Better error messages for unknown variant type (#2377)

* Better error messages for unknown variant type

* MSVC fix

* precompute length for compilers

Author: stephenberry

include/glaze/core/meta.hpp

@@ -721,6 +721,60 @@ namespace glz
       }
    }();
 
+   // Compile-time string listing the variant type IDs (e.g. "TYPE_A, TYPE_B, TYPE_C")
+   // Used for error messages when no matching variant type is found
+   namespace detail
+   {
+      template <is_variant T>
+      inline constexpr size_t variant_ids_string_len = [] {
+         constexpr auto& ids = ids_v<T>;
+         constexpr auto N = ids.size();
+         size_t len = 17; // "supported types: "
+         for (size_t i = 0; i < N; ++i) {
+            if (i > 0) len += 2; // ", "
+            len += ids[i].size();
+         }
+         return len;
+      }();
+
+      template <is_variant T, size_t Len>
+      inline constexpr auto variant_ids_joined = [] {
+         constexpr auto& ids = ids_v<T>;
+         constexpr auto N = ids.size();
+         std::array<char, Len + 1> arr{};
+         const char* prefix = "supported types: ";
+         size_t pos = 0;
+         for (size_t i = 0; i < 17; ++i) arr[pos++] = prefix[i];
+         for (size_t i = 0; i < N; ++i) {
+            if (i > 0) {
+               arr[pos++] = ',';
+               arr[pos++] = ' ';
+            }
+            for (auto c : ids[i]) arr[pos++] = c;
+         }
+         arr[Len] = '\0';
+         return arr;
+      }();
+   }
+
+   template <is_variant T>
+   inline constexpr std::string_view variant_ids_string_v = [] {
+      constexpr auto& ids = ids_v<T>;
+      constexpr auto N = ids.size();
+
+      if constexpr (N == 0) {
+         return std::string_view{};
+      }
+      else if constexpr (std::is_same_v<std::decay_t<decltype(ids[0])>, std::string_view>) {
+         constexpr auto Len = detail::variant_ids_string_len<T>;
+         auto& static_arr = detail::make_static<detail::variant_ids_joined<T, Len>>::value;
+         return std::string_view{static_arr.data(), Len};
+      }
+      else {
+         return std::string_view{};
+      }
+   }();
+
    template <class T>
    concept versioned = requires { meta<std::decay_t<T>>::version; };
 

include/glaze/json/read.hpp

@@ -3916,6 +3916,7 @@ namespace glz
                                  }
                                  else {
                                     ctx.error = error_code::no_matching_variant_type;
+                                    ctx.custom_error_message = variant_ids_string_v<T>;
                                     return;
                                  }
                               }
@@ -3972,6 +3973,7 @@ namespace glz
                               }
                               else {
                                  ctx.error = error_code::no_matching_variant_type;
+                                 ctx.custom_error_message = variant_ids_string_v<T>;
                                  return;
                               }
                            }

tests/json_test/variant_ambiguous_test.cpp

@@ -1100,6 +1100,44 @@ suite variant_default_tests = [] {
    };
 };
 
+// ============================================================================
+// Variant error message tests - verify supported types in error messages
+// ============================================================================
+
+// Variant without a default handler (ids_size == variant_size)
+using StrictActionVariant = std::variant<CreateAction, UpdateAction>;
+
+template <>
+struct glz::meta<StrictActionVariant>
+{
+   static constexpr std::string_view tag = "action";
+   static constexpr auto ids = std::array{"CREATE", "UPDATE"};
+};
+
+suite variant_error_message_tests = [] {
+   "variant error message includes supported types - tag first"_test = [] {
+      std::string_view json = R"({"action":"DELETE","id":"456"})";
+      StrictActionVariant av{};
+      auto ec = glz::read_json(av, json);
+      expect(ec == glz::error_code::no_matching_variant_type);
+      auto error_msg = glz::format_error(ec, json);
+      expect(error_msg == R"(1:19: no_matching_variant_type
+   {"action":"DELETE","id":"456"}
+                     ^ supported types: CREATE, UPDATE)") << error_msg;
+   };
+
+   "variant error message includes supported types - tag last"_test = [] {
+      std::string_view json = R"({"id":"456","action":"UNKNOWN"})";
+      StrictActionVariant av{};
+      auto ec = glz::read_json(av, json);
+      expect(ec == glz::error_code::no_matching_variant_type);
+      auto error_msg = glz::format_error(ec, json);
+      expect(error_msg == R"(1:31: no_matching_variant_type
+   {"id":"456","action":"UNKNOWN"}
+                                 ^ supported types: CREATE, UPDATE)") << error_msg;
+   };
+};
+
 // ============================================================================
 // Numeric variant tests - int64_t and double
 // ============================================================================