[v3] constexpr array additions

[Benjamin Kosnik]

Also pointed out by Daniel. 

It's kind of hokey to work around -fno-exceptions like this, but seems
harmless. In this particular case, deferring the throw to a function
like __throw_out_of_range makes the ::at member an invalid constant
expression.

Yay! std::array with compile time operator []!

tested x86/linux

-benjamin
2011-07-20  Benjamin Kosnik  <b...@redhat.com>
	    Daniel Krugler  <daniel.krueg...@googlemail.com>

	* include/std/array (array::at, array::operator[]): Mark constexpr.
	* testsuite/23_containers/array/requirements/
	constexpr_element_access.cc: Add.

Index: include/std/array
===================================================================
--- include/std/array	(revision 176549)
+++ include/std/array	(working copy)
@@ -35,6 +35,7 @@
 # include <bits/c++0x_warning.h>
 #else
 
+#include <stdexcept>
 #include <bits/stl_algobase.h>
 #include <bits/range_access.h>
 
@@ -150,8 +151,8 @@
       operator[](size_type __n)
       { return _M_instance[__n]; }
 
-      const_reference
-      operator[](size_type __n) const
+      constexpr const_reference
+      operator[](size_type __n) const noexcept
       { return _M_instance[__n]; }
 
       reference
@@ -162,12 +163,15 @@
 	return _M_instance[__n];
       }
 
-      const_reference
+      constexpr const_reference
       at(size_type __n) const
       {
-	if (__n >= _Nm)
-	  std::__throw_out_of_range(__N("array::at"));
-	return _M_instance[__n];
+	return __n < _Nm ? _M_instance[__n] : 
+#ifdef __EXCEPTIONS
+	                   throw out_of_range(__N("array::at"));
+#else
+	                   _M_instance[0];
+#endif
       }
 
       reference 
Index: testsuite/23_containers/array/requirements/constexpr_element_access.cc
===================================================================
--- testsuite/23_containers/array/requirements/constexpr_element_access.cc	(revision 0)
+++ testsuite/23_containers/array/requirements/constexpr_element_access.cc	(revision 0)
@@ -0,0 +1,31 @@
+// { dg-do compile }
+// { dg-options "-std=gnu++0x" }
+
+// Copyright (C) 2011 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>
+
+int main()
+{
+  // array
+  typedef std::array<std::size_t, 6> array_type;
+  constexpr array_type a = { 0, 55, 66, 99, 4115, 2 };
+  constexpr auto v1 = a[1];
+  constexpr auto v2 = a.at(2);
+  return 0;
+}