From f81e9bdcfe27ff289bb9212082020365889d5194 Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Tue, 20 Feb 2024 19:49:22 +0100
Subject: [PATCH] refactor: `kind_of_` refactored to take a type to improve
 error messages

---
 .../mp-units/bits/quantity_spec_concepts.h    |  4 +--
 src/core/include/mp-units/quantity_spec.h     | 28 +++++++++----------
 test/static/quantity_spec_test.cpp            | 20 ++++++-------
 3 files changed, 25 insertions(+), 27 deletions(-)

diff --git a/src/core/include/mp-units/bits/quantity_spec_concepts.h b/src/core/include/mp-units/bits/quantity_spec_concepts.h
index 9ee2b690..6ddff15c 100644
--- a/src/core/include/mp-units/bits/quantity_spec_concepts.h
+++ b/src/core/include/mp-units/bits/quantity_spec_concepts.h
@@ -34,7 +34,7 @@ template<typename, auto...>
 #endif
 struct quantity_spec;
 
-template<auto Q>
+template<typename Q>
 struct kind_of_;
 
 namespace detail {
@@ -42,7 +42,7 @@ namespace detail {
 template<typename T>
 inline constexpr bool is_specialization_of_kind_of = false;
 
-template<auto Q>
+template<typename Q>
 inline constexpr bool is_specialization_of_kind_of<kind_of_<Q>> = true;
 
 template<typename T>
diff --git a/src/core/include/mp-units/quantity_spec.h b/src/core/include/mp-units/quantity_spec.h
index 2b25b8a7..15e87547 100644
--- a/src/core/include/mp-units/quantity_spec.h
+++ b/src/core/include/mp-units/quantity_spec.h
@@ -450,30 +450,28 @@ template<QuantitySpec Q>
 }  // namespace detail
 
 #ifdef __cpp_explicit_this_parameter
-template<auto Q>
-  requires(detail::QuantitySpecWithNoSpecifiers<std::remove_const_t<decltype(Q)>>) &&
-          (detail::get_kind_tree_root(Q) == Q)
-struct kind_of_<Q> : std::remove_const_t<decltype(Q)> {
-  static constexpr auto _quantity_spec_ = Q;
+template<typename Q>
+  requires detail::QuantitySpecWithNoSpecifiers<Q> && (detail::get_kind_tree_root(Q{}) == Q{})
+struct kind_of_<Q> : Q {
+  static constexpr auto _quantity_spec_ = Q{};
 };
 #else
 
 #if MP_UNITS_COMP_CLANG
-template<auto Q>
-  requires detail::QuantitySpecWithNoSpecifiers<std::remove_cvref_t<decltype(Q)>> &&
-           (detail::get_kind_tree_root(Q) == Q)
+template<typename Q>
+  requires detail::QuantitySpecWithNoSpecifiers<Q> && (detail::get_kind_tree_root(Q{}) == Q{})
 #else
-template<detail::QuantitySpecWithNoSpecifiers auto Q>
-  requires(detail::get_kind_tree_root(Q) == Q)
+template<detail::QuantitySpecWithNoSpecifiers Q>
+  requires(detail::get_kind_tree_root(Q{}) == Q{})
 #endif
-struct kind_of_<Q> : quantity_spec<kind_of_<Q>, Q> {
-  static constexpr auto _quantity_spec_ = Q;
+struct kind_of_<Q> : quantity_spec<kind_of_<Q>, Q{}> {
+  static constexpr auto _quantity_spec_ = Q{};
 };
 #endif
 
 template<detail::QuantitySpecWithNoSpecifiers auto Q>
   requires(detail::get_kind_tree_root(Q) == Q)
-inline constexpr kind_of_<Q> kind_of;
+inline constexpr kind_of_<std::remove_const_t<decltype(Q)>> kind_of;
 
 namespace detail {
 
@@ -601,10 +599,10 @@ template<QuantitySpec Q, int... Ints>
   return get_complexity(Q{});
 }
 
