libstdc++: Add test that tracks range adaptors' sizes

libstdc++-v3/ChangeLog:

	* testsuite/std/ranges/adaptors/sizeof.cc: New test.

libstdc++-v3/testsuite/std/ranges/adaptors/sizeof.cc

+// 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 } }
+
+#include <ranges>
+#include <string_view>
+
+namespace ranges = std::ranges;
+
+auto pred_f(int x) { return x%2 == 0; };
+auto pred_l = [] (int x) { return x%2 == 0; };
+
+auto func_f(int x) { return x*x; }
+auto func_l = [] (int x) { return x*x; };
+
+using V = ranges::subrange<int*, int*>;
+constexpr auto ptr = sizeof(int*);
+static_assert(sizeof(V) == 2*ptr);
+
+static_assert(sizeof(ranges::take_view<V>) == 4*ptr);
+static_assert(sizeof(ranges::drop_view<V>) == 4*ptr);
+
+static_assert(sizeof(ranges::filter_view<V, decltype(&pred_f)>) == 5*ptr);
+static_assert(sizeof(ranges::take_while_view<V, decltype(&pred_f)>) == 4*ptr);
+static_assert(sizeof(ranges::drop_while_view<V, decltype(&pred_f)>) == 5*ptr);
+static_assert(sizeof(ranges::transform_view<V, decltype(&func_f)>) == 4*ptr);
+
+static_assert(sizeof(ranges::filter_view<V, decltype(pred_l)>) == 4*ptr);
+static_assert(sizeof(ranges::take_while_view<V, decltype(pred_l)>) == 3*ptr);
+static_assert(sizeof(ranges::drop_while_view<V, decltype(pred_l)>) == 4*ptr);
+static_assert(sizeof(ranges::transform_view<V, decltype(func_l)>) == 3*ptr);
+
+static_assert(sizeof(ranges::split_view<V, std::string_view>) == 5*ptr);
+
+static_assert
+ (sizeof(ranges::reverse_view<ranges::filter_view<V, decltype(pred_l)>>) == 5*ptr);