re PR libstdc++/87619 (sizeof(std::variant) can be reduced if its variant_size is UCHAR_MAX)

PR libstdc++/87619

* include/std/variant (__select_index): Fix an off-by-one.
* testsuite/20_util/variant/87619.cc: New.

From-SVN: r265247

libstdc++-v3/ChangeLog

+2018-10-17  Ville Voutilainen  <ville.voutilainen@gmail.com>
+
+	PR libstdc++/87619
+	* include/std/variant (__select_index): Fix an off-by-one.
+	* testsuite/20_util/variant/87619.cc: New.
+
 2018-10-16  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/bits/random.h (random_device) [!_GLIBCXX_USE_DEV_RANDOM]:

libstdc++-v3/include/std/variant

@@ -362,7 +362,7 @@ namespace __variant
 
   template <typename... _Types>
   using __select_index =
-    typename __select_int::_Select_int_base<sizeof...(_Types) + 1,
+    typename __select_int::_Select_int_base<sizeof...(_Types),
 					    unsigned char,
 					    unsigned short>::type::value_type;
 

libstdc++-v3/testsuite/20_util/variant/87619.cc

+// Copyright (C) 2018 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++17" }
+// { dg-do compile { target c++17 } }
+
+#include <variant>
+#include <utility>
+#include <limits>
+
+template<std::size_t I>
+struct S {
+};
+
+template <std::size_t... Is>
+void f_impl(std::index_sequence<Is...>)
+{
+	using V = std::variant<S<Is>...>;
+	// For a variant of 255 alternatives the valid indices are [0,254]
+	// and index 255 means valueless-by-exception, so fits in one byte.
+	if constexpr (std::variant_size_v<V> <=
+		      std::numeric_limits<unsigned char>::max())
+	  static_assert(sizeof(V) == 2);
+	else
+	  static_assert(sizeof(V) > 2);
+}
+
+template <std::size_t I>
+void f()
+{
+	f_impl(std::make_index_sequence<I>{});
+}
+
+int main()
+{
+	f<std::numeric_limits<unsigned char>::max() - 1>();
+	f<std::numeric_limits<unsigned char>::max()>();
+	f<std::numeric_limits<unsigned char>::max() + 1>();
+}