Provide additional checks for event handlers

include/SFML/Window/Event.hpp

@@ -389,6 +389,19 @@ private:
 
     template <typename T>
     static constexpr bool isEventSubtype = isInParameterPack<T>(decltype (&m_data)(nullptr));
+
+    template <typename Handler, typename... Ts>
+    static constexpr bool isInvokableWithAnyOf(std::variant<Ts...>*)
+    {
+        return (std::is_invocable_v<Handler, const Ts&> || ...);
+    }
+
+public:
+    template <typename Handler>
+    static constexpr bool isValidHandler()
+    {
+        return isInvokableWithAnyOf<Handler>(static_cast<decltype(m_data)*>(nullptr));
+    }
 };
 
 } // namespace sf

include/SFML/Window/WindowBase.inl

@@ -61,6 +61,8 @@ template <typename... Ts>
 void WindowBase::handleEvents(Ts&&... handlers) // NOLINT(cppcoreguidelines-missing-std-forward)
 {
     static_assert(sizeof...(Ts) > 0, "Must provide at least one handler");
+    static_assert((Event::isValidHandler<Ts>() && ...),
+                  "All event handlers must accept a single parameter, either a const reference or a value");
 
     // Disable misc-const-correctness for this line since clang-tidy
     // complains about it even though the code would become uncompilable

test/Window/WindowBase.test.cpp

@@ -213,6 +213,7 @@ TEST_CASE("[Window] sf::WindowBase", runDisplayTests())
 
         // Should compile if user provides only a specific handler
         windowBase.handleEvents([](const sf::Event::Closed&) {});
+        windowBase.handleEvents([](sf::Event::Closed) {});
 
         // Should compile if user provides only a catch-all
         windowBase.handleEvents([](const auto&) {});