refactor: make_reference added and used in constraints for quantity and quantity_point

`quantity_spec[unit]` syntax will not work for natural units and we want the interface to be widely applicable to all domains.

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

@@ -143,15 +143,15 @@ public:
 
   // conversions
   template<Unit U>
-    requires detail::QuantityConvertibleTo<quantity, quantity<quantity_spec[U{}], Rep>>
+    requires detail::QuantityConvertibleTo<quantity, quantity<detail::make_reference(quantity_spec, U{}), Rep>>
-  [[nodiscard]] constexpr quantity<quantity_spec[U{}], Rep> in(U) const
+  [[nodiscard]] constexpr quantity<detail::make_reference(quantity_spec, U{}), Rep> in(U) const
   {
-    return quantity<quantity_spec[U{}], Rep>{*this};
+    return quantity<detail::make_reference(quantity_spec, U{}), Rep>{*this};
   }
 
   template<Unit U>
     requires requires(quantity q) { value_cast<U{}>(q); }
-  [[nodiscard]] constexpr quantity<quantity_spec[U{}], Rep> force_in(U) const
+  [[nodiscard]] constexpr quantity<detail::make_reference(quantity_spec, U{}), Rep> force_in(U) const
   {
     return value_cast<U{}>(*this);
   }

src/core/include/mp-units/quantity_point.h

@@ -169,15 +169,15 @@ public:
   }
 
   template<Unit U>
-    requires detail::QuantityConvertibleTo<quantity_type, quantity<quantity_spec[U{}], Rep>>
+    requires detail::QuantityConvertibleTo<quantity_type, quantity<detail::make_reference(quantity_spec, U{}), Rep>>
-  [[nodiscard]] constexpr quantity_point<quantity_spec[U{}], PO, Rep> in(U) const
+  [[nodiscard]] constexpr quantity_point<detail::make_reference(quantity_spec, U{}), PO, Rep> in(U) const
   {
     return make_quantity_point<PO>(quantity_ref_from(PO).in(U{}));
   }
 
   template<Unit U>
     requires requires(quantity_type q) { value_cast<U{}>(q); }
-  [[nodiscard]] constexpr quantity_point<quantity_spec[U{}], PO, Rep> force_in(U) const
+  [[nodiscard]] constexpr quantity_point<detail::make_reference(quantity_spec, U{}), PO, Rep> force_in(U) const
   {
     return make_quantity_point<PO>(quantity_ref_from(PO).force_in(U{}));
   }

src/core/include/mp-units/quantity_spec.h

@@ -38,6 +38,16 @@ namespace mp_units {
 
 namespace detail {
 
+
+template<QuantitySpec QS, Unit U>
+[[nodiscard]] consteval Reference auto make_reference(QS qs, U u)
+{
+  if constexpr (detail::QuantityKindSpec<QS>)
+    return u;
+  else
+    return reference<qs, u>{};
+}
+
 // TODO revise the note in the below comment
 /**
  * @brief Returns the most restrictive character from the list
@@ -100,10 +110,7 @@ struct quantity_spec_interface {
   template<typename Self, UnitOf<Self{}> U>
   [[nodiscard]] consteval Reference auto operator[](this Self self, U u)
   {
-    if constexpr (detail::QuantityKindSpec<Self>)
+    return detail::make_reference(self, u);
-      return u;
-    else
-      return reference<self, u>{};
   }
 
   template<typename Self, typename Q>
@@ -117,10 +124,7 @@ struct quantity_spec_interface {
   template<typename Self_ = Self, UnitOf<Self_{}> U>
   [[nodiscard]] MP_UNITS_CONSTEVAL Reference auto operator[](U u) const
   {
-    if constexpr (detail::QuantityKindSpec<Self_>)
+    return detail::make_reference(Self{}, u);
-      return u;
-    else
-      return reference<Self{}, u>{};
   }
 
   template<typename Q, typename Self_ = Self>
@@ -296,10 +300,7 @@ struct quantity_spec<Self, QS, Args...> : std::remove_const_t<decltype(QS)> {
   template<typename Self_ = Self, UnitOf<Self_{}> U>
   [[nodiscard]] MP_UNITS_CONSTEVAL Reference auto operator[](U u) const
   {
-    if constexpr (detail::QuantityKindSpec<Self>)
+    return detail::make_reference(Self{}, u);
-      return u;
-    else
-      return reference<Self{}, u>{};
   }
 
   template<typename Q, typename Self_ = Self>