From e9c5f7236ebfe1b287a5a4a49aaf57fbefa87233 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Tue, 11 Feb 2025 21:28:15 +0100
Subject: [PATCH] feat: `ConvertibleWithNumber` introduced to improve
 convertibility of unit `one` with raw numbers

Resolves #675
---
 .../dimensionless_quantities.md               |  7 +--
 .../include/mp-units/framework/quantity.h     | 51 +++++++++++--------
 test/static/quantity_test.cpp                 | 24 ++++++---
 3 files changed, 53 insertions(+), 29 deletions(-)

diff --git a/docs/users_guide/framework_basics/dimensionless_quantities.md b/docs/users_guide/framework_basics/dimensionless_quantities.md
index 6d7b8117..b5c3fca2 100644
--- a/docs/users_guide/framework_basics/dimensionless_quantities.md
+++ b/docs/users_guide/framework_basics/dimensionless_quantities.md
@@ -166,7 +166,8 @@ inline constexpr auto ppm = parts_per_million;
 
 ### Superpowers of the unit `one`
 
-Quantities of the unit `one` are the only ones that are:
+Quantities implicitly convertible to `dimensionless` with the unit equivalent to `one` are the only
+ones that are:
 
 - implicitly constructible from the raw value,
 - explicitly convertible to a raw value,
@@ -185,8 +186,8 @@ This property also expands to usual arithmetic operators.
 !!! note
 
     Those rules do not apply to all the dimensionless quantities. It would be unsafe and misleading
-    to allow such operations on units with a magnitude different than `1` (e.g., `percent` or
-    `radian`).
+    to allow such operations on units with a magnitude different than `1` (e.g., `percent`) or
+    for quantities that are not implicitly convertible to `dimensionless` (e.g., `angular_measure`).
 
 
 ## Angular quantities
diff --git a/src/core/include/mp-units/framework/quantity.h b/src/core/include/mp-units/framework/quantity.h
index c5bc36ca..914e72f4 100644
--- a/src/core/include/mp-units/framework/quantity.h
+++ b/src/core/include/mp-units/framework/quantity.h
@@ -128,6 +128,11 @@ using quantity_like_type = quantity<quantity_like_traits<T>::reference, typename
 template<typename T, typename U, typename TT = std::remove_reference_t<T>>
 concept Mutable = (!std::is_const_v<TT>) && std::derived_from<TT, U>;
 
+template<auto R>
+concept ConvertibleWithNumber =
+  Reference<MP_UNITS_REMOVE_CONST(decltype(R))> && (implicitly_convertible(get_quantity_spec(R), dimensionless)) &&
+  (equivalent(get_unit(R), one));
+
 }  // namespace detail
 
 MP_UNITS_EXPORT_BEGIN
@@ -192,7 +197,7 @@ public:
   }
 
   template<detail::ValuePreservingTo<Rep> FwdValue>
-    requires(unit == ::mp_units::one)
+    requires detail::ConvertibleWithNumber<reference>
   constexpr explicit(false) quantity(FwdValue&& val) :
       numerical_value_is_an_implementation_detail_(std::forward<FwdValue>(val))
   {
@@ -218,7 +223,7 @@ public:
   quantity& operator=(quantity&&) = default;
 
   template<detail::ValuePreservingTo<Rep> FwdValue>
-    requires(unit == ::mp_units::one)
+    requires detail::ConvertibleWithNumber<reference>
   constexpr quantity& operator=(FwdValue&& val)
   {
     numerical_value_is_an_implementation_detail_ = std::forward<FwdValue>(val);
@@ -308,7 +313,7 @@ public:
 
   // conversion operators
   template<typename V_, std::constructible_from<Rep> Value = std::remove_cvref_t<V_>>
-    requires(unit == ::mp_units::one)
+    requires detail::ConvertibleWithNumber<reference>
   [[nodiscard]] explicit operator V_() const& noexcept
   {
     return numerical_value_is_an_implementation_detail_;
@@ -436,10 +441,10 @@ public:
   }
 
   template<detail::Mutable<quantity> Q1, QuantityOf<dimensionless> Q2>
-    requires(Q2::unit == ::mp_units::one) && detail::ValuePreservingTo<typename Q2::rep, Rep> &&
-            requires(rep& a, const Q2::rep b) {
-              { a *= b } -> std::same_as<rep&>;
-            }
+    requires detail::ConvertibleWithNumber<Q2::reference> && detail::ValuePreservingTo<typename Q2::rep, Rep> &&
+             requires(rep& a, const Q2::rep b) {
+               { a *= b } -> std::same_as<rep&>;
+             }
   friend constexpr decltype(auto) operator*=(Q1&& lhs, const Q2& rhs)
   {
     return std::forward<Q1>(lhs) *= rhs.numerical_value_is_an_implementation_detail_;
@@ -459,10 +464,10 @@ public:
   }
 
