Re: [PATCH] PR libstdc++/78939 make tuple_size depend on tuple_size

[Jonathan Wakely]

On 12/05/17 15:49 +0100, Jonathan Wakely wrote:

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.

This is the patch for gcc-7-branch, which only uses the new
tuple_size<cv T> implementation for C++17 (because it's only needed to
make structured bindings work, and there's a chance this new
implementation could cause regressions for C++14 code).

Tested x86_64-linux, committed to gcc-7-branch.



commit ff2b5b263f9a7f7a48ae5de24d9d97bf81dc116c
Author: redi <redi@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Fri May 12 14:43:11 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>) [__cplusplus > 201402L]:
    	Only define partial specializations when tuple_size<T>::value is
    	valid.
    	* testsuite/20_util/tuple/78939.cc: New.

diff --git a/libstdc++-v3/include/std/utility b/libstdc++-v3/include/std/utility
index 188fcc2..1d9b8eb 100644
--- a/libstdc++-v3/include/std/utility
+++ b/libstdc++-v3/include/std/utility
@@ -89,6 +89,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   // _GLIBCXX_RESOLVE_LIB_DEFECTS
   // 2770. tuple_size<const T> specialization is not SFINAE compatible
+
+#if __cplusplus <= 201402L
   template<typename _Tp, typename = void>
     struct __tuple_size_cv_impl { };
 
@@ -106,6 +108,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   template<typename _Tp>
     struct tuple_size<const volatile _Tp> : __tuple_size_cv_impl<_Tp> { };
+#else
+  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<const __enable_if_has_tuple_size<_Tp>>
+    : public tuple_size<_Tp> { };
+
+  template<typename _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> { };
+#endif
 
   /// 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;
+}