From 9c140f4b3ad7e35f8486cf6f7bcf822e33ab7274 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Fri, 20 Jun 2025 17:10:34 +0200
Subject: [PATCH] feat: explicit constructor now can perform an explicit
 `quantity_spec` conversion

---
 src/core/include/mp-units/framework/quantity.h       | 9 +++++----
 src/core/include/mp-units/framework/quantity_point.h | 8 +++-----
 test/static/quantity_point_test.cpp                  | 8 ++++----
 test/static/quantity_test.cpp                        | 6 ++++++
 4 files changed, 18 insertions(+), 13 deletions(-)

diff --git a/src/core/include/mp-units/framework/quantity.h b/src/core/include/mp-units/framework/quantity.h
index c44be919..e9bc2813 100644
--- a/src/core/include/mp-units/framework/quantity.h
+++ b/src/core/include/mp-units/framework/quantity.h
@@ -95,7 +95,7 @@ concept ValuePreservingScaling2Reps =
 
 template<typename QTo, typename QFrom>
 concept QuantityConstructibleFrom =
-  Quantity<QTo> && Quantity<QFrom> && implicitly_convertible(QFrom::quantity_spec, QTo::quantity_spec) &&
+  Quantity<QTo> && Quantity<QFrom> && explicitly_convertible(QFrom::quantity_spec, QTo::quantity_spec) &&
   ValuePreservingConstruction<typename QTo::rep, typename QFrom::rep> &&
   ValuePreservingScaling2Reps<QFrom::unit, typename QFrom::rep, QTo::unit, typename QTo::rep>;
 
@@ -218,7 +218,8 @@ public:
   template<auto R2, typename Rep2>
     requires detail::QuantityConstructibleFrom<quantity, quantity<R2, Rep2>>
   // NOLINTNEXTLINE(google-explicit-constructor, hicpp-explicit-conversions)
-  constexpr explicit(!std::convertible_to<Rep2, rep>) quantity(const quantity<R2, Rep2>& q) :
+  constexpr explicit(!implicitly_convertible(get_quantity_spec(R2), quantity_spec) || !std::convertible_to<Rep2, rep>)
+    quantity(const quantity<R2, Rep2>& q) :
       quantity(detail::sudo_cast<quantity>(q))
   {
   }
@@ -227,7 +228,7 @@ public:
     requires detail::QuantityConstructibleFrom<quantity, detail::quantity_like_type<Q>>
   constexpr explicit(quantity_like_traits<Q>::explicit_import ||
                      // NOLINTNEXTLINE(google-explicit-constructor, hicpp-explicit-conversions)
-                     !std::convertible_to<typename quantity_like_traits<Q>::rep, Rep>) quantity(const Q& q) :
+                     !std::convertible_to<detail::quantity_like_type<Q>, quantity>) quantity(const Q& q) :
       quantity(::mp_units::quantity{quantity_like_traits<Q>::to_numerical_value(q), quantity_like_traits<Q>::reference})
   {
   }
@@ -335,7 +336,7 @@ public:
   template<typename Q_, QuantityLike Q = std::remove_cvref_t<Q_>>
     requires detail::QuantityConstructibleFrom<detail::quantity_like_type<Q>, quantity>
   [[nodiscard]] explicit(quantity_like_traits<Q>::explicit_export ||
-                         !std::convertible_to<rep, typename quantity_like_traits<Q>::rep>) constexpr
+                         !std::convertible_to<quantity, detail::quantity_like_type<Q>>) constexpr
   // NOLINTNEXTLINE(google-explicit-constructor, hicpp-explicit-conversions)
   operator Q_() const
     noexcept(noexcept(quantity_like_traits<Q>::from_numerical_value(numerical_value_is_an_implementation_detail_)) &&
diff --git a/src/core/include/mp-units/framework/quantity_point.h b/src/core/include/mp-units/framework/quantity_point.h
index dd535a7b..a67c57e4 100644
--- a/src/core/include/mp-units/framework/quantity_point.h
+++ b/src/core/include/mp-units/framework/quantity_point.h
@@ -215,8 +215,7 @@ public:
   ~quantity_point() = default;
 