   template<detail::Mutable<quantity> Q1, QuantityOf<dimensionless> Q2>
-    requires(Q2::unit == ::mp_units::one) && detail::ValuePreservingTo<typename Q2::rep, Rep> &&
-            requires(rep& a, const Q2::rep b) {
-              { a /= b } -> std::same_as<rep&>;
-            }
+    requires detail::ConvertibleWithNumber<Q2::reference> && detail::ValuePreservingTo<typename Q2::rep, Rep> &&
+             requires(rep& a, const Q2::rep b) {
+               { a /= b } -> std::same_as<rep&>;
+             }
   friend constexpr decltype(auto) operator/=(Q1&& lhs, const Q2& rhs)
   {
     return std::forward<Q1>(lhs) /= rhs.numerical_value_is_an_implementation_detail_;
@@ -481,14 +486,16 @@ public:
   }
 
   template<std::derived_from<quantity> Q, Representation Value>
-    requires(Q::unit == ::mp_units::one) && detail::InvokeResultOf<quantity_spec, std::plus<>, Rep, const Value&>
+    requires detail::ConvertibleWithNumber<Q::reference> &&
+             detail::InvokeResultOf<quantity_spec, std::plus<>, Rep, const Value&>
   [[nodiscard]] friend constexpr Quantity auto operator+(const Q& lhs, const Value& rhs)
   {
     return lhs + ::mp_units::quantity{rhs};
   }
 
   template<std::derived_from<quantity> Q, Representation Value>
-    requires(Q::unit == ::mp_units::one) && detail::InvokeResultOf<quantity_spec, std::plus<>, Rep, const Value&>
+    requires detail::ConvertibleWithNumber<Q::reference> &&
+             detail::InvokeResultOf<quantity_spec, std::plus<>, Rep, const Value&>
   [[nodiscard]] friend constexpr Quantity auto operator+(const Value& lhs, const Q& rhs)
   {
     return ::mp_units::quantity{lhs} + rhs;
@@ -506,14 +513,16 @@ public:
   }
 
   template<std::derived_from<quantity> Q, Representation Value>
-    requires(Q::unit == ::mp_units::one) && detail::InvokeResultOf<quantity_spec, std::minus<>, Rep, const Value&>
+    requires detail::ConvertibleWithNumber<Q::reference> &&
+             detail::InvokeResultOf<quantity_spec, std::minus<>, Rep, const Value&>
   [[nodiscard]] friend constexpr Quantity auto operator-(const Q& lhs, const Value& rhs)
   {
     return lhs - ::mp_units::quantity{rhs};
   }
 
   template<std::derived_from<quantity> Q, Representation Value>
-    requires(Q::unit == ::mp_units::one) && detail::InvokeResultOf<quantity_spec, std::minus<>, Rep, const Value&>
+    requires detail::ConvertibleWithNumber<Q::reference> &&
+             detail::InvokeResultOf<quantity_spec, std::minus<>, Rep, const Value&>
   [[nodiscard]] friend constexpr Quantity auto operator-(const Value& lhs, const Q& rhs)
   {
     return ::mp_units::quantity{lhs} - rhs;
@@ -533,14 +542,16 @@ public:
   }
 
   template<std::derived_from<quantity> Q, Representation Value>
-    requires(Q::unit == ::mp_units::one) && detail::InvokeResultOf<quantity_spec, std::modulus<>, Rep, const Value&>
+    requires detail::ConvertibleWithNumber<Q::reference> &&
+             detail::InvokeResultOf<quantity_spec, std::modulus<>, Rep, const Value&>
   [[nodiscard]] friend constexpr Quantity auto operator%(const Q& lhs, const Value& rhs)
   {
     return lhs % ::mp_units::quantity{rhs};
   }
 
   template<std::derived_from<quantity> Q, Representation Value>
-    requires(Q::unit == ::mp_units::one) && detail::InvokeResultOf<quantity_spec, std::modulus<>, Rep, const Value&>
+    requires detail::ConvertibleWithNumber<Q::reference> &&
+             detail::InvokeResultOf<quantity_spec, std::modulus<>, Rep, const Value&>
   [[nodiscard]] friend constexpr Quantity auto operator%(const Value& lhs, const Q& rhs)
   {
     return ::mp_units::quantity{lhs} % rhs;
@@ -592,7 +603,7 @@ public:
   [[nodiscard]] friend constexpr Quantity auto operator/(const Value& val, const Q& q)
   {
     MP_UNITS_EXPECTS_DEBUG(is_neq_zero(q));
-    return ::mp_units::quantity{val / q.numerical_value_ref_in(unit), ::mp_units::one / R};
+    return ::mp_units::quantity{val / q.numerical_value_ref_in(unit), one / R};
   }
 
