Revert "refactor: QuantitySpec convertibility concepts replaced wit…

…h direct function calls"

This reverts commit 0948840.

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

@@ -71,7 +71,7 @@ concept ValuePreservingTo = Representation<std::remove_cvref_t<FromRep>> && Repr
 
 template<typename QFrom, typename QTo>
 concept QuantityConvertibleTo =
-  Quantity<QFrom> && Quantity<QTo> && implicitly_convertible(QFrom::quantity_spec, QTo::quantity_spec) &&
+  Quantity<QFrom> && Quantity<QTo> && QuantitySpecConvertibleTo<QFrom::quantity_spec, QTo::quantity_spec> &&
   UnitConvertibleTo<QFrom::unit, QTo::unit> &&
   ValuePreservingTo<typename QFrom::rep, typename QTo::rep, QFrom::unit, QTo::unit> &&
   // TODO consider providing constraints of sudo_cast here rather than testing if it can be called (its return type is

src/core/include/mp-units/framework/quantity_cast.h

@@ -57,7 +57,7 @@ namespace mp_units {
  * @tparam ToQS a quantity specification to use for a target quantity
  */
 template<QuantitySpec auto ToQS, typename FwdQ, Quantity Q = std::remove_cvref_t<FwdQ>>
-  requires(castable(Q::quantity_spec, ToQS))
+  requires detail::QuantitySpecCastableTo<Q::quantity_spec, ToQS>
 [[nodiscard]] constexpr Quantity auto quantity_cast(FwdQ&& q)
 {
   return quantity{std::forward<FwdQ>(q).numerical_value_is_an_implementation_detail_, make_reference(ToQS, Q::unit)};
@@ -81,7 +81,7 @@ template<QuantitySpec auto ToQS, typename FwdQ, Quantity Q = std::remove_cvref_t
  * @tparam ToQS a quantity specification to use for a target quantity point
  */
 template<QuantitySpec auto ToQS, typename FwdQP, QuantityPoint QP = std::remove_cvref_t<FwdQP>>
-  requires(castable(QP::quantity_spec, ToQS))
+  requires detail::QuantitySpecCastableTo<QP::quantity_spec, ToQS>
 [[nodiscard]] constexpr QuantityPoint auto quantity_cast(FwdQP&& qp)
 {
   return QP{quantity_cast<ToQS>(std::forward<FwdQP>(qp).quantity_from_origin_is_an_implementation_detail_),

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

@@ -151,7 +151,7 @@ struct quantity_spec_interface : quantity_spec_interface_base {
   }
 
   template<typename Self, typename FwdQ, Quantity Q = std::remove_cvref_t<FwdQ>>
-    requires(explicitly_convertible(Q::quantity_spec, Self{}))
+    requires QuantitySpecExplicitlyConvertibleTo<Q::quantity_spec, Self{}>
   [[nodiscard]] constexpr Quantity auto operator()(this Self self, FwdQ&& q)
   {
     return quantity{std::forward<FwdQ>(q).numerical_value_is_an_implementation_detail_, make_reference(self, Q::unit)};
@@ -164,7 +164,7 @@ struct quantity_spec_interface : quantity_spec_interface_base {
   }
 
   template<typename FwdQ, Quantity Q = std::remove_cvref_t<FwdQ>, typename Self_ = Self>
-    requires(explicitly_convertible(Q::quantity_spec, Self_{}))
+    requires QuantitySpecExplicitlyConvertibleTo<Q::quantity_spec, Self_{}>
   [[nodiscard]] constexpr Quantity auto operator()(FwdQ&& q) const
   {
     return quantity{std::forward<FwdQ>(q).numerical_value_is_an_implementation_detail_,
@@ -357,7 +357,7 @@ struct quantity_spec<Self, QS, Args...> : detail::propagate_equation<QS>, detail
   }
 