   template<typename FwdQ, QuantityOf<quantity_spec> Q = std::remove_cvref_t<FwdQ>>
-    requires std::constructible_from<quantity_type, FwdQ> && (point_origin == default_point_origin(R)) &&
-             (implicitly_convertible(Q::quantity_spec, quantity_spec))
+    requires std::constructible_from<quantity_type, FwdQ> && (point_origin == default_point_origin(R))
   constexpr explicit quantity_point(FwdQ&& q) : quantity_from_origin_is_an_implementation_detail_(std::forward<FwdQ>(q))
   {
   }
@@ -252,9 +251,8 @@ public:
 
   template<QuantityPointLike QP>
     requires(quantity_point_like_traits<QP>::point_origin == point_origin) &&
-            std::convertible_to<
-              quantity<quantity_point_like_traits<QP>::reference, typename quantity_point_like_traits<QP>::rep>,
-              quantity_type>
+            std::constructible_from<quantity_type, quantity<quantity_point_like_traits<QP>::reference,
+                                                            typename quantity_point_like_traits<QP>::rep>>
   constexpr explicit(
     quantity_point_like_traits<QP>::explicit_import ||
     !std::convertible_to<
diff --git a/test/static/quantity_point_test.cpp b/test/static/quantity_point_test.cpp
index 67fc8e80..866ce99c 100644
--- a/test/static/quantity_point_test.cpp
+++ b/test/static/quantity_point_test.cpp
@@ -705,8 +705,8 @@ static_assert(
   !std::convertible_to<quantity_point<si::kelvin, si::ice_point>, quantity_point<isq::height[m], mean_sea_level>>);
 
 // non-convertible quantity_specs
-static_assert(!std::constructible_from<quantity_point<special_height[m], mean_sea_level>,
-                                       quantity_point<isq::height[m], mean_sea_level>>);
+static_assert(std::constructible_from<quantity_point<special_height[m], mean_sea_level>,
+                                      quantity_point<isq::height[m], mean_sea_level>>);
 static_assert(!std::convertible_to<quantity_point<isq::height[m], mean_sea_level>,
                                    quantity_point<special_height[m], mean_sea_level>>);
 
@@ -766,8 +766,8 @@ static_assert(std::constructible_from<quantity_point<isq::height[m], mean_sea_le
 static_assert(std::convertible_to<quantity_point<special_height[m], mean_sea_level>,
                                   quantity_point<isq::height[m], mean_sea_level>>);
 
-static_assert(!std::constructible_from<quantity_point<special_height[m], mean_sea_level>,
-                                       quantity_point<isq::height[m], mean_sea_level>>);
+static_assert(std::constructible_from<quantity_point<special_height[m], mean_sea_level>,
+                                      quantity_point<isq::height[m], mean_sea_level>>);
 static_assert(!std::convertible_to<quantity_point<isq::height[m], mean_sea_level>,
                                    quantity_point<special_height[m], mean_sea_level>>);
 
diff --git a/test/static/quantity_test.cpp b/test/static/quantity_test.cpp
index 8b31a0c4..56485bf3 100644
--- a/test/static/quantity_test.cpp
+++ b/test/static/quantity_test.cpp
@@ -224,6 +224,7 @@ static_assert(std::convertible_to<quantity<si::metre, cartesian_vector<double>>,
                                   quantity<isq::position_vector[m], cartesian_vector<double>>>);
 #endif
 
+
 // conversion between different quantities not allowed
 static_assert(!std::constructible_from<quantity<isq::length[m]>, quantity<isq::time[s]>>);
 static_assert(!std::convertible_to<quantity<isq::time[s]>, quantity<isq::length[m]>>);
@@ -263,6 +264,11 @@ static_assert(!std::convertible_to<quantity<rad>, quantity<one>>);
 static_assert(!std::convertible_to<quantity<isq::angular_measure[one]>, quantity<dimensionless[one]>>);
 static_assert(!std::convertible_to<quantity<isq::angular_measure[one]>, quantity<one>>);
 
+// explicit-construction only
+static_assert(std::constructible_from<quantity<isq::distance[m]>, quantity<isq::length[m]>>);
+static_assert(!std::convertible_to<quantity<isq::length[m]>, quantity<isq::distance[m]>>);
+
+
 ///////////////////////
 // obtaining a number
 ///////////////////////