refactor: SymbolicConstant moved to _expression_template.h_

src/core/include/mp-units/ext/type_traits.h

@@ -193,11 +193,4 @@ template<template<auto...> typename T, typename T1, typename T2, typename... Ts>
     return get<T, T2, Ts...>();
 }
 
-namespace detail {
-
-template<typename T>
-concept SymbolicConstant = std::is_empty_v<T> && std::is_final_v<T>;
-
-}
-
 }  // namespace mp_units

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

@@ -31,13 +31,28 @@
 #ifdef MP_UNITS_IMPORT_STD
 import std;
 #else
+#include <concepts>
 #include <cstdint>
 #include <functional>
+#include <type_traits>
 #endif
 #endif
 
 namespace mp_units {
 
+namespace detail {
+
+// `SymbolicArg` is provided because `SymbolicConstant` requires a complete type which is not the case
+// for `OneType` below.
+template<typename T>
+concept SymbolicArg = (!std::is_const_v<T>) && (!std::is_reference_v<T>);
+
+template<typename T>
+concept SymbolicConstant =
+  SymbolicArg<T> && std::is_empty_v<T> && std::is_trivial_v<T> && std::semiregular<T> && std::is_final_v<T>;
+
+}  // namespace detail
+
 /**
  * @brief Type list type used by the expression template framework
  *