fix: common_unit handling fixed for some corner cases

src/core/include/mp-units/bits/get_associated_quantity.h

@@ -26,7 +26,12 @@
 #include <mp-units/framework/quantity_spec.h>
 #include <mp-units/framework/unit_concepts.h>
 
-namespace mp_units::detail {
+namespace mp_units {
+
+template<Unit U1, Unit U2, Unit... Rest>
+struct common_unit;
+
+namespace detail {
 
 template<AssociatedUnit U>
 [[nodiscard]] consteval auto all_are_kinds(U);
@@ -61,6 +66,12 @@ template<AssociatedUnit U>
 template<AssociatedUnit U>
 using to_quantity_spec = decltype(get_associated_quantity_impl(U{}));
 
+template<typename... Us>
+[[nodiscard]] consteval auto get_associated_quantity_impl(common_unit<Us...>)
+{
+  return get_common_quantity_spec(get_associated_quantity_impl(Us{})...);
+}
+
 template<AssociatedUnit U>
 [[nodiscard]] consteval auto get_associated_quantity_impl(U u)
 {
@@ -83,4 +94,6 @@ template<AssociatedUnit U>
     return get_associated_quantity_impl(u);
 }
 
-}  // namespace mp_units::detail
+}  // namespace detail
+
+}  // namespace mp_units

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

@@ -93,16 +93,17 @@ concept HaveCommonReferenceImpl = requires { get_common_reference(R1, R2); };
 template<auto R1, auto R2>
 concept HaveCommonReference = HaveCommonReferenceImpl<R1, R2>;
 
+template<typename Func, Quantity Q1, Quantity Q2>
+using common_quantity_for = quantity<get_common_reference(Q1::reference, Q2::reference),
+                                     std::invoke_result_t<Func, typename Q1::rep, typename Q2::rep>>;
+
 template<typename Func, typename Q1, typename Q2>
 concept CommonlyInvocableQuantities =
   Quantity<Q1> && Quantity<Q2> && HaveCommonReference<Q1::reference, Q2::reference> &&
+  std::convertible_to<Q1, common_quantity_for<Func, Q1, Q2>> &&
+  std::convertible_to<Q2, common_quantity_for<Func, Q1, Q2>> &&
   InvocableQuantities<Func, Q1, Q2, get_common_quantity_spec(Q1::quantity_spec, Q2::quantity_spec).character>;
 
-template<typename Func, Quantity Q1, Quantity Q2>
-  requires CommonlyInvocableQuantities<Func, Q1, Q2>
-using common_quantity_for = quantity<get_common_reference(Q1::reference, Q2::reference),
-                                     std::invoke_result_t<Func, typename Q1::rep, typename Q2::rep>>;
-
 template<auto R1, auto R2, typename Rep1, typename Rep2>
 concept SameValueAs = SameReference<R1, R2> && std::same_as<Rep1, Rep2>;
 

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

@@ -60,9 +60,6 @@ import std;
 
 namespace mp_units {
 
-template<Unit U1, Unit U2, Unit... Rest>
-struct common_unit;
-
 namespace detail {
 
 template<Magnitude auto M, Unit U>
@@ -784,9 +781,18 @@ constexpr Out unit_symbol_impl(Out out, const scaled_unit_impl<M, U>& u, const u
 template<typename... Us, Unit U>
 [[nodiscard]] consteval Unit auto get_common_unit_in(common_unit<Us...>, U u)
 {
+  auto get_magnitude = [&]() {
+    if constexpr (requires { common_unit<Us...>::mag; })
+      return common_unit<Us...>::mag;
+    else
+      return mag<1>;
+  };
   constexpr auto canonical_u = get_canonical_unit(u);
-  constexpr Magnitude auto mag = common_unit<Us...>::mag / canonical_u.mag;
-  return scaled_unit<mag, U>{};
+  constexpr Magnitude auto cmag = get_magnitude() / canonical_u.mag;
+  if constexpr (cmag == mag<1>)
+    return u;
+  else
+    return scaled_unit<cmag, U>{};
 }
 
 template<typename CharT, std::output_iterator<CharT> Out, typename U, typename... Rest>