diff --git a/src/core/include/mp_units/unit.h b/src/core/include/mp_units/unit.h
index da4d324a..776191c7 100644
--- a/src/core/include/mp_units/unit.h
+++ b/src/core/include/mp_units/unit.h
@@ -602,10 +602,10 @@ inline constexpr struct per_mille : named_unit<basic_symbol_text{"‰", "%o"}, m
 }
 
 // Common unit
-[[nodiscard]] consteval auto common_unit(Unit auto u) { return u; }
+[[nodiscard]] consteval Unit auto common_unit(Unit auto u) { return u; }
 
 template<Unit U1, Unit U2>
-[[nodiscard]] consteval auto common_unit(U1 u1, U2 u2)
+[[nodiscard]] consteval Unit auto common_unit(U1 u1, U2 u2)
   requires(detail::have_same_canonical_reference_unit(u1, u2))
 {
   if constexpr (U1{} == U2{}) {
@@ -628,7 +628,7 @@ template<Unit U1, Unit U2>
   }
 }
 
-[[nodiscard]] consteval auto common_unit(Unit auto u1, Unit auto u2, Unit auto u3, Unit auto... rest)
+[[nodiscard]] consteval Unit auto common_unit(Unit auto u1, Unit auto u2, Unit auto u3, Unit auto... rest)
   requires requires { common_unit(common_unit(u1, u2), u3, rest...); }
 {
   return common_unit(common_unit(u1, u2), u3, rest...);