libstdc++: Fix _GLIBCXX_DEBUG mode constexpr compatibility

The __glibcxx_check_can_[increment|decrement]_range macros are using the
_GLIBCXX_DEBUG_VERIFY_COND_AT macro which is not constexpr compliant and will produce nasty
diagnostics rather than the std::__failed_assertion dedicated to constexpr. Replace it with
correct _GLIBCXX_DEBUG_VERIFY_AT_F.

libstdc++-v3/ChangeLog:

	* include/debug/macros.h (__glibcxx_check_can_increment_range): Replace
	_GLIBCXX_DEBUG_VERIFY_COND_AT usage with _GLIBCXX_DEBUG_VERIFY_AT_F.
	(__glibcxx_check_can_decrement_range): Likewise.
	* testsuite/25_algorithms/copy_backward/constexpr.cc (test03): New.
	* testsuite/25_algorithms/copy/debug/constexpr_neg.cc: New test.
	* testsuite/25_algorithms/copy_backward/debug/constexpr_neg.cc: New test.
	* testsuite/25_algorithms/equal/constexpr_neg.cc: New test.
	* testsuite/25_algorithms/equal/debug/constexpr_neg.cc: New test.

1 files changed, 51 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/25_algorithms/equal/debug/constexpr_neg.cc b/libstdc++-v3/testsuite/25_algorithms/equal/debug/constexpr_neg.cc
new file mode 100644
index 00000000000..3a6903c9d00
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/equal/debug/constexpr_neg.cc
@@ -0,0 +1,51 @@
+// 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++2a" }
+// { dg-do compile { target c++2a xfail *-*-* } }
+// { dg-require-debug-mode "" }
+
+#include <algorithm>
+#include <array>
+
+constexpr bool
+test01()
+{
+  constexpr std::array<int, 12> ca0{{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}};
+  constexpr std::array<int, 12> ca1{{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}};
+
+  const auto outa = std::equal(ca0.end(), ca0.begin(), ca1.begin());
+  return outa;
+}
+
+static_assert(test01()); // { dg-error }
+
+constexpr bool
+test02()
+{
+  constexpr std::array<int, 12> ca0{{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}};
+  constexpr std::array<int, 11> ca1{{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}};
+
+  const auto outa = std::equal(ca0.begin(), ca0.end(), ca1.begin());
+  return outa;
+}
+
+static_assert(test02()); // { dg-error "outside the bounds" }
+
+// { dg-prune-output "non-constant condition" }
+// { dg-prune-output "in 'constexpr'" }
+// { dg-prune-output "failed_assertion" }