From 6b2606a8aaae8f2c39c4be6ae5abe13d98c8c7c7 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Mon, 13 Feb 2023 18:33:19 +0100
Subject: [PATCH] feat: explicit `quantity_spec` conversion support added to
 `quantity` constructor

---
 src/core/include/mp_units/quantity.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/core/include/mp_units/quantity.h b/src/core/include/mp_units/quantity.h
index a33ec85c..543a261d 100644
--- a/src/core/include/mp_units/quantity.h
+++ b/src/core/include/mp_units/quantity.h
@@ -61,7 +61,8 @@ concept Harmonic =  // exposition only
 
 template<typename QFrom, typename QTo>
 concept QuantityConvertibleTo =  // exposition only
-  Quantity<QFrom> && Quantity<QTo> && requires(QFrom q) { detail::sudo_cast<QTo>(q); } &&
+  Quantity<QFrom> && Quantity<QTo> && explicitly_convertible_to(QFrom::quantity_spec, QTo::quantity_spec) &&
+  requires(QFrom q) { detail::sudo_cast<QTo>(q); } &&
   (treat_as_floating_point<typename QTo::rep> ||
    (!treat_as_floating_point<typename QFrom::rep> && Harmonic<QFrom, QTo>));
 
@@ -122,7 +123,8 @@ public:
   quantity(quantity&&) = default;
 
   template<detail::QuantityConvertibleTo<quantity> Q>
-  constexpr explicit(false) quantity(const Q& q) : number_(detail::sudo_cast<quantity>(q).number())
+  constexpr explicit(!implicitly_convertible_to(Q::quantity_spec, quantity_spec)) quantity(const Q& q) :
+      number_(detail::sudo_cast<quantity>(q).number())
   {
   }