test: shared transparent_equal_to type

test/entt/common/transparent_equal_to.h

@@ -0,0 +1,20 @@
+#ifndef ENTT_COMMON_TRANSPARENT_EQUAL_TO_HPP
+#define ENTT_COMMON_TRANSPARENT_EQUAL_TO_HPP
+
+#include <type_traits>
+
+namespace test {
+
+struct transparent_equal_to {
+    using is_transparent = void;
+
+    template<typename Type, typename Other>
+    constexpr std::enable_if_t<std::is_convertible_v<Other, Type>, bool>
+    operator()(const Type &lhs, const Other &rhs) const {
+        return lhs == static_cast<Type>(rhs);
+    }
+};
+
+} // namespace test
+
+#endif