[filter] add internal deducer Eigen3 concept support (#313)

Author: FrancoisCarouge

include/fcarouge/internal/utility.hpp

@@ -50,6 +50,11 @@ concept arithmetic = std::integral<Type> || std::floating_point<Type>;
 template <typename Type>
 concept algebraic = not arithmetic<Type>;
 
+template <typename Type>
+concept eigen = requires {
+  typename Type::PlainMatrix;
+};
+
 struct empty {
   inline constexpr explicit empty([[maybe_unused]] auto &&...any) noexcept {
     // Constructs from anything for all initializations compatibility.
@@ -122,36 +127,30 @@ struct transpose final {
 //! correctly sized resulting matrix but the correctness of the units have yet
 //! to be proven, nor whether its systematic usage is in fact appropriate.
 //! Hypothesis: units are incorrect, usage may be incorrect, for example
-//! `state_transition` may actually be unit-less. Note the lhs column size and
-//! rhs row size are the resulting type's column and row sizes, respectively:
+//! `state_transition` may actually be unit-less. Note the `lhs` column size and
+//! `rhs` row size are the resulting type's column and row sizes, respectively:
 //! Lhs [m by n] and Rhs [o by n] -> Result [m by o].
 //! @todo Is there a better, simpler, canonical, standard way of doing this type
-//! deductions?
+//! deduction?
 struct deducer final {
-  // Built-in's types deductions.
+  // Built-in, arithmetic, standard division support.
   template <arithmetic Lhs, arithmetic Rhs>
   [[nodiscard]] inline constexpr auto operator()(const Lhs &lhs,
                                                  const Rhs &rhs) const
       -> decltype(lhs / rhs);
 
-  // Eigen's types deductions.
-  template <typename Lhs, typename Rhs>
-  requires requires(Lhs lhs, Rhs rhs) {
-    typename Lhs::PlainMatrix;
-    typename Lhs::PlainMatrix;
-  }
+  // Type-erased Eigen third party linear algebra support.
+  template <eigen Lhs, eigen Rhs>
   [[nodiscard]] inline constexpr auto operator()(const Lhs &lhs,
                                                  const Rhs &rhs) const ->
       typename decltype(lhs * rhs.transpose())::PlainMatrix;
 
-  template <typename Lhs, arithmetic Rhs>
-  requires requires(Lhs lhs) { typename Lhs::PlainMatrix; }
+  template <eigen Lhs, arithmetic Rhs>
   [[nodiscard]] inline constexpr auto operator()(const Lhs &lhs,
                                                  const Rhs &rhs) const ->
       typename Lhs::PlainMatrix;
 
-  template <arithmetic Lhs, typename Rhs>
-  requires requires(Rhs rhs) { typename Rhs::PlainMatrix; }
+  template <arithmetic Lhs, eigen Rhs>
   [[nodiscard]] inline constexpr auto operator()(const Lhs &lhs,
                                                  const Rhs &rhs) const ->
       typename decltype(rhs.transpose())::PlainMatrix;