refactor: Quantity no longer can be constructed with a raw value

Resolves #434

src/core/include/mp_units/bits/sudo_cast.h

@@ -51,10 +51,10 @@ template<Quantity To, auto R, typename Rep>
 {
   if constexpr (get_unit(R) == To::unit) {
     // no scaling of the number needed
-    return To(static_cast<TYPENAME To::rep>(q.number()));  // this is the only (and recommended) way to do
-                                                           // a truncating conversion on a number, so we are
-                                                           // using static_cast to suppress all the compiler
-                                                           // warnings on conversions
+    return static_cast<TYPENAME To::rep>(q.number()) * To::reference;  // this is the only (and recommended) way to do
+                                                                       // a truncating conversion on a number, so we are
+                                                                       // using static_cast to suppress all the compiler
+                                                                       // warnings on conversions
   } else {
     // scale the number
     using rep_type = decltype([] {
@@ -85,7 +85,8 @@ template<Quantity To, auto R, typename Rep>
     constexpr Magnitude auto irr = c_mag * (den / num);
 
     constexpr auto val = [](Magnitude auto m) { return get_value<multiplier_type>(m); };
-    return To(static_cast<TYPENAME To::rep>(static_cast<rep_type>(q.number()) * val(num) / val(den) * val(irr)));
+    return static_cast<TYPENAME To::rep>(static_cast<rep_type>(q.number()) * val(num) / val(den) * val(irr)) *
+           To::reference;
   }
 }
 

src/core/include/mp_units/quantity.h

@@ -121,12 +121,6 @@ public:
   quantity(const quantity&) = default;
   quantity(quantity&&) = default;
 
-  template<typename Value>
-    requires detail::RepSafeConstructibleFrom<rep, Value>
-  constexpr explicit(!detail::QuantityOne<quantity>) quantity(Value&& v) : number_(std::forward<Value>(v))
-  {
-  }
-
   template<detail::QuantityConvertibleTo<quantity> Q>
   constexpr explicit(false) quantity(const Q& q) : number_(detail::sudo_cast<quantity>(q).number())
   {
@@ -439,6 +433,17 @@ public:
 #endif
 
   [[nodiscard]] friend constexpr bool operator==(const quantity& lhs, const quantity& rhs) = default;
+
+private:
+  template<Reference R2, typename Rep2>
+    requires RepresentationOf<std::remove_cvref_t<Rep2>, get_quantity_spec(R2{}).character>
+  friend constexpr quantity<R2{}, std::remove_cvref_t<Rep2>> operator*(Rep2&& lhs, R2);
+
+  template<typename Value>
+    requires detail::RepSafeConstructibleFrom<rep, Value>
+  constexpr explicit quantity(Value&& v) : number_(std::forward<Value>(v))
+  {
+  }
 };
 
 // CTAD
@@ -465,7 +470,7 @@ template<Quantity Q1, Quantity Q2>
 {
   constexpr auto ref = common_reference(Q1::reference, Q2::reference);
   using ret = quantity<ref, decltype(lhs.number() + rhs.number())>;
-  return ret(ret(lhs).number() + ret(rhs).number());
+  return (ret(lhs).number() + ret(rhs).number()) * ref;
 }
 
 template<Quantity Q1, Quantity Q2>
@@ -476,7 +481,7 @@ template<Quantity Q1, Quantity Q2>
 {
   constexpr auto ref = common_reference(Q1::reference, Q2::reference);
   using ret = quantity<ref, decltype(lhs.number() - rhs.number())>;
-  return ret(ret(lhs).number() - ret(rhs).number());
+  return (ret(lhs).number() - ret(rhs).number()) * ref;
 }
 
 template<Quantity Q1, Quantity Q2>