[filter] support input control optional api

include/fcarouge/internal/format.hpp

@@ -73,7 +73,7 @@ struct std::formatter<
     format_context.advance_to(
         format_to(format_context.out(), R"({{"f": {}, )", filter.f()));
 
-    if constexpr (requires { filter.g(); }) {
+    if constexpr (fcarouge::internal::has_input_control_method<kalman>) {
       format_context.advance_to(
           format_to(format_context.out(), R"("g": {}, )", filter.g()));
     }

include/fcarouge/internal/kalman.tpp

@@ -241,26 +241,29 @@ kalman<State, Output, Input, UpdateTypes, PredictionTypes>::h(
 template <typename State, typename Output, typename Input, typename UpdateTypes,
           typename PredictionTypes>
 [[nodiscard("The returned control transition matrix G is unexpectedly "
-            "discarded.")]] inline constexpr auto
+            "discarded.")]] inline constexpr const auto &
 kalman<State, Output, Input, UpdateTypes, PredictionTypes>::g() const
-    -> const input_control &requires(requires { filter.g; }) {
-      return filter.g;
-    }
+  requires(has_input_control<implementation>)
+{
+  return filter.g;
+}
 
 template <typename State, typename Output, typename Input, typename UpdateTypes,
           typename PredictionTypes>
 [[nodiscard("The returned control transition matrix G is unexpectedly "
-            "discarded.")]] inline constexpr auto kalman<State, Output, Input,
-                                                         UpdateTypes,
-                                                         PredictionTypes>::g()
-    -> input_control &requires(requires { filter.g; }) { return filter.g; }
+            "discarded.")]] inline constexpr auto &
+kalman<State, Output, Input, UpdateTypes, PredictionTypes>::g()
+  requires(has_input_control<implementation>)
+{
+  return filter.g;
+}
 
 template <typename State, typename Output, typename Input, typename UpdateTypes,
           typename PredictionTypes>
-inline constexpr void kalman<State, Output, Input, UpdateTypes,
-                             PredictionTypes>::g(const auto &value,
-                                                 const auto &...values)
-  requires(requires { filter.g; })
+inline constexpr void
+kalman<State, Output, Input, UpdateTypes, PredictionTypes>::g(
+    const auto &value, const auto &...values)
+  requires(has_input_control<implementation>)
 {
   using transition_control_function = decltype(filter.transition_control_g);
   filter.transition_control_g =

include/fcarouge/internal/utility.hpp

@@ -62,6 +62,17 @@ concept has_input_method = requires(Filter filter) { filter.u(); };
 template <typename Filter>
 concept has_input = has_input_member<Filter> || has_input_method<Filter>;
 
+template <typename Filter>
+concept has_input_control_member = requires(Filter filter) { filter.g; };
+
+template <typename Filter>
+concept has_input_control_method = requires(Filter filter) { filter.g(); };
+
+//! @todo Shorten when MSVC has better if-constexpr-requires support.
+template <typename Filter>
+concept has_input_control =
+    has_input_control_member<Filter> || has_input_control_method<Filter>;
+
 struct empty {
   inline constexpr explicit empty([[maybe_unused]] auto &&...any) noexcept {
     // Constructs from anything for all initializations compatibility.

include/fcarouge/kalman.hpp

@@ -485,9 +485,10 @@ class kalman final {
   //! @return The control transition matrix G.
   //!
   //! @complexity Constant.
-  inline constexpr auto g() const
-      -> const input_control &requires(requires { filter.g; });
-  inline constexpr auto g() -> input_control &requires(requires { filter.g; });
+  inline constexpr const auto &g() const
+    requires(has_input_control<implementation>);
+  inline constexpr auto &g()
+    requires(has_input_control<implementation>);
 
   //! @brief Sets the control transition matrix G.
   //!
@@ -506,7 +507,7 @@ class kalman final {
   //!
   //! @complexity Constant.
   inline constexpr void g(const auto &value, const auto &...values)
-    requires(requires { filter.g; });
+    requires(has_input_control<implementation>);
 
   //! @brief Returns the gain matrix K.
   //!

include/fcarouge/utility.hpp

@@ -76,6 +76,13 @@ concept eigen = internal::eigen<Type>;
 //! member and `u()` input method.
 template <typename Filter>
 concept has_input = internal::has_input<Filter>;
+
+//! @brief Filter input control support concept.
+//!
+//! @details The filter supports the input control related functionality:
+//! `input_control` type member and `g()` input method.
+template <typename Filter>
+concept has_input_control = internal::has_input_control<Filter>;
 //! @}
 
 //! @name Types