delegate: prepare to support filtering on both sides

src/entt/signal/delegate.hpp

@@ -75,6 +75,7 @@ class delegate<Ret(Args...)> {
     template<auto Candidate, std::size_t... Index>
     [[nodiscard]] auto wrap(std::index_sequence<Index...>) noexcept {
         return [](const void *, Args... args) -> Ret {
+            static_assert(std::is_invocable_r_v<Ret, decltype(Candidate), type_list_element_t<Index, type_list<Args...>>...>);
             [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
             return static_cast<Ret>(std::invoke(Candidate, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));
         };
@@ -83,6 +84,7 @@ class delegate<Ret(Args...)> {
     template<auto Candidate, typename Type, std::size_t... Index>
     [[nodiscard]] auto wrap(Type &, std::index_sequence<Index...>) noexcept {
         return [](const void *payload, Args... args) -> Ret {
+            static_assert(std::is_invocable_r_v<Ret, decltype(Candidate), Type &, type_list_element_t<Index, type_list<Args...>>...>);
             [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
             Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
             return static_cast<Ret>(std::invoke(Candidate, *curr, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));
@@ -92,6 +94,7 @@ class delegate<Ret(Args...)> {
     template<auto Candidate, typename Type, std::size_t... Index>
     [[nodiscard]] auto wrap(Type *, std::index_sequence<Index...>) noexcept {
         return [](const void *payload, Args... args) -> Ret {
+            static_assert(std::is_invocable_r_v<Ret, decltype(Candidate), Type *, type_list_element_t<Index, type_list<Args...>>...>);
             [[maybe_unused]] const auto arguments = std::forward_as_tuple(std::forward<Args>(args)...);
             Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
             return static_cast<Ret>(std::invoke(Candidate, curr, std::forward<type_list_element_t<Index, type_list<Args...>>>(std::get<Index>(arguments))...));