Add possibility to use lambda in is-statements

Author: filipsajdak

include/cpp2util.h

@@ -637,6 +637,25 @@ auto is( X const& x ) -> bool {
         return false;
 }
 
+template <typename F, typename X>
+struct lambda_wrapper {
+    F f;
+    X x;
+
+    constexpr lambda_wrapper(F f, X x)
+    : f{f}, x{x} {}
+
+    template <typename T>
+    auto operator()(T&& t) const {
+        return f(std::forward<T>(t), x);
+    }
+};
+
+template< lambda_wrapper value, typename X >
+auto is( X const& x ) -> bool {
+        return value(x);
+}
+
 //-------------------------------------------------------------------------------------------------------------
 //  Built-in as (partial)
 //