Add test/variant_index_type.cpp. Refs #50.

test/Jamfile

@@ -143,3 +143,5 @@ run variant_derived_construct2.cpp ;
 compile variant_visit_cx.cpp : [ requires cxx14_constexpr ] ;
 compile variant_visit_cx_2.cpp : [ requires cxx14_constexpr ] ;
 compile variant_visit_r_cx.cpp : [ requires cxx14_constexpr ] ;
+
+run variant_index_type.cpp : : : <variant>release ;

test/variant_index_type.cpp

@@ -0,0 +1,92 @@
+// Copyright 2024 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#if defined(_MSC_VER)
+# pragma warning( disable: 4503 ) // decorated name length exceeded
+#endif
+
+#include <boost/variant2/variant.hpp>
+#include <boost/mp11.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+using namespace boost::mp11;
+
+template<std::size_t I> struct X
+{
+};
+
+template<class N> using Xf = X< N::value >;
+
+template<std::size_t I> struct Y
+{
+    Y() = default;
+    Y( Y&& ) {}
+};
+
+template<class N> using Yf = Y< N::value >;
+
+template<std::size_t I> struct Z
+{
+    ~Z() {}
+};
+
+template<class N> using Zf = Z< N::value >;
+
+template<std::size_t I> struct W
+{
+    ~W() {}
+
+    W() = default;
+    W( W&& ) {}
+};
+
+template<class N> using Wf = W< N::value >;
+
+template<template<class...> class F, std::size_t N> void test_()
+{
+    using V = mp_rename< mp_transform<F, mp_iota_c<N>>, boost::variant2::variant >;
+
+    {
+        V v( mp_back<V>{} );
+        BOOST_TEST_EQ( v.index(), mp_size<V>() - 1 );
+    }
+
+    {
+        V v;
+        BOOST_TEST_EQ( v.index(), 0 );
+
+        v.template emplace< mp_back<V> >();
+        BOOST_TEST_EQ( v.index(), mp_size<V>() - 1 );
+    }
+}
+
+template<template<class...> class F> void test()
+{
+    test_<F, 126>();
+    test_<F, 127>();
+    test_<F, 128>();
+    test_<F, 129>();
+
+    test_<F, 254>();
+    test_<F, 255>();
+    test_<F, 256>();
+    test_<F, 257>();
+}
+
+int main()
+{
+    static_assert( !boost::variant2::variant< X<0>, X<1> >::uses_double_storage(), "" );
+    test<Xf>();
+
+    static_assert( boost::variant2::variant< Y<0>, Y<1> >::uses_double_storage(), "" );
+    test<Yf>();
+
+    static_assert( !boost::variant2::variant< Z<0>, Z<1> >::uses_double_storage(), "" );
+    test<Zf>();
+
+    static_assert( boost::variant2::variant< W<0>, W<1> >::uses_double_storage(), "" );
+    test<Wf>();
+
+    return boost::report_errors();
+}