return types for meta_func are no longer ambiguous (close #274)

src/entt/meta/meta.hpp

@@ -2269,6 +2269,7 @@ invoke(const meta_handle &, meta_any *args, std::index_sequence<Indexes...>) {
     {
         if constexpr(std::is_void_v<typename helper_type::return_type>) {
             std::invoke(Func, (args+Indexes)->cast<typename helper_type::template arg_type<Indexes>>()...);
+            any.emplace<void>();
         } else {
             any = meta_any{std::invoke(Func, (args+Indexes)->cast<typename helper_type::template arg_type<Indexes>>()...)};
         }
@@ -2291,6 +2292,7 @@ invoke(meta_handle &handle, meta_any *args, std::index_sequence<Indexes...>) {
     {
         if constexpr(std::is_void_v<typename helper_type::return_type>) {
             std::invoke(Member, clazz, (args+Indexes)->cast<typename helper_type::template arg_type<Indexes>>()...);
+            any.emplace<void>();
         } else {
             any = meta_any{std::invoke(Member, clazz, (args+Indexes)->cast<typename helper_type::template arg_type<Indexes>>()...)};
         }

test/entt/meta/meta.cpp

@@ -1373,7 +1373,8 @@ TEST_F(Meta, MetaFuncRetVoid) {
 
     auto any = func.invoke(instance, 5);
 
-    ASSERT_FALSE(any);
+    ASSERT_TRUE(any);
+    ASSERT_TRUE(any.can_cast<void>());
     ASSERT_EQ(func_type::value, 25);
 
     func.prop([](auto prop) {
@@ -1442,7 +1443,8 @@ TEST_F(Meta, MetaFuncStaticRetVoid) {
 
     auto any = func.invoke({}, 42);
 
-    ASSERT_FALSE(any);
+    ASSERT_TRUE(any);
+    ASSERT_TRUE(any.can_cast<void>());
     ASSERT_EQ(func_type::value, 42);
 
     func.prop([](auto *prop) {