registry/storage/sparse_set: minor changes to get around an issue of gcc7.5

src/entt/entity/registry.hpp

@@ -111,7 +111,7 @@ public:
     }
 
     template<typename ILhs, typename IRhs>
-    friend auto operator-(const storage_proxy_iterator<ILhs> &, const storage_proxy_iterator<IRhs> &) ENTT_NOEXCEPT;
+    friend std::ptrdiff_t operator-(const storage_proxy_iterator<ILhs> &, const storage_proxy_iterator<IRhs> &) ENTT_NOEXCEPT;
 
     template<typename ILhs, typename IRhs>
     friend bool operator==(const storage_proxy_iterator<ILhs> &, const storage_proxy_iterator<IRhs> &) ENTT_NOEXCEPT;
@@ -124,7 +124,7 @@ private:
 };
 
 template<typename ILhs, typename IRhs>
-[[nodiscard]] auto operator-(const storage_proxy_iterator<ILhs> &lhs, const storage_proxy_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
+[[nodiscard]] std::ptrdiff_t operator-(const storage_proxy_iterator<ILhs> &lhs, const storage_proxy_iterator<IRhs> &rhs) ENTT_NOEXCEPT {
     return lhs.it - rhs.it;
 }
 

src/entt/entity/sparse_set.hpp

@@ -96,7 +96,7 @@ private:
 };
 
 template<typename Type, typename Other>
-[[nodiscard]] auto operator-(const sparse_set_iterator<Type> &lhs, const sparse_set_iterator<Other> &rhs) ENTT_NOEXCEPT {
+[[nodiscard]] std::ptrdiff_t operator-(const sparse_set_iterator<Type> &lhs, const sparse_set_iterator<Other> &rhs) ENTT_NOEXCEPT {
     return rhs.index() - lhs.index();
 }
 

src/entt/entity/storage.hpp

@@ -122,7 +122,7 @@ private:
 };
 
 template<typename CLhs, typename CRhs>
-[[nodiscard]] auto operator-(const storage_iterator<CLhs> &lhs, const storage_iterator<CRhs> &rhs) ENTT_NOEXCEPT {
+[[nodiscard]] std::ptrdiff_t operator-(const storage_iterator<CLhs> &lhs, const storage_iterator<CRhs> &rhs) ENTT_NOEXCEPT {
     return rhs.index() - lhs.index();
 }