This solves a conflict between late changes to the C++17 library and
core language, which make tuple_size<const T>::value cause errors for
const structured binding declarations. The problem is that LWG 2770
wants tuple_size<cv T> to be complete, but sometimes have no value
member, but the core language for structured bindings barfs on a
complete type with no value member.
It's possible this will be fixed in core (e.g. by only trying to use
tuple_size when it's complete _and_ has a usable value member) but for
now let's just fix it in the library. This is consistent with the
solution in libc++ too.
PR libstdc++/78939
* include/std/utility (tuple_size<cv T>): Only define partial
specializations when tuple_size<T>::value is valid.
* testsuite/20_util/tuple/78939.cc: New.
* testsuite/20_util/tuple/cv_tuple_size_neg.cc: New.
Tested powerpc64le-linux, committed to trunk.
commit f72d64f99385f51efb8f437618f084335fe64f89
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Wed Mar 8 03:07:48 2017 +0000
PR libstdc++/78939 make tuple_size<cv T> depend on tuple_size<T>
PR libstdc++/78939
* include/std/utility (tuple_size<cv T>): Only define partial
specializations when tuple_size<T>::value is valid.
* testsuite/20_util/tuple/78939.cc: New.
* testsuite/20_util/tuple/cv_tuple_size_neg.cc: New.
diff --git a/libstdc++-v3/include/std/utility b/libstdc++-v3/include/std/utility
index 188fcc2..c18bcb6 100644
--- a/libstdc++-v3/include/std/utility
+++ b/libstdc++-v3/include/std/utility
@@ -88,24 +88,26 @@ _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> : __tuple_size_cv_impl<_Tp> { };
+ // 2770. tuple_size<const T> specialization is not SFINAE compatible
+
+ template<typename _Tp,
+ typename _Up = typename remove_cv<_Tp>::type,
+ typename = typename enable_if<is_same<_Tp, _Up>::value>::type,
+ size_t = tuple_size<_Tp>::value>
+ using __enable_if_has_tuple_size = _Tp;
template<typename _Tp>
- struct tuple_size<volatile _Tp> : __tuple_size_cv_impl<_Tp> { };
+ struct tuple_size<const __enable_if_has_tuple_size<_Tp>>
+ : public tuple_size<_Tp> { };
template<typename _Tp>
- struct tuple_size<const volatile _Tp> : __tuple_size_cv_impl<_Tp> { };
+ struct tuple_size<volatile __enable_if_has_tuple_size<_Tp>>
+ : public tuple_size<_Tp> { };
+
+ template<typename _Tp>
+ struct tuple_size<const volatile __enable_if_has_tuple_size<_Tp>>
+ : public tuple_size<_Tp> { };
/// Gives the type of the ith element of a given tuple type.
template<std::size_t __i, typename _Tp>
diff --git a/libstdc++-v3/testsuite/20_util/tuple/78939.cc
b/libstdc++-v3/testsuite/20_util/tuple/78939.cc
new file mode 100644
index 0000000..bab143b
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/tuple/78939.cc
@@ -0,0 +1,49 @@
+// Copyright (C) 2017 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++17" }
+// { dg-do compile { target c++1z } }
+
+// PR libstdc++/78939
+
+#include <utility>
+
+struct A { int i, j; };
+
+int
+test01()
+{
+ A a{};
+ const auto [i, j] = a;
+ return i + j;
+}
+
+int
+test02()
+{
+ A a{};
+ volatile auto [i, j] = a;
+ return i + j;
+}
+
+int
+test03()
+{
+ A a{};
+ const volatile auto [i, j] = a;
+ return i + j;
+}
diff --git a/libstdc++-v3/testsuite/20_util/tuple/cv_tuple_size_neg.cc
b/libstdc++-v3/testsuite/20_util/tuple/cv_tuple_size_neg.cc
new file mode 100644
index 0000000..450ed89
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/tuple/cv_tuple_size_neg.cc
@@ -0,0 +1,35 @@
+// Copyright (C) 2017 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-do compile { target c++11 } }
+
+#include <tuple>
+
+// PR libstdc++/78939
+
+std::tuple_size<const int> ic; // { dg-error "incomplete" }
+std::tuple_size<volatile int> iv; // { dg-error "incomplete" }
+std::tuple_size<const volatile int> icv; // { dg-error "incomplete" }
+
+struct A { };
+std::tuple_size<const A> ac; // { dg-error "incomplete" }
+std::tuple_size<volatile A> av; // { dg-error "incomplete" }
+std::tuple_size<const volatile A> acv; // { dg-error "incomplete" }
+
+std::tuple_size<const void> vc; // { dg-error "incomplete" }
+std::tuple_size<volatile void> vv; // { dg-error "incomplete" }
+std::tuple_size<const volatile void> vcv; // { dg-error "incomplete" }
