fix: recursive concepts evaluation fixed

Resolves #740

Author: mpusz

src/core/include/mp-units/framework/representation_concepts.h

@@ -118,7 +118,7 @@ void real() = delete;  // poison pill
 
 struct real_t {
   // TODO how to constrain the return with RealScalar?
-  [[nodiscard]] constexpr auto operator()(const WeaklyRegular auto& clx) const
+  [[nodiscard]] constexpr auto operator()(const auto& clx) const
     requires requires { clx.real(); } || requires { real(clx); }
   {
     if constexpr (requires { clx.real(); })
@@ -142,7 +142,7 @@ void imag() = delete;  // poison pill
 
 struct imag_t {
   // TODO how to constrain the return with RealScalar?
-  [[nodiscard]] constexpr auto operator()(const WeaklyRegular auto& clx) const
+  [[nodiscard]] constexpr auto operator()(const auto& clx) const
     requires requires { clx.imag(); } || requires { imag(clx); }
   {
     if constexpr (requires { clx.imag(); })
@@ -167,7 +167,7 @@ void abs() = delete;      // poison pill
 
 struct modulus_t {
   // TODO how to constrain the return with RealScalar?
-  [[nodiscard]] constexpr auto operator()(const WeaklyRegular auto& clx) const
+  [[nodiscard]] constexpr auto operator()(const auto& clx) const
     requires requires { clx.modulus(); } || requires { modulus(clx); } || requires { clx.abs(); } ||
              requires { abs(clx); }
   {
@@ -216,7 +216,7 @@ void magnitude() = delete;  // poison pill
 void abs() = delete;        // poison pill
 
 struct magnitude_t {
-  template<WeaklyRegular T>
+  template<typename T>
   [[nodiscard]] constexpr Scalar auto operator()(const T& vec) const
     requires requires { vec.magnitude(); } || requires { magnitude(vec); } ||
              (RealScalar<T> && (std::is_arithmetic_v<T> || requires { vec.abs(); } || requires { abs(vec); }))

test/static/quantity_test.cpp

@@ -38,6 +38,7 @@ import std;
 #include <concepts>
 #include <cstdint>
 #include <limits>
+#include <optional>
 #include <type_traits>
 #include <utility>
 #if MP_UNITS_HOSTED
@@ -1406,4 +1407,14 @@ concept overflowing_unit_conversion = requires {
 };
 static_assert(overflowing_unit_conversion<std::int8_t(1) * km>);
 
+// std::optional
+static_assert(
+  std::equality_comparable_with<std::optional<quantity<isq::length[m], int>>, quantity<isq::length[m], int>>);
+#if MP_UNITS_HOSTED
+static_assert(std::equality_comparable_with<std::optional<quantity<si::volt, std::complex<double>>>,
+                                            quantity<si::volt, std::complex<double>>>);
+static_assert(std::equality_comparable_with<std::optional<quantity<si::metre, cartesian_vector<double>>>,
+                                            quantity<si::metre, cartesian_vector<double>>>);
+#endif
+
 }  // namespace