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Re: [v3 PATCH] PR libstdc++/87619


On Wed, 17 Oct 2018 at 12:20, Ville Voutilainen
<ville.voutilainen@gmail.com> wrote:
>
> On Wed, 17 Oct 2018 at 12:07, Jonathan Wakely <jwakely@redhat.com> wrote:
> > It might be worth also calling f<256>() and doing:
> >
> > template <std::size_t... Is>
> > void f_impl(std::index_sequence<Is...> 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> <= 255)
> >     static_assert(sizeof(V) == 2)
> >   else
> >     static_assert(sizeof(V) > 2)
> > }
> >
> > Just to check we don't introduce an off-by-one error in the *other*
> > direction in future. What do you think?
>
> Agreed. I'll patch that in.
>
> > To be really portable we would use numeric_limits<unsigned char>::max()
> > but we don't need to worry about non-8-bit char in our implementation.
>
> I can change that as well so that we don't need to re-think it every time.

Here.
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 2d86a70..55b6440 100644
--- a/libstdc++-v3/include/std/variant
+++ b/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;
 
diff --git a/libstdc++-v3/testsuite/20_util/variant/87619.cc b/libstdc++-v3/testsuite/20_util/variant/87619.cc
new file mode 100644
index 0000000..e6f48fe
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/variant/87619.cc
@@ -0,0 +1,53 @@
+// 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>();
+}

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