Added the discriminative test for the function bug

tests/math.cpp

@@ -146,4 +146,24 @@ struct render_test : public test_scaffold {
 	}
 };
 
-append_test dummy_ml8576f(new render_test{});
+append_test dummy_pzj6f(new render_test{});
+
+struct function_test : public test_scaffold {
+	function_test() {
+		name = __FILE__ ":3";
+	}
+
+
+	virtual int run() {
+		int res = 0;
+		gp::function<float(vec3)> l_sdf_b = gp::sphere_sdf<float>({0.0,0.0,0.0}, 1.0);
+
+		gp::function<float(vec3)> sdf = l_sdf_b;
+
+		gp_config::assertion(sdf(vec3(0,0,0)) == -1, "Bad sdf");
+
+		return res;
+	}
+};
+
+append_test dummy_ml8576f(new function_test{});