feat: kind_of_qs[unit] now returns a bare unit as a reference

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

@@ -104,6 +104,9 @@ struct quantity_spec_interface {
   template<typename Self, UnitOf<Self{}> U>
   [[nodiscard]] consteval Reference auto operator[](this Self self, U u)
   {
+    if constexpr (detail::QuantityKindSpec<Self>)
+      return u;
+    else
       return reference<self, u>{};
   }
 
@@ -118,6 +121,9 @@ struct quantity_spec_interface {
   template<typename Self_ = Self, UnitOf<Self_{}> U>
   [[nodiscard]] consteval Reference auto operator[](U u) const
   {
+    if constexpr (detail::QuantityKindSpec<Self_>)
+      return u;
+    else
       return reference<Self{}, u>{};
   }
 
@@ -294,6 +300,9 @@ struct quantity_spec<Self, QS, Args...> : std::remove_const_t<decltype(QS)> {
   template<typename Self_ = Self, UnitOf<Self_{}> U>
   [[nodiscard]] consteval Reference auto operator[](U u) const
   {
+    if constexpr (detail::QuantityKindSpec<Self>)
+      return u;
+    else
       return reference<Self{}, u>{};
   }
 

test/unit_test/static/reference_test.cpp

@@ -105,6 +105,13 @@ inline constexpr struct kilometre_ : decltype(si::kilo<metre>) {} kilometre;
 inline constexpr struct bit_ : named_unit<"bit", one, kind_of<storage_capacity>> {} bit;
 // clang-format on
 
+
+static_assert(is_of_type<length[metre], reference<length, metre>>);
+static_assert(is_of_type<kind_of<length>[metre], metre_>);
+
+static_assert(is_of_type<(length / time)[metre / second], reference<length / time, metre / second>>);
+static_assert(is_of_type<(kind_of<length> / kind_of<time>)[metre / second], derived_unit<metre_, per<second_>>>);
+
 // Unit as a reference
 static_assert(is_of_type<42 * metre, quantity<metre, int>>);
 static_assert(quantity<metre, int>::quantity_spec == length);