message: Specify return type of reference operators again
diff: @@ -127,22 +127,26 @@ struct enable_reference_operators {
  using arithmetic_type = std::remove_cv_t<ArithmeticType>;
  #define GKO_REFERENCE_BINARY_OPERATOR_OVERLOAD(_op) \
  - friend GKO_ENABLE_REFERENCE_CONSTEXPR GKO_INLINE GKO_ATTRIBUTES auto \
  + friend GKO_ENABLE_REFERENCE_CONSTEXPR GKO_INLINE GKO_ATTRIBUTES \
  + arithmetic_type \
  operator _op(const Reference &ref1, const Reference &ref2) \
  { \
  return to_value_type<arithmetic_type>(ref1) \
  _op to_value_type<arithmetic_type>(ref2); \
  } \
  - friend GKO_ENABLE_REFERENCE_CONSTEXPR GKO_INLINE GKO_ATTRIBUTES auto \
  + friend GKO_ENABLE_REFERENCE_CONSTEXPR GKO_INLINE GKO_ATTRIBUTES \
  + arithmetic_type \
  operator _op(const Reference &ref, const arithmetic_type &a) \
  { \
  return to_value_type<arithmetic_type>(ref) _op a; \
  } \
  - friend GKO_ENABLE_REFERENCE_CONSTEXPR GKO_INLINE GKO_ATTRIBUTES auto \
  + friend GKO_ENABLE_REFERENCE_CONSTEXPR GKO_INLINE GKO_ATTRIBUTES \
  + arithmetic_type \
  operator _op(const arithmetic_type &a, const Reference &ref) \
  { \
  return a _op to_value_type<arithmetic_type>(ref); \
  }
  +
  GKO_REFERENCE_BINARY_OPERATOR_OVERLOAD(*)
  GKO_REFERENCE_BINARY_OPERATOR_OVERLOAD(/)
  GKO_REFERENCE_BINARY_OPERATOR_OVERLOAD(+)
  @@ -170,8 +174,6 @@ struct enable_reference_operators {
  GKO_REFERENCE_ASSIGNMENT_OPERATOR_OVERLOAD(operator-=, -)
  #undef GKO_REFERENCE_ASSIGNMENT_OPERATOR_OVERLOAD
  - // TODO Add overloads for comparison operators if needed
  -
  friend GKO_ENABLE_REFERENCE_CONSTEXPR GKO_INLINE GKO_ATTRIBUTES
  arithmetic_type
  operator-(const Reference &ref)
  