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Make std::tuple_size<cv T> SFINAE-friendly (LWG 2770) 

* doc/xml/manual/intro.xml: Document LWG 2770 status. Remove entries
	for 2742 and 2748.
	* doc/html/*: Regenerate.
	* include/std/utility (__tuple_size_cv_impl): New helper to safely
	detect tuple_size<T>::value, as per LWG 2770.
	(tuple_size<cv T>): Adjust partial specializations to derive from
	__tuple_size_cv_impl.
	* testsuite/20_util/tuple/cv_tuple_size.cc: Test SFINAE-friendliness.

From-SVN: r242452
This commit is contained in:
Jonathan Wakely 2016-11-15 19:32:44 +00:00 committed by Jonathan Wakely
parent 168126e598
commit 6ae2ae3b84
5 changed files with 41 additions and 27 deletions
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9
libstdc++-v3/ChangeLog
View File

@ -1,5 +1,14 @@
2016-11-15 Jonathan Wakely <jwakely@redhat.com>
* doc/xml/manual/intro.xml: Document LWG 2770 status. Remove entries
for 2742 and 2748.
* doc/html/*: Regenerate.
* include/std/utility (__tuple_size_cv_impl): New helper to safely
detect tuple_size<T>::value, as per LWG 2770.
(tuple_size<cv T>): Adjust partial specializations to derive from
__tuple_size_cv_impl.
* testsuite/20_util/tuple/cv_tuple_size.cc: Test SFINAE-friendliness.
* testsuite/libstdc++-prettyprinters/cxx17.cc: Adjust test for
variant<T&>.

14
libstdc++-v3/doc/html/manual/bugs.html
View File

@ -503,14 +503,10 @@
<span class="emphasis"><em><code class="code">priority_queue</code> lacking comparator typedef
</em></span>
</span></dt><dd><p>Define the <code class="code">value_compare</code> typedef.
</p></dd><dt><span class="term"><a class="link" href="../ext/lwg-defects.html#2742" target="_top">2742</a>:
<span class="emphasis"><em>Inconsistent <code class="code">string</code> interface taking <code class="code">string_view</code>
</p></dd><dt><span class="term"><a class="link" href="../ext/lwg-defects.html#2770" target="_top">2770</a>:
<span class="emphasis"><em><code class="code">tuple_size&lt;const T&gt;</code> specialization is not
SFINAE compatible and breaks decomposition declarations
</em></span>
</span></dt><dd><p>Add the new constructor and additionally constrain it
to avoid ambiguities with non-const <code class="code">charT*</code>.
</p></dd><dt><span class="term"><a class="link" href="../ext/lwg-defects.html#2748" target="_top">2748</a>:
<span class="emphasis"><em>swappable traits for optionals
</em></span>
</span></dt><dd><p>Disable the non-member <code class="code">swap</code> overload when
the contained object is not swappable.
</span></dt><dd><p>Safely detect <code class="code">tuple_size&lt;T&gt;::value</code> and
only use it if valid.
</p></dd></dl></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="license.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="status.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="setup.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">License </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 2. Setup</td></tr></table></div></body></html>

17
libstdc++-v3/doc/xml/manual/intro.xml
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@ -1107,20 +1107,13 @@ requirements of the license of GCC.
<listitem><para>Define the <code>value_compare</code> typedef.
</para></listitem></varlistentry>
<varlistentry><term><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="../ext/lwg-defects.html#2742">2742</link>:
<emphasis>Inconsistent <code>string</code> interface taking <code>string_view</code>
<varlistentry><term><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="../ext/lwg-defects.html#2770">2770</link>:
<emphasis><code>tuple_size&lt;const T&gt;</code> specialization is not
SFINAE compatible and breaks decomposition declarations
</emphasis>
</term>
<listitem><para>Add the new constructor and additionally constrain it
to avoid ambiguities with non-const <code>charT*</code>.
</para></listitem></varlistentry>
<varlistentry><term><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="../ext/lwg-defects.html#2748">2748</link>:
<emphasis>swappable traits for optionals
</emphasis>
</term>
<listitem><para>Disable the non-member <code>swap</code> overload when
the contained object is not swappable.
<listitem><para>Safely detect <code>tuple_size&lt;T&gt;::value</code> and
only use it if valid.
</para></listitem></varlistentry>
</variablelist>

18
libstdc++-v3/include/std/utility
View File

@ -87,19 +87,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename _Tp>
struct tuple_size;
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 2770. tuple_size<const T> specialization is not SFINAE compatible
template<typename _Tp, typename = void>
struct __tuple_size_cv_impl { };
template<typename _Tp>
struct __tuple_size_cv_impl<_Tp, __void_t<decltype(tuple_size<_Tp>::value)>>
: integral_constant<size_t, tuple_size<_Tp>::value> { };
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 2313. tuple_size should always derive from integral_constant<size_t, N>
template<typename _Tp>
struct tuple_size<const _Tp>
: integral_constant<size_t, tuple_size<_Tp>::value> { };
struct tuple_size<const _Tp> : __tuple_size_cv_impl<_Tp> { };
template<typename _Tp>
struct tuple_size<volatile _Tp>
: integral_constant<size_t, tuple_size<_Tp>::value> { };
struct tuple_size<volatile _Tp> : __tuple_size_cv_impl<_Tp> { };
template<typename _Tp>
struct tuple_size<const volatile _Tp>
: integral_constant<size_t, tuple_size<_Tp>::value> { };
struct tuple_size<const volatile _Tp> : __tuple_size_cv_impl<_Tp> { };
/// Gives the type of the ith element of a given tuple type.
template<std::size_t __i, typename _Tp>

10
libstdc++-v3/testsuite/20_util/tuple/cv_tuple_size.cc
View File

@ -42,3 +42,13 @@ int main()
test01();
return 0;
}
// LWG DR 2770. tuple_size<const T> specialization is not SFINAE compatible
template<typename T, typename = void>
struct has_value : std::false_type { };
template<typename T>
struct has_value<T, std::__void_t<decltype(T::value)>> : std::true_type { };
static_assert( !has_value<std::tuple_size<int>>::value, "" );
static_assert( !has_value<std::tuple_size<const int>>::value, "" );