Add operator<< for variant

test/Jamfile

@@ -123,3 +123,5 @@ run variant_visit_r.cpp : : :
 compile variant_derived_construct.cpp ;
 
 run variant_visit_by_index.cpp ;
+
+run variant_ostream_insert.cpp ;

test/variant_ostream_insert.cpp

@@ -0,0 +1,36 @@
+// Copyright 2021 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <sstream>
+#include <string>
+
+using namespace boost::variant2;
+
+template<class T> std::string to_string( T const& t )
+{
+    std::ostringstream os;
+
+    os << t;
+
+    return os.str();
+}
+
+int main()
+{
+    variant<int, float, std::string> v( 1 );
+
+    BOOST_TEST_EQ( to_string( v ), to_string( 1 ) );
+
+    v = 3.14f;
+
+    BOOST_TEST_EQ( to_string( v ), to_string( 3.14f ) );
+
+    v = "test";
+
+    BOOST_TEST_EQ( to_string( v ), to_string( "test" ) );
+
+    return boost::report_errors();
+}