First attempt

tests/test_traits.cpp

@@ -0,0 +1,42 @@
+#include <catch2/catch_test_macros.hpp>
+#include <kpn/traits.hpp>
+#include <tuple>
+
+using namespace kpn;
+
+static int    free_func(int a, float b) { return a; }
+static void   sink_func(int a)          {}
+static std::tuple<int, float> multi_func(int a) { return {a, 1.f}; }
+
+TEST_CASE("arity of free function", "[traits]") {
+    STATIC_REQUIRE(arity_v<decltype(free_func)> == 2);
+}
+
+TEST_CASE("return type of free function", "[traits]") {
+    STATIC_REQUIRE(std::is_same_v<return_t<decltype(free_func)>, int>);
+}
+
+TEST_CASE("normalised return: single value", "[traits]") {
+    STATIC_REQUIRE(std::is_same_v<normalised_return_t<int>, std::tuple<int>>);
+}
+
+TEST_CASE("normalised return: void", "[traits]") {
+    STATIC_REQUIRE(std::is_same_v<normalised_return_t<void>, std::tuple<>>);
+}
+
+TEST_CASE("normalised return: tuple passthrough", "[traits]") {
+    using T = std::tuple<int, float>;
+    STATIC_REQUIRE(std::is_same_v<normalised_return_t<T>, T>);
+}
+
+TEST_CASE("output_count: single return", "[traits]") {
+    STATIC_REQUIRE(output_count_v<decltype(free_func)> == 1);
+}
+
+TEST_CASE("output_count: void return", "[traits]") {
+    STATIC_REQUIRE(output_count_v<decltype(sink_func)> == 0);
+}
+
+TEST_CASE("output_count: tuple return", "[traits]") {
+    STATIC_REQUIRE(output_count_v<decltype(multi_func)> == 2);
+}