improve Result type constraints

Author: liam-teale

include/common/result.hpp

@@ -13,12 +13,13 @@ namespace zest {
  *
  * @note the "normal" value is always valid, but an "error" value may or may not be contained.
  *
- * @tparam T Type of the expected value. Must have
- * @tparam Errs List of possible error types (must inherit from ResultError).
+ * @tparam T Type of the expected value. Must have a Sentinel Value.
+ * @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>
-    requires(traits::HasSentinel<T> || std::same_as<void, T>) && (sizeof...(Errs) > 0)
-class Result {
+template<traits::HasSentinel T, traits::IsResultError... Errs>
+    requires traits::NotEmpty<Errs...> && traits::AllUnique<Errs...>
+class Result<T, Errs...> {
   public:
     // instead of wrapping the variant in std::optional, we can use std::monostate
     std::variant<std::monostate, Errs...> error;
@@ -211,12 +212,9 @@ operator==(const Result<LhsT, LhsErrs...>& lhs, const Result<RhsT, RhsErrs...>&
     return lhs.get_value() == rhs.get_value();
 }
 
-/**
- * @brief Result specialization for void value type (no stored value).
- * @tparam Errs List of possible error types (must inherit from ResultError).
- */
+// Result void specialization
 template<traits::IsResultError... Errs>
-    requires(sizeof...(Errs) > 0)
+    requires traits::NotEmpty<Errs...> && traits::AllUnique<Errs...>
 class Result<void, Errs...> {
   public:
     using value_type = void;

include/common/result_impl.hpp

@@ -79,6 +79,37 @@ class ResultError {
 
 namespace traits {
 
+/**
+ * @brief Check if a type pack is empty
+ *
+ * @tparam Ts the type pack to check
+ */
+template<typename... Ts>
+concept NotEmpty = sizeof...(Ts) > 0;
+
+/**
+ * @brief Check if all types in a type pack are unique
+ *
+ * @tparam Ts types to check
+ */
+template<typename... Ts>
+struct all_unique : std::true_type {};
+
+template<typename T, typename... Ts>
+struct all_unique<T, Ts...> :
+    std::conditional_t<
+        (!std::same_as<T, Ts> && ...) && all_unique<Ts...>::type,
+        std::true_type,
+        std::false_type> {};
+
+/**
+ * @brief Check if all types in a type pack are unique
+ *
+ * @tparam Ts types to check
+ */
+template<typename... Ts>
+concept AllUnique = all_unique<Ts...>::value;
+
 /**
  * @brief Check if a given type is derived from ResultError
  */
@@ -310,8 +341,7 @@ struct invocable_indirect_v {
 } // namespace traits
 
 // forward-declare Result
-template<typename T, traits::IsResultError... Errs>
-    requires(traits::HasSentinel<T> || std::same_as<void, T>) && (sizeof...(Errs) > 0)
+template<typename T, typename... Errs>
 class Result;
 
 namespace traits {