libstdc++: Disable std::array assertions for C++11 constexpr

The recent changes to add assertions to std::array broke the functions
that need to be constexpr in C++11, because of the restrictive rules for
constexpr functions in C++11.

This simply disables the assertions for C++11 mode, so the functions can
be constexpr again.

libstdc++-v3/ChangeLog:

	* include/std/array (array::operator[](size_t) const, array::front() const)
	(array::back() const) [__cplusplus == 201103]: Disable
	assertions.
	* testsuite/23_containers/array/element_access/constexpr_element_access.cc:
	Check for correct values.
	* testsuite/23_containers/array/tuple_interface/get_neg.cc:
	Adjust dg-error line numbers.
	* testsuite/23_containers/array/debug/constexpr_c++11.cc: New test.

4 files changed, 48 insertions, 11 deletions

diff --git a/libstdc++-v3/include/std/array b/libstdc++-v3/include/std/array
index 3c4c88a..8075099 100644
--- a/libstdc++-v3/include/std/array
+++ b/libstdc++-v3/include/std/array
@@ -192,7 +192,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr const_reference
       operator[](size_type __n) const noexcept
       {
+#if __cplusplus >= 201402L
 	__glibcxx_requires_subscript(__n);
+#endif
 	return _AT_Type::_S_ref(_M_elems, __n);
       }
 
@@ -228,7 +230,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr const_reference
       front() const noexcept
       {
+#if __cplusplus >= 201402L
 	__glibcxx_requires_nonempty();
+#endif
 	return _AT_Type::_S_ref(_M_elems, 0);
       }
 
@@ -242,7 +246,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr const_reference
       back() const noexcept
       {
+#if __cplusplus >= 201402L
 	__glibcxx_requires_nonempty();
+#endif
 	return _Nm ? _AT_Type::_S_ref(_M_elems, _Nm - 1)
  	           : _AT_Type::_S_ref(_M_elems, 0);
       }
diff --git a/libstdc++-v3/testsuite/23_containers/array/debug/constexpr_c++11.cc b/libstdc++-v3/testsuite/23_containers/array/debug/constexpr_c++11.cc
new file mode 100644
index 0000000..c8b3599
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/array/debug/constexpr_c++11.cc
@@ -0,0 +1,32 @@
+// { dg-options "-std=gnu++11 -D_GLIBCXX_DEBUG" }
+// { dg-do compile { target c++11 } }
+
+// Copyright (C) 2011-2020 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/>.
+
+#include <array>
+
+void test01()
+{
+  // array
+  constexpr std::array<std::size_t, 6> a = { { 0, 55, 66, 99, 4115, 2 } };
+  constexpr auto v1 = a[1];
+  constexpr auto v2 = a.at(2);
+  constexpr auto v3 = a.front();
+  constexpr auto v4 = a.back();
+  static_assert( (v1 + v2 + v3 + v4) == (55 + 66 + 0 + 2), "" );
+}
diff --git a/libstdc++-v3/testsuite/23_containers/array/element_access/constexpr_element_access.cc b/libstdc++-v3/testsuite/23_containers/array/element_access/constexpr_element_access.cc
index c1dd905..037f6ea 100644
--- a/libstdc++-v3/testsuite/23_containers/array/element_access/constexpr_element_access.cc
+++ b/libstdc++-v3/testsuite/23_containers/array/element_access/constexpr_element_access.cc
@@ -19,14 +19,13 @@
 
 #include <array>
 
-int main()
+void test01()
 {
   // array
-  typedef std::array<std::size_t, 6> array_type;
-  constexpr array_type a = { { 0, 55, 66, 99, 4115, 2 } };
-  constexpr auto v1 __attribute__((unused)) = a[1];
-  constexpr auto v2 __attribute__((unused)) = a.at(2);
-  constexpr auto v3 __attribute__((unused)) = a.front();
-  constexpr auto v4 __attribute__((unused)) = a.back();
-  return 0;
+  constexpr std::array<std::size_t, 6> a = { { 0, 55, 66, 99, 4115, 2 } };
+  constexpr auto v1 = a[1];
+  constexpr auto v2 = a.at(2);
+  constexpr auto v3 = a.front();
+  constexpr auto v4 = a.back();
+  static_assert( (v1 + v2 + v3 + v4) == (55 + 66 + 0 + 2), "" );
 }
diff --git a/libstdc++-v3/testsuite/23_containers/array/tuple_interface/get_neg.cc b/libstdc++-v3/testsuite/23_containers/array/tuple_interface/get_neg.cc
index 88416cc..e287eef 100644
--- a/libstdc++-v3/testsuite/23_containers/array/tuple_interface/get_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/array/tuple_interface/get_neg.cc
@@ -26,6 +26,6 @@ int n1 = std::get<1>(a);
 int n2 = std::get<1>(std::move(a));
 int n3 = std::get<1>(ca);
 
-// { dg-error "static assertion failed" "" { target *-*-* } 357 }
-// { dg-error "static assertion failed" "" { target *-*-* } 365 }
-// { dg-error "static assertion failed" "" { target *-*-* } 373 }
+// { dg-error "static assertion failed" "" { target *-*-* } 363 }
+// { dg-error "static assertion failed" "" { target *-*-* } 371 }
+// { dg-error "static assertion failed" "" { target *-*-* } 379 }