   template<typename FwdQ, Quantity Q = std::remove_cvref_t<FwdQ>, typename Self_ = Self>
-    requires(explicitly_convertible(Q::quantity_spec, Self_{}))
+    requires detail::QuantitySpecExplicitlyConvertibleTo<Q::quantity_spec, Self_{}>
   [[nodiscard]] constexpr Quantity auto operator()(FwdQ&& q) const
   {
     return quantity{std::forward<FwdQ>(q).numerical_value_is_an_implementation_detail_,
@@ -398,12 +398,12 @@ struct quantity_spec<Self, QS, Args...> : detail::propagate_equation<QS>, detail
 // clang-format on
 #if MP_UNITS_API_NO_CRTP
 template<detail::NamedQuantitySpec auto QS, detail::DerivedQuantitySpec auto Eq, detail::QSProperty auto... Args>
-  requires(explicitly_convertible(Eq, QS))
+  requires(detail::QuantitySpecExplicitlyConvertibleTo<Eq, QS>)
 struct quantity_spec<QS, Eq, Args...> : detail::quantity_spec_interface {
 #else
 template<typename Self, detail::NamedQuantitySpec auto QS, detail::DerivedQuantitySpec auto Eq,
          detail::QSProperty auto... Args>
-  requires(explicitly_convertible(Eq, QS))
+  requires(detail::QuantitySpecExplicitlyConvertibleTo<Eq, QS>)
 struct quantity_spec<Self, QS, Eq, Args...> : detail::quantity_spec_interface<Self> {
 #endif
   using _base_type_ = quantity_spec;
@@ -1547,8 +1547,8 @@ template<QuantitySpec Q>
 [[nodiscard]] consteval QuantitySpec auto get_common_quantity_spec(QuantitySpec auto q) { return q; }
 
 template<QuantitySpec Q1, QuantitySpec Q2>
-  requires(implicitly_convertible(detail::get_kind_tree_root(Q1{}), detail::get_kind_tree_root(Q2{}))) ||
-          (implicitly_convertible(detail::get_kind_tree_root(Q2{}), detail::get_kind_tree_root(Q1{})))
+  requires detail::QuantitySpecConvertibleTo<detail::get_kind_tree_root(Q1{}), detail::get_kind_tree_root(Q2{})> ||
+           detail::QuantitySpecConvertibleTo<detail::get_kind_tree_root(Q2{}), detail::get_kind_tree_root(Q1{})>
 [[nodiscard]] consteval QuantitySpec auto get_common_quantity_spec(Q1 q1, Q2 q2)
 {
   using QQ1 = decltype(detail::remove_kind(q1));

src/core/include/mp-units/framework/quantity_spec_concepts.h

@@ -134,7 +134,7 @@ concept QuantitySpecCastableTo = QuantitySpec<MP_UNITS_REMOVE_CONST(decltype(Fro
 
 MP_UNITS_EXPORT template<typename T, auto QS>
 concept QuantitySpecOf =
-  QuantitySpec<T> && QuantitySpec<MP_UNITS_REMOVE_CONST(decltype(QS))> && (implicitly_convertible(T{}, QS)) &&
+  QuantitySpec<T> && QuantitySpec<MP_UNITS_REMOVE_CONST(decltype(QS))> && detail::QuantitySpecConvertibleTo<T{}, QS> &&
   // the below is to make the following work
   // static_assert(ReferenceOf<si::radian, isq::angular_measure>);
   // static_assert(!ReferenceOf<si::radian, dimensionless>);

src/core/include/mp-units/framework/unit_concepts.h

@@ -106,7 +106,7 @@ concept AssociatedUnit = Unit<U> && detail::has_associated_quantity(U{});
 MP_UNITS_EXPORT template<typename U, auto QS>
 concept UnitOf =
   AssociatedUnit<U> && QuantitySpec<MP_UNITS_REMOVE_CONST(decltype(QS))> &&
-  (implicitly_convertible(get_quantity_spec(U{}), QS)) &&
+  detail::QuantitySpecConvertibleTo<get_quantity_spec(U{}), QS> &&
   // the below is to make `dimensionless[radian]` invalid
   (get_kind(QS) == get_kind(get_quantity_spec(U{})) || !detail::NestedQuantityKindSpecOf<get_quantity_spec(U{}), QS>);