From 49da1ced3a2144c7850442ae28974b60c943b124 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Wed, 20 Sep 2023 20:31:49 +0200
Subject: [PATCH] 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       |  8 +++----
 src/core/include/mp-units/quantity_point.h |  6 +++---
 src/core/include/mp-units/quantity_spec.h  | 25 +++++++++++-----------
 3 files changed, 20 insertions(+), 19 deletions(-)

diff --git a/src/core/include/mp-units/quantity.h b/src/core/include/mp-units/quantity.h
index c8d1d3196..e1e989cd5 100644
--- a/src/core/include/mp-units/quantity.h
+++ b/src/core/include/mp-units/quantity.h
@@ -143,15 +143,15 @@ class quantity {
 
   // conversions
   template<Unit U>
-    requires detail::QuantityConvertibleTo<quantity, quantity<quantity_spec[U{}], Rep>>
-  [[nodiscard]] constexpr quantity<quantity_spec[U{}], Rep> in(U) const
+    requires detail::QuantityConvertibleTo<quantity, quantity<detail::make_reference(quantity_spec, U{}), Rep>>
+  [[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);
   }
diff --git a/src/core/include/mp-units/quantity_point.h b/src/core/include/mp-units/quantity_point.h
index 910620233..3a8dd8bd1 100644
--- a/src/core/include/mp-units/quantity_point.h
+++ b/src/core/include/mp-units/quantity_point.h
@@ -169,15 +169,15 @@ class quantity_point {
   }
 
   template<Unit U>
-    requires detail::QuantityConvertibleTo<quantity_type, quantity<quantity_spec[U{}], Rep>>
-  [[nodiscard]] constexpr quantity_point<quantity_spec[U{}], PO, Rep> in(U) const
+    requires detail::QuantityConvertibleTo<quantity_type, quantity<detail::make_reference(quantity_spec, U{}), Rep>>
+  [[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{}));
   }
diff --git a/src/core/include/mp-units/quantity_spec.h b/src/core/include/mp-units/quantity_spec.h
index b6e2e2a1a..1a55f50cc 100644
--- a/src/core/include/mp-units/quantity_spec.h
+++ b/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 u;
-    else
-      return reference<self, u>{};
+    return detail::make_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 u;
-    else
-      return reference<Self{}, u>{};
+    return detail::make_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 u;
-    else
-      return reference<Self{}, u>{};
+    return detail::make_reference(Self{}, u);
   }
 
   template<typename Q, typename Self_ = Self>