diff --git a/src/core/include/mp-units/bits/quantity_spec_concepts.h b/src/core/include/mp-units/bits/quantity_spec_concepts.h
index 27543dde..c74f565a 100644
--- a/src/core/include/mp-units/bits/quantity_spec_concepts.h
+++ b/src/core/include/mp-units/bits/quantity_spec_concepts.h
@@ -143,4 +143,17 @@ template<typename T>
 concept QuantitySpec =
   detail::NamedQuantitySpec<T> || detail::IntermediateDerivedQuantitySpec<T> || detail::QuantityKindSpec<T>;
 
+template<QuantitySpec Q>
+[[nodiscard]] consteval QuantitySpec auto get_kind(Q q);
+
+namespace detail {
+
+template<auto To, auto From>
+concept DerivedFromQuantityKindSpecOf =
+  QuantitySpec<std::remove_const_t<decltype(From)>> && QuantitySpec<std::remove_const_t<decltype(To)>> &&
+  get_kind(From) != get_kind(To) &&
+  std::derived_from<std::remove_cvref_t<decltype(To)>, std::remove_cvref_t<decltype(get_kind(From))>>;
+
+}
+
 }  // namespace mp_units
diff --git a/src/core/include/mp-units/bits/unit_concepts.h b/src/core/include/mp-units/bits/unit_concepts.h
index 152ca819..bc4154ee 100644
--- a/src/core/include/mp-units/bits/unit_concepts.h
+++ b/src/core/include/mp-units/bits/unit_concepts.h
@@ -188,7 +188,6 @@ concept UnitOf = AssociatedUnit<U> && QuantitySpec<std::remove_const_t<decltype(
                  implicitly_convertible(get_quantity_spec(U{}), QS) &&
                  // the below is to make `dimensionless[radian]` invalid
                  (get_kind(QS) == get_kind(get_quantity_spec(U{})) ||
-                  !std::derived_from<std::remove_const_t<decltype(get_quantity_spec(U{}))>,
-                                     std::remove_const_t<decltype(get_kind(QS))>>);
+                  !detail::DerivedFromQuantityKindSpecOf<get_quantity_spec(U{}), QS>);
 
 }  // namespace mp_units
diff --git a/src/core/include/mp-units/quantity_spec.h b/src/core/include/mp-units/quantity_spec.h
index 4412c099..c8386cdc 100644
--- a/src/core/include/mp-units/quantity_spec.h
+++ b/src/core/include/mp-units/quantity_spec.h
@@ -433,9 +433,6 @@ struct derived_quantity_spec :
  */
 QUANTITY_SPEC(dimensionless, derived_quantity_spec<>{});
 
-template<QuantitySpec Q>
-[[nodiscard]] consteval QuantitySpec auto get_kind(Q q);
-
 /**
  * @brief Quantity kind specifier
  *
@@ -1331,10 +1328,7 @@ template<QuantitySpec From, QuantitySpec To>
     else
       return exploded_kind_result(
         convertible_impl(from_kind, get_kind(explode<get_complexity(from_kind)>(to_kind).quantity)));
-  } else if constexpr (get_kind(from) != get_kind(to) &&
-                       std::derived_from<std::remove_cvref_t<decltype(get_kind(to))>,
-                                         std::remove_cvref_t<decltype(get_kind(from))>> &&
-                       get_kind(to) == to)
+  } else if constexpr (DerivedFromQuantityKindSpecOf<get_kind(to), from> && get_kind(to) == to)
     return yes;
   else if constexpr (NamedQuantitySpec<From> && NamedQuantitySpec<To>) {
     if constexpr (have_common_base(from, to)) {
@@ -1461,11 +1455,9 @@ template<QuantitySpec Q1, QuantitySpec Q2>
   using QQ2 = std::remove_const_t<decltype(remove_kind(q2))>;
   if constexpr (is_same_v<Q1, Q2>)
     return q1;
-  else if constexpr (get_kind(q1) != get_kind(q2) && std::derived_from<std::remove_const_t<decltype(get_kind(q1))>,
-                                                                       std::remove_const_t<decltype(get_kind(q2))>>)
+  else if constexpr (detail::DerivedFromQuantityKindSpecOf<q1, q2>)
     return remove_kind(q1);
-  else if constexpr (get_kind(q1) != get_kind(q2) && std::derived_from<std::remove_const_t<decltype(get_kind(q2))>,
-                                                                       std::remove_const_t<decltype(get_kind(q1))>>)
+  else if constexpr (detail::DerivedFromQuantityKindSpecOf<q2, q1>)
     return remove_kind(q2);
   else if constexpr ((detail::QuantityKindSpec<Q1> && !detail::QuantityKindSpec<Q2>) ||
                      (detail::IntermediateDerivedQuantitySpec<QQ1> && detail::NamedQuantitySpec<QQ2> &&