feat: = delete should have a reason

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

@@ -24,6 +24,7 @@
 #pragma once
 
 // IWYU pragma: private, include <mp-units/framework.h>
+#include <mp-units/bits/hacks.h>
 #include <mp-units/bits/module_macros.h>
 #include <mp-units/bits/sudo_cast.h>
 #include <mp-units/compat_macros.h>
@@ -229,7 +230,12 @@ public:
 
   template<Unit U>
     requires(U{} == unit)
-  constexpr const rep&& numerical_value_ref_in(U) const&& noexcept = delete;
+  constexpr const rep&& numerical_value_ref_in(U) const&& noexcept
+#if __cpp_deleted_function
+    = delete("Can't form a reference to a temporary");
+#else
+    = delete;
+#endif
 
   template<UnitCompatibleWith<unit, quantity_spec> U>
     requires requires(quantity q) { q.in(U{}); }

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

@@ -23,6 +23,7 @@
 #pragma once
 
 // IWYU pragma: private, include <mp-units/framework.h>
+#include <mp-units/bits/hacks.h>
 #include <mp-units/bits/module_macros.h>
 #include <mp-units/framework/compare.h>
 #include <mp-units/framework/customization_points.h>
@@ -241,7 +242,12 @@ public:
 
   template<PointOrigin PO2>
     requires(PO2{} == point_origin)
-  constexpr const quantity_type&& quantity_ref_from(PO2) const&& noexcept = delete;
+  constexpr const quantity_type&& quantity_ref_from(PO2) const&& noexcept
+#if __cpp_deleted_function
+    = delete("Can't form a reference to a temporary");
+#else
+    = delete;
+#endif
 
   template<PointOrigin PO2>
     requires requires { quantity_point{} - PO2{}; }

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

@@ -240,12 +240,22 @@ template<typename Rep, Reference R>
 template<Reference R, typename Rep>
   requires RepresentationOf<std::remove_cvref_t<Rep>, get_quantity_spec(R{}).character>
 // NOLINTNEXTLINE(cppcoreguidelines-missing-std-forward)
-constexpr auto operator*(R, Rep&&) = delete;
+constexpr auto operator*(R, Rep&&)
+#if __cpp_deleted_function
+  = delete("To create a `quantity` or `quantity_point` use `Rep * R`");
+#else
+  = delete;
+#endif
 
 template<Reference R, typename Rep>
   requires RepresentationOf<std::remove_cvref_t<Rep>, get_quantity_spec(R{}).character>
 // NOLINTNEXTLINE(cppcoreguidelines-missing-std-forward)
-constexpr auto operator/(R, Rep&&) = delete;
+constexpr auto operator/(R, Rep&&)
+#if __cpp_deleted_function
+  = delete("To create a `quantity` or `quantity_point` use `Rep / R`");
+#else
+  = delete;
+#endif
 
 template<typename Q, Reference R>
   requires Quantity<std::remove_cvref_t<Q>>

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

@@ -449,7 +449,12 @@ template<Magnitude M, Unit U>
     return scaled_unit<M{}, U>{};
 }
 
-[[nodiscard]] consteval Unit auto operator*(Unit auto, Magnitude auto) = delete;
+[[nodiscard]] consteval Unit auto operator*(Unit auto, Magnitude auto)
+#if __cpp_deleted_function
+  = delete("To scale a unit use `mag * unit` syntax");
+#else
+  = delete;
+#endif
 
 /**
  * Returns the result of multiplication with an inverse unit.