Add proper type deduction to visit_by_index

include/boost/variant2/variant.hpp

@@ -2213,27 +2213,27 @@ void swap( variant<T...> & v, variant<T...> & w )
 namespace detail
 {
 
-template<class V, class... F> struct visit_by_index_L
+template<class R, class V, class... F> using Vret2 = mp11::mp_eval_if_not< std::is_same<R, deduced>, R, front_if_same, mp11::mp_transform<mp11::mp_invoke_q, mp11::mp_list<Qret<F>...>, apply_cv_ref<V>> >;
+
+template<class R, class V, class... F> struct visit_by_index_L
 {
     V&& v;
     std::tuple<F&&...> tp;
 
-    template<class I> void operator()( I ) const
+    template<class I> constexpr detail::Vret2<R, V, F...> operator()( I ) const
     {
-        std::get<I::value>( std::move(tp) )( unsafe_get<I::value>( std::forward<V>(v) ) );
+        return std::get<I::value>( std::move(tp) )( unsafe_get<I::value>( std::forward<V>(v) ) );
     }
 };
 
 } // namespace detail
 
-template<class V, class... F> void visit_by_index( V&& v, F&&... f )
+template<class R = detail::deduced, class V, class... F> constexpr auto visit_by_index( V&& v, F&&... f ) -> detail::Vret2<R, V, F...>
 {
-    constexpr auto N = variant_size<V>::value;
+    static_assert( variant_size<V>::value == sizeof...(F), "Incorrect number of function objects" );
 
-    static_assert( N == sizeof...(F), "Incorrect number of function objects" );
-
-    mp11::mp_with_index<N>( v.index(),
-        detail::visit_by_index_L<V, F...>{ std::forward<V>(v), std::tuple<F&&...>( std::forward<F>(f)... ) } );
+    return mp11::mp_with_index<variant_size<V>::value>( v.index(),
+        detail::visit_by_index_L<R, V, F...>{ std::forward<V>(v), std::tuple<F&&...>( std::forward<F>(f)... ) } );
 }
 
 // hashing support

test/variant_visit_by_index.cpp

@@ -4,9 +4,32 @@
 
 #include <boost/variant2/variant.hpp>
 #include <boost/core/lightweight_test.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <boost/mp11.hpp>
 #include <boost/config.hpp>
 
 using namespace boost::variant2;
+using boost::mp11::mp_int;
+
+struct X
+{
+};
+
+struct F1
+{
+    int operator()( X& ) const { return 1; }
+    int operator()( X const& ) const { return 2; }
+    int operator()( X&& ) const { return 3; }
+    int operator()( X const&& ) const { return 4; }
+};
+
+struct F2
+{
+    mp_int<1> operator()( X& ) const { return {}; }
+    mp_int<2> operator()( X const& ) const { return {}; }
+    mp_int<3> operator()( X&& ) const { return {}; }
+    mp_int<4> operator()( X const&& ) const { return {}; }
+};
 
 int main()
 {
@@ -20,7 +43,7 @@ int main()
     }
 
     {
-        variant<int const, int, float const> v( 1 );
+        variant<int const, int, float const> v( in_place_index_t<0>(), 1 );
 
         visit_by_index( v,
             []( int const& x ){ BOOST_TEST_EQ( x, 1 ); },
@@ -46,5 +69,58 @@ int main()
             []( float const& x ){ BOOST_TEST_EQ( x, 3.14f ); } );
     }
 
+    {
+        variant<int, float> const v( 7 );
+
+        auto r = visit_by_index( v,
+            []( int const& x   ) -> double { return x; },
+            []( float const& x ) -> double { return x; } );
+
+        BOOST_TEST_TRAIT_SAME( decltype(r), double );
+        BOOST_TEST_EQ( r, 7.0 );
+    }
+
+    {
+        variant<int, float> const v( 2.0f );
+
+        auto r = visit_by_index( v,
+            []( int const& x   ) { return x + 0.0; },
+            []( float const& x ) { return x + 0.0; } );
+
+        BOOST_TEST_TRAIT_SAME( decltype(r), double );
+        BOOST_TEST_EQ( r, 2.0 );
+    }
+
+    {
+        variant<int, float, double> const v( 3.0 );
+
+        auto r = visit_by_index<double>( v,
+            []( int const& x    ) { return x; },
+            []( float const& x  ) { return x; },
+            []( double const& x ) { return x; } );
+
+        BOOST_TEST_TRAIT_SAME( decltype(r), double );
+        BOOST_TEST_EQ( r, 3.0 );
+    }
+
+    {
+        variant<X> v;
+        variant<X> const cv;
+
+        F1 f1;
+
+        BOOST_TEST_EQ( visit_by_index( v, f1 ), 1 );
+        BOOST_TEST_EQ( visit_by_index( cv, f1 ), 2 );
+        BOOST_TEST_EQ( visit_by_index( std::move( v ), f1 ), 3 );
+        BOOST_TEST_EQ( visit_by_index( std::move( cv ), f1 ), 4 );
+
+        F2 f2;
+
+        BOOST_TEST_EQ( visit_by_index<int>( v, f2 ), 1 );
+        BOOST_TEST_EQ( visit_by_index<int>( cv, f2 ), 2 );
+        BOOST_TEST_EQ( visit_by_index<int>( std::move( v ), f2 ), 3 );
+        BOOST_TEST_EQ( visit_by_index<int>( std::move( cv ), f2 ), 4 );
+    }
+
     return boost::report_errors();
 }