Add hashing support

test/Jamfile

@@ -105,3 +105,5 @@ run variant_get_by_type.cpp throw_exception.cpp : : : $(NX) : variant_get_by_typ
 compile variant_get_by_type_cx.cpp : $(NX) : variant_get_by_type_cx_nx ;
 
 run variant_subset.cpp throw_exception.cpp : : : $(NX) : variant_subset_nx ;
+
+run variant_hash.cpp ;

test/variant_hash.cpp

@@ -0,0 +1,73 @@
+
+// Copyright 2020 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#if defined(_MSC_VER)
+# pragma warning( disable: 4244 ) // conversion from int to float, possible loss of data
+#endif
+
+#include <boost/variant2/variant.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <boost/container_hash/hash.hpp>
+#include <boost/config/workaround.hpp>
+
+using namespace boost::variant2;
+
+template<template<class...> class Hash, class T1, class T2, class T3> void test()
+{
+    variant<T1, T2, T3> v1( in_place_index_t<0>{} );
+    std::size_t h1 = Hash<decltype(v1)>()( v1 );
+
+    variant<T1, T2, T3> v2( in_place_index_t<1>{} );
+    std::size_t h2 = Hash<decltype(v2)>()( v2 );
+
+    variant<T1, T2, T3> v3( in_place_index_t<2>{} );
+    std::size_t h3 = Hash<decltype(v3)>()( v3 );
+
+    BOOST_TEST_NE( h1, h2 );
+    BOOST_TEST_NE( h1, h3 );
+    BOOST_TEST_NE( h2, h3 );
+}
+
+template<template<class...> class Hash, class T> void test2()
+{
+    variant<T> v1( 0 );
+    std::size_t h1 = Hash<decltype(v1)>()( v1 );
+
+    variant<T> v2( 1 );
+    std::size_t h2 = Hash<decltype(v2)>()( v2 );
+
+    variant<T> v3( 2 );
+    std::size_t h3 = Hash<decltype(v3)>()( v3 );
+
+    BOOST_TEST_NE( h1, h2 );
+    BOOST_TEST_NE( h1, h3 );
+    BOOST_TEST_NE( h2, h3 );
+}
+
+struct X {};
+
+int main()
+{
+    test<std::hash, monostate, monostate, monostate>();
+    test<std::hash, int, int, float>();
+
+    test<boost::hash, monostate, monostate, monostate>();
+    test<boost::hash, int, int, float>();
+
+    test2<std::hash, int>();
+    test2<std::hash, float>();
+
+    test2<boost::hash, int>();
+    test2<boost::hash, float>();
+
+#if !BOOST_WORKAROUND(BOOST_MSVC, < 1910)
+
+    BOOST_TEST_TRAIT_FALSE(( detail::is_hash_enabled<X> ));
+
+#endif
+
+    return boost::report_errors();
+}