refactor: integral conversion factor optimized for the case of the sa…

…me unit type

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

@@ -45,17 +45,22 @@ namespace mp_units {
 
 namespace detail {
 
-template<auto UFrom, auto UTo>
-concept IntegralConversionFactor = Unit<decltype(UFrom)> && Unit<decltype(UTo)> &&
-                                   decltype(is_integral(decltype(decltype(get_canonical_unit(UFrom))::mag /
-                                                                 decltype(get_canonical_unit(UTo))::mag){}))::value;
+template<Unit UFrom, Unit UTo>
+[[nodiscard]] consteval bool integral_conversion_factor(UFrom from, UTo to)
+{
+  if constexpr (is_same_v<UFrom, UTo>)
+    return true;
+  else
+    return decltype(is_integral(
+      decltype(decltype(get_canonical_unit(from))::mag / decltype(get_canonical_unit(to))::mag){}))::value;
+}
 
 template<typename QFrom, typename QTo>
 concept QuantityConvertibleTo =
   Quantity<QFrom> && Quantity<QTo> && implicitly_convertible(QFrom::quantity_spec, QTo::quantity_spec) &&
   convertible(QFrom::unit, QTo::unit) &&
   (treat_as_floating_point<typename QTo::rep> ||
-   (!treat_as_floating_point<typename QFrom::rep> && IntegralConversionFactor<QFrom::unit, QTo::unit>)) &&
+   (!treat_as_floating_point<typename QFrom::rep> && (integral_conversion_factor(QFrom::unit, QTo::unit)))) &&
   // TODO consider providing constraints of sudo_cast here rather than testing if it can be called (its return type is
   // deduced thus the function is evaluated here and may emit truncating conversion or other warnings)
   requires(QFrom q) { detail::sudo_cast<QTo>(q); };