From defc69b346c8a6366876d790e13005887863f8cd Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Wed, 2 Oct 2024 19:05:45 +0200
Subject: [PATCH] refactor: :boom: `mag_constant` now takes a symbol and a
 value and the class deriving from it must be final

---
 .../framework_basics/systems_of_units.md      |  4 +--
 .../include/mp-units/framework/magnitude.h    | 28 +++++++++++++++----
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/docs/users_guide/framework_basics/systems_of_units.md b/docs/users_guide/framework_basics/systems_of_units.md
index 6f61cc10..57d4d592 100644
--- a/docs/users_guide/framework_basics/systems_of_units.md
+++ b/docs/users_guide/framework_basics/systems_of_units.md
@@ -184,9 +184,7 @@ For some units, a magnitude might also be irrational. The best example here is a
 is defined using a floating-point magnitude having a factor of the number π (Pi):
 
 ```cpp
-inline constexpr struct pi : mag_constant {
-  static constexpr auto value = std::numbers::pi_v<long double>;
-} pi;
+inline constexpr struct pi final : mag_constant<symbol_text{u8"𝜋", "pi"}, std::numbers::pi_v<long double>> {} pi;
 ```
 
 ```cpp
diff --git a/src/core/include/mp-units/framework/magnitude.h b/src/core/include/mp-units/framework/magnitude.h
index 7159effc..3ec0254c 100644
--- a/src/core/include/mp-units/framework/magnitude.h
+++ b/src/core/include/mp-units/framework/magnitude.h
@@ -56,13 +56,27 @@ using factorizer = wheel_factorizer<4>;
 
 }  // namespace detail
 
-MP_UNITS_EXPORT struct mag_constant {};
+#if defined MP_UNITS_COMP_CLANG || MP_UNITS_COMP_CLANG < 18
 
-template<typename T>
-concept MagConstant = std::derived_from<T, mag_constant> && std::is_empty_v<T> && requires {
-  { +T::value } -> std::same_as<long double>;
+MP_UNITS_EXPORT template<symbol_text Symbol>
+struct mag_constant {};
+
+#else
+
+MP_UNITS_EXPORT template<symbol_text Symbol, auto Value>
+struct mag_constant {
+  static constexpr auto value = Value;
 };
 
+#endif
+
+MP_UNITS_EXPORT template<typename T>
+concept MagConstant =
+  is_derived_from_specialization_of_v<T, mag_constant> && std::is_empty_v<T> && std::is_final_v<T> && requires {
+    { +T::value } -> std::same_as<long double>;
+  };
+
+
 /**
  * @brief  Any type which can be used as a basis vector in a PowerV.
  *
@@ -598,8 +612,12 @@ constexpr Magnitude auto mag_power = pow<Pow>(mag<Base>);
 /**
  * @brief  A convenient Magnitude constant for pi, which we can manipulate like a regular number.
  */
-inline constexpr struct pi : mag_constant {
+#if defined MP_UNITS_COMP_CLANG || MP_UNITS_COMP_CLANG < 18
+inline constexpr struct pi final : mag_constant<symbol_text{u8"𝜋", "pi"}> {
   static constexpr auto value = std::numbers::pi_v<long double>;
+#else
+inline constexpr struct pi final : mag_constant<symbol_text{u8"𝜋", "pi"}, std::numbers::pi_v<long double>> {
+#endif
 } pi;
 
 [[deprecated("Use `mag<pi>` instead")]] inline constexpr Magnitude auto mag_pi = mag<pi>;