...

mcc/mcc_generics.h

@@ -203,6 +203,7 @@ concept mcc_coord_epoch_c = std::derived_from<T, mcc_coord_epoch_interface_t> &&
 
 template <typename T>
 concept mcc_error_c = std::default_initializable<T> && (std::convertible_to<T, bool> || requires(const T t) {
+                          { t.value() } -> std::formattable<char>;
                           { t.operator bool() };
                           (bool)T() == false;  // default constucted value must be a "non-error"!
                       });