   template<std::derived_from<quantity> Q, auto R2, typename Rep2>
@@ -607,7 +618,7 @@ public:
   }
 
   template<std::derived_from<quantity> Q, Representation Value>
-    requires(Q::unit == ::mp_units::one) && std::equality_comparable_with<Rep, Value>
+    requires detail::ConvertibleWithNumber<Q::reference> && std::equality_comparable_with<Rep, Value>
   [[nodiscard]] friend constexpr bool operator==(const Q& lhs, const Value& rhs)
   {
     return lhs.numerical_value_ref_in(unit) == rhs;
@@ -625,7 +636,7 @@ public:
   }
 
   template<std::derived_from<quantity> Q, Representation Value>
-    requires(Q::unit == ::mp_units::one) && std::three_way_comparable_with<Rep, Value>
+    requires detail::ConvertibleWithNumber<Q::reference> && std::three_way_comparable_with<Rep, Value>
   [[nodiscard]] friend constexpr auto operator<=>(const Q& lhs, const Value& rhs)
   {
     return lhs.numerical_value_ref_in(unit) <=> rhs;
diff --git a/test/static/quantity_test.cpp b/test/static/quantity_test.cpp
index 8d9a1e0c..5a41d0dc 100644
--- a/test/static/quantity_test.cpp
+++ b/test/static/quantity_test.cpp
@@ -279,16 +279,16 @@ static_assert(quantity<isq::length[km]>(1500 * m).numerical_value_in(km) == 1.5)
 
 static_assert(!std::convertible_to<quantity<one>, double>);
 static_assert(std::constructible_from<double, quantity<one>>);
-static_assert(!std::convertible_to<quantity<isq::angular_measure[one]>, double>);
-static_assert(std::constructible_from<double, quantity<isq::angular_measure[one]>>);
 static_assert(!std::convertible_to<quantity<one>, int>);
 static_assert(std::constructible_from<int, quantity<one>>);
-static_assert(!std::convertible_to<quantity<isq::angular_measure[one]>, int>);
-static_assert(std::constructible_from<int, quantity<isq::angular_measure[one]>>);
 static_assert(!std::convertible_to<quantity<one, int>, double>);
 static_assert(std::constructible_from<double, quantity<one, int>>);
-static_assert(!std::convertible_to<quantity<isq::angular_measure[one], int>, double>);
-static_assert(std::constructible_from<double, quantity<isq::angular_measure[one], int>>);
+static_assert(!std::convertible_to<quantity<isq::rotation[one]>, double>);
+static_assert(std::constructible_from<double, quantity<isq::rotation[one]>>);
+static_assert(!std::convertible_to<quantity<isq::rotation[one]>, int>);
+static_assert(std::constructible_from<int, quantity<isq::rotation[one]>>);
+static_assert(!std::convertible_to<quantity<isq::rotation[one], int>, double>);
+static_assert(std::constructible_from<double, quantity<isq::rotation[one], int>>);
 #if MP_UNITS_HOSTED
 static_assert(!std::convertible_to<quantity<one, std::complex<double>>, std::complex<double>>);
 static_assert(std::constructible_from<std::complex<double>, quantity<one, std::complex<double>>>);
@@ -300,6 +300,18 @@ static_assert(!std::convertible_to<quantity<one, double>, cartesian_vector<doubl
 static_assert(std::constructible_from<cartesian_vector<double>, quantity<one, double>>);
 #endif
 
+static_assert(!std::convertible_to<quantity<rad>, double>);
+static_assert(!std::constructible_from<double, quantity<rad>>);
+static_assert(!std::convertible_to<quantity<rad>, int>);
+static_assert(!std::constructible_from<int, quantity<rad>>);
+static_assert(!std::convertible_to<quantity<rad, int>, double>);
+static_assert(!std::constructible_from<double, quantity<rad, int>>);
+static_assert(!std::convertible_to<quantity<isq::angular_measure[one]>, double>);
+static_assert(!std::constructible_from<double, quantity<isq::angular_measure[one]>>);
+static_assert(!std::convertible_to<quantity<isq::angular_measure[one]>, int>);
+static_assert(!std::constructible_from<int, quantity<isq::angular_measure[one]>>);
+static_assert(!std::convertible_to<quantity<isq::angular_measure[one], int>, double>);
+static_assert(!std::constructible_from<double, quantity<isq::angular_measure[one], int>>);
 
 ///////////////////////////////////
 // converting to a different unit