</pre></code>
+
 <h2>-Wmisleading-indentation</h2>
 <p>
 A new warning <code>-Wmisleading-indentation</code> was added

	Marek