@@ -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<const T></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<T>::value</code> and
+ only use it if valid.
</para></listitem></varlistentry>
</variablelist>
@@ -88,18 +88,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
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>
@@ -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, "" );
This is needed to avoid problems with the new structured bindings feature that landed in trunk yesterday. As part of this patch I'm removing the docs for the DR 2742 and 2748 changes that I added earlier today. The manual doesn't need to track changes against new features that only appear in trunk builds of GCC and working drafts of the standard. DR 2770 should be documented though, because it affects a C++11 feature that is in several GCC releases. * 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. Tested powerpc64le-linux, committed to trunk. commit 6182fd1774f8129f127b213586807080fdc58d1b Author: Jonathan Wakely <jwakely@redhat.com> Date: Tue Nov 15 18:27:15 2016 +0000 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.