Add visit_by_index

test/Jamfile

@@ -121,3 +121,5 @@ run variant_visit_r.cpp : : :
     ;
 
 compile variant_derived_construct.cpp ;
+
+run variant_visit_by_index.cpp ;

test/variant_visit_by_index.cpp

@@ -0,0 +1,50 @@
+// Copyright 2017, 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 <boost/config.hpp>
+
+using namespace boost::variant2;
+
+int main()
+{
+    {
+        variant<int, int, float> v;
+
+        visit_by_index( v,
+            []( int& x ){ BOOST_TEST_EQ( x, 0 ); },
+            []( int&   ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( float& ){ BOOST_ERROR( "incorrect alternative" ); } );
+    }
+
+    {
+        variant<int const, int, float const> v( 1 );
+
+        visit_by_index( v,
+            []( int const& x ){ BOOST_TEST_EQ( x, 1 ); },
+            []( int&         ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( float const& ){ BOOST_ERROR( "incorrect alternative" ); } );
+    }
+
+    {
+        variant<int, int, float> const v( in_place_index_t<1>(), 2 );
+
+        visit_by_index( v,
+            []( int const&   ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( int const& x ){ BOOST_TEST_EQ( x, 2 ); },
+            []( float const& ){ BOOST_ERROR( "incorrect alternative" ); } );
+    }
+
+    {
+        variant<int const, int, float const> const v( 3.14f );
+
+        visit_by_index( v,
+            []( int const&     ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( int const&     ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( float const& x ){ BOOST_TEST_EQ( x, 3.14f ); } );
+    }
+
+    return boost::report_errors();
+}