fix: compilation-error fixed

src/core/include/mp_units/unit.h

@@ -598,7 +598,7 @@ inline constexpr struct per_mille : named_unit<basic_symbol_text{"‰", "%o"}, m
 // convertible_to
 [[nodiscard]] consteval bool convertible(Unit auto from, Unit auto to)
 {
-  return detail::have_same_canonical_reference_unit(u1, u2);
+  return detail::have_same_canonical_reference_unit(from, to);
 }
 
 // Common unit