feat: explicit constructor now can perform an explicit quantity_spec conversion

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>>);
 

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
 ///////////////////////