[committed] libstdc++: Fix function that can't be constexpr in C++11 (PR 95289)

[Jonathan Wakely via Gcc-patches]

The body of this function isn't just a return statement, so it can't be
constexpr until C++14.

        PR libstdc++/95289
        * include/debug/helper_functions.h (__get_distance): Only declare
        as a constexpr function for C++14 and up.
        * testsuite/25_algorithms/copy/debug/95289.cc: New test.

Tested powerpc64le-linux, committed to master.

I'll backport to gcc-10 too.


commit 3cb0c7cc160a50f830bfa9aa5a3264b773a28bf8
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Sat May 23 18:27:35 2020 +0100

    libstdc++: Fix function that can't be constexpr in C++11 (PR 95289)
    
    The body of this function isn't just a return statement, so it can't be
    constexpr until C++14.
    
            PR libstdc++/95289
            * include/debug/helper_functions.h (__get_distance): Only declare
            as a constexpr function for C++14 and up.
            * testsuite/25_algorithms/copy/debug/95289.cc: New test.

diff --git a/libstdc++-v3/include/debug/helper_functions.h 
b/libstdc++-v3/include/debug/helper_functions.h
index 251582d5922..62d5309257f 100644
--- a/libstdc++-v3/include/debug/helper_functions.h
+++ b/libstdc++-v3/include/debug/helper_functions.h
@@ -96,7 +96,7 @@ namespace __gnu_debug
     { return std::make_pair(__rhs - __lhs, __dp_exact); }
 
   template<typename _Iterator>
-    _GLIBCXX_CONSTEXPR
+    _GLIBCXX14_CONSTEXPR
     inline typename _Distance_traits<_Iterator>::__type
     __get_distance(_Iterator __lhs, _Iterator __rhs,
                   std::input_iterator_tag)
diff --git a/libstdc++-v3/testsuite/25_algorithms/copy/debug/95289.cc 
b/libstdc++-v3/testsuite/25_algorithms/copy/debug/95289.cc
new file mode 100644
index 00000000000..bcdab19761f
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/copy/debug/95289.cc
@@ -0,0 +1,31 @@
+// Copyright (C) 2020 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++11 -D_GLIBCXX_DEBUG" }
+// { dg-do compile { target c++11_only } }
+
+#include <algorithm>
+#include <iterator>
+#include <istream>
+
+// PR libstdc++/95289
+
+void
+test01(std::istream_iterator<char> i, char* out)
+{
+  std::copy(i, decltype(i){}, out);
+}