[filter] support input optional api

include/fcarouge/internal/format.hpp

@@ -58,14 +58,16 @@ template <typename State, typename Output, typename Input, typename UpdateTypes,
 struct std::formatter<
     fcarouge::kalman<State, Output, Input, UpdateTypes, PredictionTypes>,
     Char> {
+  using kalman =
+      fcarouge::kalman<State, Output, Input, UpdateTypes, PredictionTypes>;
+
   constexpr auto parse(std::basic_format_parse_context<Char> &parse_context) {
     return parse_context.begin();
   }
 
   //! @todo P2585 may be useful in simplifying and standardizing the support.
   template <typename OutputIt>
-  auto format(const fcarouge::kalman<State, Output, Input, UpdateTypes,
-                                     PredictionTypes> &filter,
+  auto format(const kalman &filter,
               std::basic_format_context<OutputIt, Char> &format_context) const
       -> OutputIt {
     format_context.advance_to(
@@ -96,7 +98,9 @@ struct std::formatter<
                                         R"("q": {}, "r": {}, "s": {}, )",
                                         filter.q(), filter.r(), filter.s()));
 
-    if constexpr (requires { filter.u(); }) {
+    //! @todo Generalize out internal method concept when MSVC has better
+    //! if-constexpr-requires support.
+    if constexpr (fcarouge::internal::has_input_method<kalman>) {
       format_context.advance_to(
           format_to(format_context.out(), R"("u": {}, )", filter.u()));
     }

include/fcarouge/internal/kalman.tpp

@@ -77,17 +77,19 @@ kalman<State, Output, Input, UpdateTypes, PredictionTypes>::z() const
 template <typename State, typename Output, typename Input, typename UpdateTypes,
           typename PredictionTypes>
 [[nodiscard("The returned control column vector U is unexpectedly "
-            "discarded.")]] inline constexpr auto
+            "discarded.")]] inline constexpr const auto &
 kalman<State, Output, Input, UpdateTypes, PredictionTypes>::u() const
-    -> const input &requires(requires { filter.u; }) { return filter.u; }
+  requires(has_input<implementation>)
+{
+  return filter.u;
+}
 
 template <typename State, typename Output, typename Input, typename UpdateTypes,
           typename PredictionTypes>
 [[nodiscard("The returned estimated covariance matrix P is unexpectedly "
-            "discarded.")]] inline constexpr auto kalman<State, Output, Input,
-                                                         UpdateTypes,
-                                                         PredictionTypes>::p()
-    const -> const estimate_uncertainty & {
+            "discarded.")]] inline constexpr auto
+kalman<State, Output, Input, UpdateTypes, PredictionTypes>::p() const
+    -> const estimate_uncertainty & {
   return filter.p;
 }
 

include/fcarouge/internal/utility.hpp

@@ -52,6 +52,16 @@ concept algebraic = not arithmetic<Type>;
 template <typename Type>
 concept eigen = requires { typename Type::PlainMatrix; };
 
+template <typename Filter>
+concept has_input_member = requires(Filter filter) { filter.u; };
+
+template <typename Filter>
+concept has_input_method = requires(Filter filter) { filter.u(); };
+
+//! @todo Shorten when MSVC has better if-constexpr-requires support.
+template <typename Filter>
+concept has_input = has_input_member<Filter> || has_input_method<Filter>;
+
 struct empty {
   inline constexpr explicit empty([[maybe_unused]] auto &&...any) noexcept {
     // Constructs from anything for all initializations compatibility.

include/fcarouge/kalman.hpp

@@ -346,8 +346,8 @@ class kalman final {
   //! @return The last control column vector U.
   //!
   //! @complexity Constant.
-  inline constexpr auto u() const
-      -> const input &requires(requires { filter.u; });
+  inline constexpr const auto &u() const
+    requires(has_input<implementation>);
 
   //! @brief Returns the estimated covariance matrix P.
   //!

include/fcarouge/utility.hpp

@@ -69,6 +69,13 @@ concept algebraic = internal::algebraic<Type>;
 //! @details A third party Eigen3 algebraic concept.
 template <typename Type>
 concept eigen = internal::eigen<Type>;
+
+//! @brief Filter input support concept.
+//!
+//! @details The filter supports the input related functionality: `input` type
+//! member and `u()` input method.
+template <typename Filter>
+concept has_input = internal::has_input<Filter>;
 //! @}
 
 //! @name Types