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.

Backport from mainline
2020-05-23  Jonathan Wakely  <jwakely@redhat.com>

	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.

libstdc++-v3/ChangeLog

+2020-05-23  Jonathan Wakely  <jwakely@redhat.com>
+
+	Backport from mainline
+	2020-05-23  Jonathan Wakely  <jwakely@redhat.com>
+
+	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.
+
 2020-05-21  Jonathan Wakely  <jwakely@redhat.com>
 
 	Backport from mainline

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)

libstdc++-v3/testsuite/25_algorithms/copy/debug/95289.cc

+// 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);
+}