diff --git a/src/core/include/mp-units/framework/quantity_spec.h b/src/core/include/mp-units/framework/quantity_spec.h
index c2c7fb8d..89eae99e 100644
--- a/src/core/include/mp-units/framework/quantity_spec.h
+++ b/src/core/include/mp-units/framework/quantity_spec.h
@@ -120,10 +120,34 @@ template<QuantitySpec auto... From, QuantitySpec Q>
 template<QuantitySpec Q>
 [[nodiscard]] consteval auto remove_kind(Q q);
 
+struct quantity_spec_interface_base {
+  template<QuantitySpec Lhs, QuantitySpec Rhs>
+  [[nodiscard]] friend consteval QuantitySpec auto operator*(Lhs lhs, Rhs rhs)
+  {
+    return clone_kind_of<Lhs{}, Rhs{}>(
+      expr_multiply<derived_quantity_spec, struct dimensionless, type_list_of_quantity_spec_less>(remove_kind(lhs),
+                                                                                                  remove_kind(rhs)));
+  }
+
+  template<QuantitySpec Lhs, QuantitySpec Rhs>
+  [[nodiscard]] friend consteval QuantitySpec auto operator/(Lhs lhs, Rhs rhs)
+  {
+    return clone_kind_of<Lhs{}, Rhs{}>(
+      expr_divide<derived_quantity_spec, struct dimensionless, type_list_of_quantity_spec_less>(remove_kind(lhs),
+                                                                                                remove_kind(rhs)));
+  }
+
+  template<QuantitySpec Lhs, QuantitySpec Rhs>
+  [[nodiscard]] friend consteval bool operator==(Lhs, Rhs)
+  {
+    return is_same_v<Lhs, Rhs>;
+  }
+};
+
 #if !MP_UNITS_API_NO_CRTP
 template<typename Self>
 #endif
-struct quantity_spec_interface {
+struct quantity_spec_interface : quantity_spec_interface_base {
 #if MP_UNITS_API_NO_CRTP
   template<typename Self, UnitOf<Self{}> U>
   [[nodiscard]] consteval Reference auto operator[](this Self self, U u)
@@ -155,28 +179,6 @@ struct quantity_spec_interface {
                     make_reference(Self{}, std::remove_cvref_t<Q>::unit)};
   }
 #endif
-
-  template<QuantitySpec Lhs, QuantitySpec Rhs>
-  [[nodiscard]] friend consteval QuantitySpec auto operator*(Lhs lhs, Rhs rhs)
-  {
-    return clone_kind_of<Lhs{}, Rhs{}>(
-      expr_multiply<derived_quantity_spec, struct dimensionless, type_list_of_quantity_spec_less>(remove_kind(lhs),
-                                                                                                  remove_kind(rhs)));
-  }
-
-  template<QuantitySpec Lhs, QuantitySpec Rhs>
-  [[nodiscard]] friend consteval QuantitySpec auto operator/(Lhs lhs, Rhs rhs)
-  {
-    return clone_kind_of<Lhs{}, Rhs{}>(
-      expr_divide<derived_quantity_spec, struct dimensionless, type_list_of_quantity_spec_less>(remove_kind(lhs),
-                                                                                                remove_kind(rhs)));
-  }
-
-  template<QuantitySpec Lhs, QuantitySpec Rhs>
-  [[nodiscard]] friend consteval bool operator==(Lhs, Rhs)
-  {
-    return is_same_v<Lhs, Rhs>;
-  }
 };
 
 }  // namespace detail
diff --git a/src/core/include/mp-units/framework/quantity_spec_concepts.h b/src/core/include/mp-units/framework/quantity_spec_concepts.h
index af939c59..9eac9f2d 100644
--- a/src/core/include/mp-units/framework/quantity_spec_concepts.h
+++ b/src/core/include/mp-units/framework/quantity_spec_concepts.h
@@ -30,30 +30,14 @@
 
 namespace mp_units {
 
-#if MP_UNITS_API_NO_CRTP
-
 namespace detail {
 
-struct quantity_spec_interface;
+struct quantity_spec_interface_base;
 
 }
 
 MP_UNITS_EXPORT template<typename T>
-concept QuantitySpec = std::derived_from<T, detail::quantity_spec_interface> && std::is_final_v<T>;
-
-#else
-
-namespace detail {
-
-template<typename Self>
-struct quantity_spec_interface;
-
-}
-
-MP_UNITS_EXPORT template<typename T>
-concept QuantitySpec = is_derived_from_specialization_of<T, detail::quantity_spec_interface> && std::is_final_v<T>;
-
-#endif
+concept QuantitySpec = std::derived_from<T, detail::quantity_spec_interface_base> && std::is_final_v<T>;
 
 MP_UNITS_EXPORT
 #if MP_UNITS_API_NO_CRTP