prevent ResultError derived class being used as the expected value in Reesult class

Author: SizzinSeal

include/common/result.hpp

@@ -19,7 +19,7 @@ namespace zest {
  * @tparam Errs List of possible error types. Must inherit from ResultError.
  * Must contain at least 1 type. All types must be unique.
  */
-template<typename T, traits::IsResultError... Errs>
+template<traits::NotResultError T, traits::IsResultError... Errs>
     requires traits::NotEmpty<Errs...> && traits::AllUnique<Errs...>
 class Result {
   public:
@@ -35,7 +35,7 @@ class Result {
      * @tparam U Type convertible to T, and U not derived from ResultError
      * @param value Value to initialize the result with.
      */
-    template<typename U>
+    template<traits::NotResultError U>
         requires std::convertible_to<U, T>
     constexpr Result(U&& value)
         : value(std::forward<U>(value)),
@@ -48,7 +48,7 @@ class Result {
      * @param error Error to store.
      * @note Requires T to have a defined sentinel value (via SentinelValue<T>).
      */
-    template<typename U, traits::IsResultError E>
+    template<traits::NotResultError U, traits::IsResultError E>
         requires std::convertible_to<U, T> && traits::InPack<E, Errs...>
     constexpr Result(U&& value, E&& error)
         : value(std::forward<U>(value)),
@@ -94,7 +94,7 @@ class Result {
      * - The fallback value must be convertible to the value type of the Result type returned by the
      * callable.
      */
-    template<typename Self, typename F, typename U>
+    template<typename Self, typename F, traits::NotResultError U>
         requires std::invocable<F, decltype((std::declval<Self>().value))>
                  && traits::IsResult<traits::value_return_t<Self, F>>
                  && traits::
@@ -181,7 +181,7 @@ class Result {
      * @param self the current Result instance
      * @param val the value to use if self contains an error
      */
-    template<typename Self, typename U>
+    template<typename Self, traits::NotResultError U>
         requires std::convertible_to<U, T>
     constexpr T or_default(this Self&& self, U&& val) {
         if (self.has_error()) {

include/common/result_impl.hpp

@@ -102,6 +102,12 @@ concept AllUnique = ([]<typename U>(std::type_identity<U>) {
 template<typename T>
 concept IsResultError = std::is_base_of_v<ResultError, std::remove_cvref_t<T>>;
 
+/**
+ * @brief Check if a given type is not derived from ResultError
+ */
+template<typename T>
+concept NotResultError = !IsResultError<T>;
+
 /**
  * @brief Check whether a given type is in a typename pack
  *
@@ -232,7 +238,7 @@ using error_return_t = invoke_result_t<F, decltype((std::get<E>(std::declval<R>(
 } // namespace traits
 
 // forward-declare Result
-template<typename T, traits::IsResultError... Errs>
+template<traits::NotResultError T, traits::IsResultError... Errs>
     requires traits::NotEmpty<Errs...> && traits::AllUnique<Errs...>
 class Result;