-template<auto Q>
+template<typename Q>
 [[nodiscard]] consteval int get_complexity(kind_of_<Q>)
 {
-  return get_complexity(Q);
+  return get_complexity(Q{});
 }
 
 template<QuantitySpec Q>
diff --git a/test/static/quantity_spec_test.cpp b/test/static/quantity_spec_test.cpp
index b845e4c8..a7e7ee65 100644
--- a/test/static/quantity_spec_test.cpp
+++ b/test/static/quantity_spec_test.cpp
@@ -111,10 +111,10 @@ static_assert(detail::NamedQuantitySpec<dimensionless_>);
 static_assert(!detail::IntermediateDerivedQuantitySpec<dimensionless_>);
 static_assert(!detail::QuantityKindSpec<dimensionless_>);
 
-static_assert(QuantitySpec<kind_of_<length>>);
-static_assert(!detail::NamedQuantitySpec<kind_of_<length>>);
-static_assert(!detail::IntermediateDerivedQuantitySpec<kind_of_<length>>);
-static_assert(detail::QuantityKindSpec<kind_of_<length>>);
+static_assert(QuantitySpec<kind_of_<length_>>);
+static_assert(!detail::NamedQuantitySpec<kind_of_<length_>>);
+static_assert(!detail::IntermediateDerivedQuantitySpec<kind_of_<length_>>);
+static_assert(detail::QuantityKindSpec<kind_of_<length_>>);
 
 static_assert(QuantitySpec<frequency_>);
 static_assert(detail::NamedQuantitySpec<frequency_>);
@@ -126,10 +126,10 @@ static_assert(!detail::NamedQuantitySpec<decltype(inverse(time))>);
 static_assert(detail::IntermediateDerivedQuantitySpec<decltype(inverse(time))>);
 static_assert(!detail::QuantityKindSpec<decltype(inverse(time))>);
 
-static_assert(QuantitySpec<kind_of_<length / time>>);
-static_assert(!detail::NamedQuantitySpec<kind_of_<length / time>>);
-static_assert(detail::IntermediateDerivedQuantitySpec<kind_of_<length / time>>);
-static_assert(detail::QuantityKindSpec<kind_of_<length / time>>);
+static_assert(QuantitySpec<kind_of_<std::remove_const_t<decltype(length / time)>>>);
+static_assert(!detail::NamedQuantitySpec<kind_of_<std::remove_const_t<decltype(length / time)>>>);
+static_assert(detail::IntermediateDerivedQuantitySpec<kind_of_<std::remove_const_t<decltype(length / time)>>>);
+static_assert(detail::QuantityKindSpec<kind_of_<std::remove_const_t<decltype(length / time)>>>);
 
 static_assert(QuantitySpec<decltype(kind_of<length> / kind_of<time>)>);
 static_assert(!detail::NamedQuantitySpec<decltype(kind_of<length> / kind_of<time>)>);
@@ -265,8 +265,8 @@ static_assert(is_of_type<acceleration * mass, derived_quantity_spec<acceleration
 static_assert(is_of_type<(acceleration * mass).dimension,
                          derived_dimension<dim_length_, dim_mass_, per<mp_units::power<dim_time_, 2>>>>);
 
-static_assert(is_of_type<kind_of<length> / kind_of<time>, kind_of_<derived_quantity_spec<length_, per<time_>>{}>>);
-static_assert(is_of_type<kind_of<length / time>, kind_of_<derived_quantity_spec<length_, per<time_>>{}>>);
+static_assert(is_of_type<kind_of<length> / kind_of<time>, kind_of_<derived_quantity_spec<length_, per<time_>>>>);
+static_assert(is_of_type<kind_of<length / time>, kind_of_<derived_quantity_spec<length_, per<time_>>>>);
 
 // power
 static_assert(is_of_type<pow<0>(length), dimensionless_>);