feat: quantities of the same kind can now be added, subtracted, or compared to each other

src/core/include/mp_units/bits/external/type_traits.h

@@ -80,4 +80,47 @@ concept is_derived_from_specialization_of = requires(T* t) { detail::to_base_spe
 template<typename T, typename... Ts>
 concept one_of = (false || ... || std::same_as<T, Ts>);
 
+template<typename T, auto... Vs>
+consteval bool contains()
+{
+  return (false || ... || is_same_v<std::remove_const_t<decltype(Vs)>, T>);
+}
+
+template<template<typename...> typename T, typename... Ts>
+consteval bool contains()
+{
+  return (false || ... || is_specialization_of<Ts, T>);
+}
+
+template<typename T, std::same_as<T> auto V>
+consteval auto get()
+{
+  return V;
+}
+
+template<typename T, auto V1, auto V2, auto... Vs>
+consteval auto get()
+{
+  if constexpr (is_same_v<T, std::remove_const_t<decltype(V1)>>)
+    return V1;
+  else
+    return get<T, V2, Vs...>();
+}
+
+template<template<typename...> typename T, typename T1>
+  requires is_specialization_of<T1, T>
+consteval auto get()
+{
+  return T1{};
+}
+
+template<template<typename...> typename T, typename T1, typename T2, typename... Ts>
+consteval auto get()
+{
+  if constexpr (is_specialization_of<T1, T2, T>)
+    return T1{};
+  else
+    return get<T, T2, Ts...>();
+}
+
 }  // namespace mp_units