Fix constness detection and writer deferral

tests/unit/tt.cc

@@ -303,6 +303,32 @@ TEST_CASE("TemplateTask", "[core]") {
             static_assert(std::is_const_v<std::remove_reference_t<decltype(datum)>>, "Const datum expected");
           },
           ttg::edges(in), ttg::edges()));
+      {
+        auto tt = ttg::make_tt(
+          [](const int &key, const auto &datum, auto &outs) {
+            static_assert(std::is_lvalue_reference_v<decltype(datum)>, "Lvalue datum expected");
+            static_assert(std::is_const_v<std::remove_reference_t<decltype(datum)>>, "Const datum expected");
+          },
+          ttg::edges(in), ttg::edges());
+        using tt_t = typename std::remove_reference_t<decltype(*tt)>;
+        static_assert(std::is_const_v<std::tuple_element_t<0, tt_t::input_args_type>>);
+      }
+      CHECK_NOTHROW(ttg::make_tt(
+          [](const int &key, auto &&datum, auto &outs) {
+            static_assert(std::is_rvalue_reference_v<decltype(datum)>, "Rvalue datum expected");
+            static_assert(!std::is_const_v<std::remove_reference_t<decltype(datum)>>, "Nonconst datum expected");
+          },
+          ttg::edges(in), ttg::edges()));
+      {
+        auto tt = ttg::make_tt(
+          [](const int &key, auto &&datum, auto &outs) {
+            static_assert(std::is_rvalue_reference_v<decltype(datum)>, "Rvalue datum expected");
+            static_assert(!std::is_const_v<std::remove_reference_t<decltype(datum)>>, "Nonconst datum expected");
+          },
+          ttg::edges(in), ttg::edges());
+        using tt_t = typename std::remove_reference_t<decltype(*tt)>;
+        static_assert(!std::is_const_v<std::tuple_element_t<0, tt_t::input_args_type>>);
+      }
 
       // and without an output terminal
       CHECK_NOTHROW(ttg::make_tt(
@@ -312,6 +338,18 @@ TEST_CASE("TemplateTask", "[core]") {
             ttg::send(0, std::decay_t<decltype(key)>{}, std::decay_t<decltype(datum)>{});
           },
           ttg::edges(in), ttg::edges()));
+      {
+        auto tt = ttg::make_tt(
+          [](const int &key, auto &datum) {
+            static_assert(std::is_lvalue_reference_v<decltype(datum)>, "Lvalue datum expected");
+            static_assert(std::is_const_v<std::remove_reference_t<decltype(datum)>>, "Const datum expected");
+            ttg::send(0, std::decay_t<decltype(key)>{}, std::decay_t<decltype(datum)>{});
+          },
+          ttg::edges(in), ttg::edges());
+        using tt_t = typename std::remove_reference_t<decltype(*tt)>;
+        static_assert(std::is_const_v<std::tuple_element_t<0, tt_t::input_args_type>>);
+      }
+
     }
   }
 

ttg/ttg/madness/ttg.h

@@ -228,6 +228,8 @@ namespace ttg_madness {
     using input_refs_full_tuple_type =
         ttg::meta::add_glvalue_reference_tuple_t<ttg::meta::void_to_Void_tuple_t<actual_input_tuple_type>>;
 
+    using input_args_type = actual_input_tuple_type;
+
     using input_values_tuple_type = ttg::meta::drop_void_t<ttg::meta::decayed_typelist_t<input_tuple_type>>;
     using input_refs_tuple_type = ttg::meta::drop_void_t<ttg::meta::add_glvalue_reference_tuple_t<input_tuple_type>>;
     static_assert(!ttg::meta::is_any_void_v<input_values_tuple_type>);

ttg/ttg/util/meta/callable.h

@@ -150,7 +150,7 @@ namespace ttg::meta {
   template <typename T>
   struct candidate_argument_bindings<T, std::enable_if_t<!std::is_reference_v<T> && !std::is_void_v<T>>> {
     using type = std::conditional_t<std::is_const_v<T>, typelist<const T&>,
-                                    typelist<const T&, T&&,
+                                    typelist<T&&, const T&,
                                              // check for T& to be able to detect (erroneous) passing non-const lvalue&
                                              T&>>;
   };