diff --git a/src/core/include/mp_units/quantity.h b/src/core/include/mp_units/quantity.h
index 11700992..07267b79 100644
--- a/src/core/include/mp_units/quantity.h
+++ b/src/core/include/mp_units/quantity.h
@@ -139,18 +139,16 @@ public:
   quantity& operator=(quantity&&) = default;
 
   // data access
-#ifdef __cpp_explicit_this_parameter
-  template<typename Self>
-  [[nodiscard]] constexpr auto&& number(this Self&& self) noexcept
-  {
-    return std::forward<Self>(self).number_;
-  }
-#else
-  [[nodiscard]] constexpr rep& number() & noexcept { return number_; }
   [[nodiscard]] constexpr const rep& number() const& noexcept { return number_; }
   [[nodiscard]] constexpr rep&& number() && noexcept { return std::move(number_); }
   [[nodiscard]] constexpr const rep&& number() const&& noexcept { return std::move(number_); }
-#endif
+
+  template<Unit U>
+    requires requires(quantity q) { q[U{}]; }
+  [[nodiscard]] constexpr rep number_in(U) const noexcept
+  {
+    return (*this)[U{}].number();
+  }
 
   template<Unit U>
     requires quantity_convertible_to_<quantity, quantity<::mp_units::reference<quantity_spec, U{}>{}, Rep>>