diff --git a/au/quantity.hh b/au/quantity.hh
index 4ca35b7d..5e216378 100644
--- a/au/quantity.hh
+++ b/au/quantity.hh
@@ -307,10 +307,12 @@ class Quantity {
     friend constexpr bool operator>=(Quantity a, Quantity b) { return a.value_ >= b.value_; }
 
     // Addition and subtraction for like quantities.
-    friend constexpr auto operator+(Quantity a, Quantity b) {
+    friend constexpr Quantity<UnitT, decltype(std::declval<RepT>() + std::declval<RepT>())>
+    operator+(Quantity a, Quantity b) {
         return make_quantity<UnitT>(a.value_ + b.value_);
     }
-    friend constexpr auto operator-(Quantity a, Quantity b) {
+    friend constexpr Quantity<UnitT, decltype(std::declval<RepT>() - std::declval<RepT>())>
+    operator-(Quantity a, Quantity b) {
         return make_quantity<UnitT>(a.value_ - b.value_);
     }