Check the number of named arguments at compile time

include/pybind11/attr.h

@@ -321,5 +321,18 @@ template <typename... Args> struct process_attributes {
     }
 };
 
+/// Compile-time integer sum
+constexpr size_t constexpr_sum() { return 0; }
+template <typename T, typename... Ts>
+constexpr size_t constexpr_sum(T n, Ts... ns) { return n + constexpr_sum(ns...); }
+
+/// Check the number of named arguments at compile time
+template <typename... Extra,
+          size_t named = constexpr_sum(std::is_base_of<arg, Extra>::value...),
+          size_t self  = constexpr_sum(std::is_same<is_method, Extra>::value...)>
+constexpr bool expected_num_args(size_t nargs) {
+    return named == 0 || (self + named) == nargs;
+}
+
 NAMESPACE_END(detail)
 NAMESPACE_END(pybind11)

include/pybind11/pybind11.h

@@ -72,6 +72,9 @@ protected:
     /// Special internal constructor for functors, lambda functions, etc.
     template <typename Func, typename Return, typename... Args, typename... Extra>
     void initialize(Func &&f, Return (*)(Args...), const Extra&... extra) {
+        static_assert(detail::expected_num_args<Extra...>(sizeof...(Args)),
+                      "The number of named arguments does not match the function signature");
+
         struct capture { typename std::remove_reference<Func>::type f; };
 
         /* Store the function including any extra state it might have (e.g. a lambda capture object) */
@@ -206,12 +209,6 @@ protected:
         }
 #endif
 
-        if (!rec->args.empty() && (int) rec->args.size() != args)
-            pybind11_fail(
-                "cpp_function(): function \"" + std::string(rec->name) + "\" takes " +
-                std::to_string(args) + " arguments, but " + std::to_string(rec->args.size()) +
-                " pybind11::arg entries were specified!");
-
         rec->signature = strdup(signature.c_str());
         rec->args.shrink_to_fit();
         rec->is_constructor = !strcmp(rec->name, "__init__") || !strcmp(rec->name, "__setstate__");